The Stovebolt Site Forums The Parking Lot Big Bolts How to adapt to dual reservoir cylinder?

I have a 49 two ton with a dump that I haul heavy loads up and down steep passes with and I need a dual cylinder master brake cylinder. The easiest way, it seems to me, to mount it would be under the floor, but that would mean cutting a much bigger hole in the floor, which I could do, but has anybody mounted it on the firewall and used a later model linkage? I could probably figure it out by looking at the linkage on my 79 chevy but would that require using the 79 booster too? Would mounting it anywhere under the hood require a booster? I don't want to experiment, but if someone has already done this I would appreciate some reference to a good youtube vid or something like that. (thats my next stop). Thanks

49TT - Welcome to Stovebolt!

Since your truck seems to fit into the "Big Bolt" category best, I've moved your thread to that forum for help from those most knowledgeable of "Big Bolt" trucks.

Others should be along soon to offer some help and pointers on your dual master cylinder brake upgrade.

Hi 49TT, welcome. If your original brake system did not have a vacuum boost and you install a new master cylinder with the exact same piston bore and stroke as the original, the brakes should feel ok. If the truck had/has a booster, you will want to incorporate one into the new system. My '57 2 ton has a divorced booster, Bendix HydoVac. Without the booster in the system it takes much more peddle force to control the braking. Just like a modern truck or car, the brakes will work manually if the engine is not running.

If you're running a loaded 2 ton without a Hydrovac booster, you're a hazard to yourself, and everyone who shares the road with you. In order to convert to a dual chamber master cylinder and do it properly, I'd suggest looking into a system that Winnebago used on some of its motorhomes, which incorporated a dual chamber MC and two remote vacuum boosters, one for each axle. Swapping to the entire dual brake setup, including the twin boosters, from a retired motorhome would allow you to take advantage of pre-existing engineering, and make the installation as safe as possible. The only advantage of a dual chamber MC over the single unit your truck was engineered for is the redundancy of having half a brake system if a line or a wheel cylinder fails on one axle. Use ALL the associated components of a dual system, such as proportioning valves, lines, booster(s), etc. not just the master cylinder. The system needs to be able to isolate a failed part in case of a problem, and still retain partial braking.
Jerry

I would be cautious using dual hydro boost may increase line pressure too great for the system those huck wheel cyclinders may leak at a higher pressure

Thanks guys for the welcome. And for moving this to the right location. 78 Buckshot my truck had no booster but I'm definitely considering it. I've replaced them before but never added one. I see there are some kits out there to do that with and when it comes to this project money is no consideration. I want the best. I'll definitely include a proportioning valve. I exchanged one once on my 79 Chevy K20 when I put a disc kit on the rears. Which turned out fine. HotrodLincoln if Hydrovac is just another way of saying "booster" or are you saying that is the ony brand that is safe enough (being best). I'll say this though about this project, I don't want a system thats going to be too difficult to use if the boost gets lost for some reason. I've seen some dual reservoirs for 47-54 that are 1 1/8" diameter and that I think might be bigger than the one thats in there now. So that's my main objective, is the bigger bore if I can find it. Then if the boost goes I'm still getting good power to the brakes without breaking a sweat.

Here are a few things I've discovered since I asked the question. Theres a good Youtube vid by Brothers auto parts on this where he replaces the under the floor factory single reservoir with a dual that has a top with two fittings that are fed from a higher up reservoir, remote under the hood, which eliminates the residual valves. Which help with the under-floor issue of drain-back, as you probably are aware of. Then I looked at the big bunch of masters on the Summit website. There are quite a few there, some with firewall mounted boosters. So I'm looking at all of that. At first I just wanted the Brothers dual under the floor but now I'm going for the big package. Will keep you updated. Maybe someone will recommend me a kit that worked well for them. Thanks again

I just ordered a bunch of parts direct from Classic Performance Products. I got the 1.125" bore dual reservoir master, with a 8" booster, adapter mount, Proportioning valve that is what the guy calls "hybrid" meaning it can work with either disc/drum or drum/drum systems, and the remote cover for the master which has two inlets that you feed from one remote reservoir on the firewall. So its all under the floor except the remote reservoir. $421 without shipping.
A lot of the Ebay kits are 1.00" bore and some say only for automatic transmissions. Some say "for disc/drum" which begs the question: what about drum/drum.

I think if I lose the front or rear with only one booster, I still have boost to the good half right? And I still have the gears to slow me. I dont really have time right now to go to firewall boosters but if you have pictures of it I'd be interested in learning more. CPP cant ship until they do inventory Jan. 6 so have time to cancel the order

"Hydrovac" is a generic name for a frame-mounted power brake booster, which virtually all big rigs were equipped with at the factory. For instance:

www.ebay.com/itm/296905636303?

