The Stovebolt Site Forums The Parking Lot Big Bolts HYDRAVACS Is there a way to get rid of them?

Over the years (2-3 decades ago) if you were working on a heavy truck with juice brakes and the brakes were doing funny things, first thing you did was replac the hydrovac and the issue usually went away.. Thankfully Hydrovacs are a thing of the past. Unfortunately so are replacement parts....and the people who knew anything about them are becomming hard to find....

So on to my current dilemma... Brought home a Coleman 4x4 Tug Tractor, 4 wheel steer rated for 130,000 pound air plane spotting. Will go about 25 miles an hour with a good tail wind. Had been sitting a few years after being rebuilt by a retired aircraft mechanic. I put brake fluid in the small fluid reservoir and let it set a couple weeks. Pushed the pedal a couple times and got a hard pedal that felt right... Started the engine and with the vacuum assistance pushed on the brake pedal a couple times, felt right then on the 3rd push, pedal went to the floor.. Pumped the brakes till the reservoir was dry and looked underneath for a leak. Nothing.. Repeated this process till I had pumped through 2 quarts of brake fluid and find no leaks underneath.. Fluid must be in the hydrovac...?

Currently there is a passenger car size vacuum booster on the firewall and a small single reservoir master cylinder with one brake line coming out of that down to the hydrovac bolted to the frame. I am assuming the function of the hydrovac is to send a large amount of fluid out to the wheel cylinders when signaled by the small master cylinder roughly figuring? The odds of finding a replacement hydrovac would likely be slim...

My question- I look at more modern high gvw trucks in the boneyard with juice brakes and they have LARGE vacuum boosters on the firewall with LARGE master cylinders with double reservoir. This is a slow moving off road vehicle that will never again be towing a 130,000 pound aircraft unassisted. I'm just gonna plow my driveway with it. Just need some brakes... Anybody have any thoughts of the success of stripping out the larger vacuum booster and master cylinder from the modern high GVW truck and having enough fluid volume movement capacity from the large master cylinder to equal the hydrovac and make the wheel brakes somewhat functional? I won't be attempting to stop any 130,000 pound aircraft... Recognized the large modern master cylinder on the 80's GMC 6500 I'm looking at as a donor vehicle is double reservoir and I"d have to take the divider block from the donor truck and feed each axel from the divider block. Am I on to a possible off road fix or completely looking down a path to wasted time/failure. What do you think guys?

There are plenty of sources for new/rebuilt hydrovacs. Betting your life on a home-brewed brake system on a rig that big is like drawing to an inside straight- - - -not a wager I'd want to make. Either fix it right or make it yard art!
Jerry

Well.... That was a fervently expressed opinion....

Anyone else have any input based on the mechanics of the idea?

If it were a road vehicle I wouldn't fool with the brakes either... As it is when plowing a person jogging liesurely would run circles around it, and after you take your foot off the gas real quick it slows to a crawl then after you push the clutch in it travels maybe 10 feet with the plow down.. Stops real quick if you pull the emergency brake/drum on driveshaft... I think I'm more intrigued by the idea of will it work than actually needing footbrakes..

Midwest Power Products in Morris, Il. 815 943 0550 can rebuild your hydrovac also he can get you a new one. He can resleeve Master cyls. ectra. I got a brand new one from him for a 57 GMC. Real nice guy, name is Rich. He can do anything with brakes from the 20's to the present.

As some one who has recently reworked the brake system on my 46 to have disc brakes and a modern master with booster I can tell you that it take a lot of time to make something that works. I probably have a 150 hours in making mine work correctly. Now, it stops extremely well and I drive it in stop and go traffic.

Find a source for the hydrovac and send it in for repair. Even at $500 repair would be much cheaper and easier than adapting something else. Now if you want it to work better that is a different story!

My disc brake thread... https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=708681#Post708681

Good luck which ever way you choose to go!

You didn't say what year your Tugger is. You can still get rebuilt Hydrovac's to fit 1950's trucks from NAPA.

Before you replace the MC you will need to know the size of the wheel cylinders and how many there are.

The Hydrovac is nothing more than a Booster, when you press on the brake pedal the Hydrovac piston pushes it's MC for added system pressure. I think I read somewhere the Hydrovac adds an additional 900-1000 pounds of pressure to the system. If your truck has a firewall MC with a booster then you get the added boost from it first and then the added boost from the Hydrovac.