It will be mounted somewhere behind the cab, under the bed. It uses engine vacuum to apply the brakes much more forcefully than the driver can do with foot pressure alone. The single chamber master cylinder provides the pressure to trigger the vacuum boost. In case of a loss of engine vacuum, the brake system still works, but without power assist. Any attempt to modify the brake system on a commercial vehicle hauling heavy loads should be made by someone who is thoroughly familiar with the physics involved. I've been an ASE certified master heavy truck technician for over 40 years, and I would be reluctant to attempt the modifications you're considering.
Jerry

All of the "kits" found on line are for cars and 1/2 ton trucks, they have the smaller diameter MC. Your truck came from the factory with a 1.25" MC. If you use a MC with a smaller diameter you end up having to push the pedal further to the floor to move the needed amount of fluid to activate the brakes. This could be dangerous if you get low on fluid or out of adjustment due to normal wear! I've never done the conversion you're asking about on a big truck (only on 1/2 tons), but have always understood you need to match the MC bore sizes when doing it.

I like Jerry's idea of finding a motorhome as a donor and moving ALL of that brake system components onto your chassis...someone else has already done the engineering for you!

Be safe!

Mike B

Thanks very much for this information, I'll cancel that order and look for the motorhome setup. I can go look at a motorhome's system but first I'd like to know what you envision it to be, would I keep my pedal and put a MC under the floor to work off the existing petal's bar? I can't imagine how any booster can be remote from a MC. If the MC has to be 1.25" this may take some time. I would think that going slightly bigger, say 1.375 might be just as good. I haven't taken the MC off yet so didn't know it was 1.250. I wouldn't have gone smaller.

Hotrod when you say the single chamber MC loses it's boost when the engine dies, but you still have pedal without boost, what about if there's a fluid leak and you lose that one piston (chamber)? What about the need for a dual chamber MC? Doesn't seem so safe to me, but I don't know much about this. I replaced all the leaking slave cylinders but the master was mushy.

BTW, do you happen to know if those hycrovac's have a rubber membrane in them that would dry rot sitting on the shelf? That was NOS in the Ebay ad. Many thanks

I'm watching Youtube vids on hydrovac, they're answering some questions. It looks not to difficult, since hydrovac just works "inline" between MC and wheels. The vids are very good. One explains why hydroboost replaced hydrovac

Maybe a hydro-boost system would work better and be easier to install. You would have to have a power steering pump, or some kind of hyd pump to make it work. Check with power brake systems on the web.

Any large vehicle (1 1/2 ton or bigger) would have come from the factory with a hydrovac, I'm pretty sure. If yours doesn't have one, it's probably been removed or disabled. That's deadly dangerous. Untold millions of commercial vehicles were driven with single line hydraulic brakes from the 1930s to the 1970s or later. There is absolutely nothing wrong with a properly maintained single line brake system with a remote booster. My grandparents' company ran tractor-trailers based on 1934 Chevy 1 1/2 ton trucks between Milwaukee or St. Louis to Nashville during the late 1930s, hauling beer, with mechanical brakes on the tractors and NO trailer brakes for many years before hydraulic brakes became popular. They eventually upgraded those trucks to vacuum-assisted mechanical brakes.

Doing major modifications to a commercial vehicle brake system could result in some massive liability issues in case of an accident. It would be much better to rebuild the entire brake system, including metal and flexible brake lines to original equipment condition, especially if you're getting paid for the hauling you're doing. People who modify brakes, suspension, or steering systems are an ambulance chaser's wet dream! My attorney brother has spent almost as much time trying those kinds of cases as I have spent fixing virtually everything that rolls, floats, or flies!

That booster I referenced was just an example- - - -there are new, or rebuilt units available from most professional level parts stores such as NAPA. Most hydrovac units have a metal vacuum piston with rubber O rings, running in a large diameter metal cylinder.
Jerry

Thanks. I'm in town and can't look at it until I get back but it might have a hydrovac on it. It's got a 13 foot flatbed. It's a 4800 with duallies. I bought it from a friend who used it to haul cows to auction for many years. I haven't used it for commerce yet, just for my own use. I've worked on a lot of brakes before. I've replaced ball joints, steering columns, tie rod ends, pinions, axles, many brake lines, proportioning valves. I have a heavy duty flaring kit that I can double flare with. If there is any question I need answered I know good mechanics and I'm not afraid to ask for help. I want to replace all the old lines metal and hose. I rebuild engines and transmissions. I have a big shop thats well outfitted. With a little advice I can do a good job.
I would buy a new hydrovac. According to one vid I''ll need a check valve for the vacuum line, and an air filter mounted on the inside of the floor, a tank. But I'll have to look at it first and see if its got one.

49TT, if you can post a couple of pictures of the firewall in the engine compartment, we might be able to see if any original vacuum plumbing is there. The check valve would have been mounted on the firewall as high above the engine as possible. Vacuum lines for the HydroVac were1/2 inch steel with rubber flex sections. The HydroVac on my truck is mounted on the left frame rail under the drivers seat/step.

Thanks for the good ideas. I'll get a picture up, thats easy once I get back home. I dont recall seeing half inch pipe but I dont always notice everything.

It just occurred to me that IF a dual reservoir MC with two one inch bore cylinders replaces my single reservoir with 1.250" bore, I'm not reducing the pedal power, I'm increasing it. Or so it seems to me. If decreasing the bore size reduces braking power, wouldn't adding an extra cylinder increase it? What do you think about that? My old MC is worn out and needs replacing in any case. Then consider that those two cylinders are getting pressure from an 8" diameter married booster under the floor, and it seems if there's no hydroboost, I've greatly increased the power of the brakes. If I do find there is a hydroboost, it seems like simply replacing the MC with a dual one would be an improvement ? There are a LOT of those duals with boost being sold as upgrades on half tons so why wouldn't it be also an upgrade for a two ton without hydrovac. I do want the best though and it does look like hydrovac or hydroboost is the best. It might be a few days until I can get out there

Quote:

All of the "kits" found on line are for cars and 1/2 ton trucks, they have the smaller diameter MC. Your truck came from the factory with a 1.25" MC. If you use a MC with a smaller diameter you end up having to push the pedal further to the floor to move the needed amount of fluid to activate the brakes. This could be dangerous if you get low on fluid or out of adjustment due to normal wear! I've never done the conversion you're asking about on a big truck (only on 1/2 tons), but have always understood you need to match the MC bore sizes when doing it.

A dual reservoir MC does not have two independent pistons- - - -they are inline with each other, so there's no increase in volume. A column of fluid between the front and rear pistons actually pushes the forward piston. If there's a leak in one circuit, the pistons come into physical contact with each other, the pedal goes down to less than half its original height, and the remaining undamaged system gives you enough braking to come to a stop- - - - -longer stopping distance, much less efficient, but not a total loss of braking effect. That requires a very much different plumbing setup, and several valves that respond to the loss of pressure on one end to isolate the failed system from the good one. At one time, there was a "dual-diagonal" setup, where each chamber of the master cylinder operated one front and one rear brake. That one was a real nightmare to fix, especially trying to bleed the brakes!
Jerry

Good to know, thanks. So it's either hydrovac or hydroboost. I'll look for the hydrovac on it asap

Most, if not all hydrovac systems have a vacuum reservoir tank mounted somewhere inside a frame rail with a one way check valve between the intake manifold and the tank. The tank stores enough vacuum to allow one or two brake applications if the engine stalls for some reason. If there's a vacuum shift two speed rear end, that same vacuum tank makes axle ratio shifting smoother.
Jerry

My mostly unmolested truck did not have a vacuum reservoir when I acquired it, does not appear to have had one from the factory. I swapped my single speed rear differential for a two speed which is vacuum operated, I added a vacuum tank to the right frame rail and plumbed into the line at a factory tee. Works very well.

That all sounds pretty simple, thanks. How big a tank is typical? Theres a brand new one on Craigslist here thats half a gallon. It's got a mounting bracket.
I have a two speed rear end which does shift pretty nice, although I haven't used it much. In fact I haven't driven the truck much since I bought it 6 years ago. I had a load of gravel on it and took a county road short cut because it wasn't insured and forgot about how steep that road got in one place. I got home OK but checked the fluid level on the MC as soon as I got back and it was completely empty. That put the fear of god into me and I haven't driven it since. I figured it was holding fluid because the inside of the tires were all dry and the MC wasn't wet anywhere, but the pressure from driving it must have opened up a leak somewhere. I since have replaced three of the slave cylinders but it's leaking somewhere just sitting that i cant find. Maybe it's leaking out a slave cylinder and coming down between two dual tires? I am determined to replace every part of that brake system. This truck only has 75,000 miles on it. The previous owner said he only drove it in the fall to bring his cows in to sell. it's completely original except I took out the knocking 216 and replaced it with a nice rebuilt 235 that runs like a charm. Altho I might put one of my 292's in it some time.

I read somewhere that it's best to mount the hydrovac within two feet of the MC? And that the check valve should go on the firewall? I'm curious to find out what is there but can't make it over for a few more days.

Due to having some on hand as I am a retired HVAC technician, I used a disposable refrigerant canister of approximately 2 gallons for my reservoir. I used some steel plumbers strap to fabricate mounts to the frame without drilling the frame rail. If your truck has the check valve, it should be on the firewall above the engine. Part of the plumbing configuration was to run the main vacuum line from the intake manifold UP to the check valve, in hopes that any condensing fuel vapor would not run down the line to the HydroVac. The MC and HydroVac on my truck are roughly 3 feet apart, connected only by the 1/4" brake line. I bought a rebuild kit for my booster and did the job myself, I rebuilt all six wheel cylinders and the master, replaced 100% of the hard lines and rubber lines. The brake system works very well with no leaks, I have only had 4500-5000 lbs on the truck so I can't say how it would handle the 16,000 GVW that it is rated for.

Many thanks, I found several rebuild kits for 49 Hydrovac's online. It will be interesting to see what is on there now. If there's no hydrovac then I'll just have to put one on. I googled and found out that the 49 series 3000 two ton (didn't know there was such a thing) did not have hydrovac, and that the 49 "heavy duty" (meaning, I guess my 4800) sometimes did and sometimes did not. I know mine is a 4800 because the numbers are still clipped onto the side of the hood.

Good luck with your project. Back when I was teaching auto mechanics, I made it a point to stress the idea to my students that where brakes, lights, steering, and other "safety of operation" systems are concerned, the very best you can do is just barely "good enough". "I'll be happy to help you with your 'go fast' modifications- - - - -but we're going to make it steer and stop safely first!" At least some of them got the message!
Jerry

Thats great advice to anyone for sure. Thanks for the offer. Was just reading what you said on Saw Mill's Rear Wheel Cylinders Assistance thread.
It seems to me if a shop could sleeve them that if you had several the cost might not be so high. I had a hard time finding a shop that could bore 2.54" cylinders but maybe he can do 1.5" too. (1.75) He uses the main bearing caps to help line the holes. But the brake cylinders maybe wouldn't need to be lined at all. (?)

Cylinder man says a vertical mill would be the way to go. I don't have one but I know where there is one

I use a lathe, with the wheel cylinders or the master cylinder mounted on a holding fixture in a 4 jaw chuck. If you have several cylinders to reline, it's easy to make an alignment fixture that plugs into the tailstock that's a slip fit in the bore- - - -then adjust the 4 jaw chuck with a "last word" dial indicator to keep the hole on center. Then use a carbide tipped boring bar to open up the cylinder for a pressed-in sleeve. Bore and hone the sleeve to a finish diameter. Setup time for the first one is probably 75% of the job.
Jerry

I'm not seeing it, can you send a picture? My tailstock just takes a morse taper, so I don't understand how the fixture can plug into the tailstock. Mine is a 1906 Logan though. Anyway, you put the cylinder in the fixture and the fixture in the 4 jaw chuck, how do you center both end of the cylinder at the same time, with the dial indicator? Seems to me as soon as you get one end of the bore centered, the other one is off.

I use a Morse taper in the tailstock with a 3/8-24 male thread that points toward the chuck, which is normally used to mount a drill chuck. Then I make a mandrel about 2" long that threads onto the taper that is a snug slip fit into the bore of the cylinder. This fixture keeps the cylinder bore aligned with the lathe ways. I use a fixture that bolts onto the mounting holes of the cylinder- - - -a piece of angle iron or flat stock welded to a piece of pipe that fits into the 4 jaw chuck. Once everything is rough-aligned, I use the last word dial indicator to do any final centering of the cylinder bore- - - -which is usually not necessary.

www.ebay.com/itm/177562654305?_

The centering device can be swapped to any diameter and/or length needed to fit master cylinders or wheel cylinders of any size. I just sent you a private message. Click on the flashing envelope icon at the top of any page.
Jerry

Thanks! It has hydrovac. All the lines are there and the check valve is on the firewall. Should I take it out and test it? I’m thinking I’ll rebuild the hydrovac the membrane inside has got to be cracked

Rather than rebuild it, you might check to see if NAPA or one of the other professional-grade parts stores can order a reman unit and use your old one as a core exchange. Hydrovacs aren't particularly brand-specific- - - -Bendix sold them to GM, Ford, MOPAR, International, and most other manufacturers by gross weight rating, rather than specific models, similar to big rig air brake systems.
Jerry

Haven’t been able to find a reman hydrovac yet, Cardone has one listed on Rock Auto but it’s out of stock. The rebuild kits I saw on EBay were not for my hydrovac, I found the tag on it…it’s 574550. I emailed a brake parts specialty supplier Accuparts, should get a reply Monday…. I was considering a hydroboost unit off a one ton, I’d have to move the MC to the firewall but the unit would get pretty close to the carb’s air filter, which wouldn’t be too hard to deal with, just elbow on the carb, altho I’d rather keep it looking stock as possible. Also I’m wondering if converting brakes might cancel an insurance policy should I have an accident, need to ask my broker. And it definitely needs some steering parts, a quick search wasn’t encouraging…kingpins are easy to find and gearbox is rebuildable IF the parts are available, need to do an inspection though to find out what’s needed.

Try contacting a place called "Then and Now Rebuilders". They do an excellent job of remanufacturing fuel pumps and water pumps, (better than new) and it's possible they might also work on Hydrovacs. Good point on insurance coverage, especially on something as large and heavy as a dump truck! Better safe than sorry!

https://www.then-now-auto.com/

Jerry

Thanks Jerry, They said they can do it so I’ll probably ship it to them, just trying to find instructions on rebuilding it right now as I may be able to get a kit.
The MC needs a kit too, and a sleeve, it’s slightly pitted.
Can’t find a new one for a 6400 series. ( I said before it was a 4800) Have only seen them for 3000 or 3800. Might take it to a machine shop.

49TT - Your hydrovac should be a Bendix 374000 that is about 15" long overall. Same as the hydrovacs for all 1946-57 "Utility" trucks (except those with the H.D. brake booster). That's according to the1929-57 Chevrolet Master Parts book (excerpt below). Also attached below from the Master Parts book is an exploded diagram of the unit. Go check out the 1948-51 Chevrolet Truck Shop Manual, starting on Page 5-29 for info on repair and replacement info.

If ypu do send off your hydrovac to "Then and Now Rebuilders", send them your master cylinder too. They can do both.

Thanks Gdads51, these pics will help if I rebuild it myself. I found two sources now for kits. This Bendix is a 547550, according to the tag. Maybe somebody put the wrong one in? Should I go ahead and order the 374000? This one looks like brand new inside except for a little sawdust type dirt inside. No corrosion. 7" diameter.

Any brake boost at all is better than none, but if the piston diameter is noticeably smaller boost will be reduced, as it is directly proportional to the piston diameter. When there are more square inches of surface area for atmospheric pressure to push on with a bigger piston, the thrust on the hydrovac piston is increased. Just an inch or two of diameter difference will make a huge difference in stopping effect.

Back in the late 1950s my father put a 3/4 ton truck hydrovac on a 49 Oldsmobile 98 sedan. That thing would stop on a dime, and give some change back!
Jerry

49TT - I couldn't find anything listing a Bendix "547550" hydrovac. Sounds like a replacement, but I also couldn't find any reference to a repair kit for the same numbered unit. You indicated you found sources for kits, but are those specifically for the Bendix 547550, or were they kits for the 374000?

After a little deeper diving through multiple Chevrolet Master Parts books, I found later books show a Bendix 378802 (GM# 3761237) listed to replace the Bendix 374000 (GM# 2208488) in a 1962 Chevrolet Master Parts Book. Both the 1957 and earlier books and the 1962 book listed these as fitting ALL Utility Trucks (1-1/2 & 2 ton), so I think either one works for your application.

With that in mind, a little further web surfing found you can get a remanufactured Bendix 374000 from Powerbrake-Drivetrain for about $795, which includes a $200 refundable core charge. Your choice on whch way you decide to proceed, but this is another option to consider.

Looking forward to hearing which way you go.

BTW - Posting some pictures of your truck and the items being discussed can often help folks help you in solving some of these hard to resolve things. Plus we love to see what your working on!

I'll put a picture up, it's a 5 window, none of the original paint is left but theres only one rust hole in the rear corner of the cab. was blue originally. Only has 75,000 original miles, I'm the third owner. It was mostly used to haul cows in the fall, so it's all original, the seat is like new, has a canvas cover. The box and bed were made by the previous owner, and he did a really nice job so it's not too heavy, he's a professional fabricator. The bed is wood and the sides are steel, with removeable panels.
Thanks for doing all the digging, I'll pass those numbers on to the kit suppliers with kits and see what they say. Accuparts said they had the kit I needed but I don't know if they found one for a 547550 but I'll ask them tomorrow. They just sent me a email. I haven't talked to the other company yet. The inside of mine is like new so I'd rather rebuild it than buy another one, unless I need a bigger than my 7" dia. Anybody know what diameter those other numbers are?