I agree with Jerry and Randy...will be way cheaper to repair the existing system than to try and engineer something else.

Mike B

I have first-hand experience in the up-grade dual-reservoir/booster swap. We tried this on a '59 Viking bus. Rear brakes do not work,and front brakes take a MILE,to stop the bus,from 30 M.P.H.! (solid pedal!!) That's an exaggeration,but you get the idea. All lines were pressure-tested,and even at 1,000#,(rear),the brakes don't engage well enough to be effective.(Yes,they,re adjusted properly.) "Billy W.,(owner),opted to discard the hydrovac,and now,wishes he hadn't. I plan to use a dual-reservoir M.C.,but I'll have my hydrovac rebuilt,since it does a GREAT job of stopping my T.F.'55 6500. It seems that the hydrovac supplies more pressure,than a "newer' firewall-mounted booster. I've read many posts,in regard to Midwest Power,in Morris,Il,and they're ALL positive reviews. That's the place I'm sending mine,when money permits. Hope this helps.

OKAY, NOW WE ARE COOKING. Some one who tried the idea and advises against it. I'm SOLD...... Pulling the hydrovac and researching how to get it identified and repaired is the route I'm looking at now. THANKS WETWILLY!!!!!!!!

As I mentioned above, the NEW Hydrovac I got from Rich at Midwest cost me under $300. The best money spent on my truck !!You just barely press the pedal and it stops like a modern truck.
That Hydro. is awesome. Actually it seems to take less foot pressure than my 2000 GMC pickup.
Call the man! 815-943-0550
Oh yeah, I sent him mine but it was beyond repair. Was rusted from hauling fertilizer. But the new one was not much more than rebuilding mine. I think $275 for a rebuild. There is a pic. of my old one in my Photobucket below.

Hi Old GMC

I am waiting to get my 1951 1 ton delivered to my house and I am reading up on as much as I can find on these trucks. So is it more veasable to just repair the original drum brakes or to try to swap out major components to make it stop better? I know I am going to want to drive my truck more than just in parades. I am looking at something I can drive to my job which is 11 miles away. So I will see stop and go type situations. I did notice NAPA sold the Hydrovacs brand new for these trucks...

MikeC

At the time I needed mine, NAPA or any of the FLAPS could not get the one that fit my truck. They may have them for it now. I think my new one originally came from Cardone. But I think you have to be a dealer to buy from them.
As to your question referring to the best way to go. I am by no means an expert on these old brakes. I just know I am very happy with re-working the original setup, and would trust it anywhere on the road.. I know brake systems on new cars and trucks are much more advanced now. I was trying to keep mine all original.
The only thing I might be concerned about, is I kept the original single reservoir master cylinder. But back in the day, they used millions of them. I know a dual would give added safety.

Mine works fine. It will squeel the back tires on a full bore stop on my GMC 450 firetruck. As far as I can tell the brakes are all original too. Bled mine with a Mighty Vac and wifes assistance. Didn't have a pressure source for brake bleeding at the time. Good luck with yours. Jim

This seems like a chance to get a dumb question answered. (I am not looking to change to a dual master cylinder because my 66 GMC 5500 truck uses a tandem master cylinder for brakes and clutch that, as far as I know, does not come in a dual configuration for brakes.) But I have still wondered how a dual master cylinder would connect to a hydrovac that has only one brake line inlet and outlet. One of the major advantages of the dual master cylinder is not losing both front and rear brakes at the same time because of a big leak, such as failure of one of the rubber hoses, and this advantage would be lost at the hydrovac because it has only a single outlet connecting to both front and rear brakes. Is this correct? How do more modern trucks, that I assume are required to have dual master cylinders, deal with this?

As I said, this is all new to me, but I think when people change to a dual reservoir master cylinder, they change to the modern booster at the same time. Therefore doing away with the old Hydrovac.
But I would think in doing this, there would have to be a proportioning valve that divides the front from the rear. In other words, a complete changeover.
Guess it just amounts to -- if you want original brakes or make it all modern?
As to the cost, I do not know the difference. Mine had 8 wheel cylinders. Including new lines and the new hydro. it cost me $1000 to put everything back new original.

Original brakes would be cheaper.... If your happy with the original brake set up you should repair it and have good brakes. I would imagine that the price of $1000 mentioned above for repair is a good number. You would at least double that number to put modern stuff on....

A dual Master could be used. Just put check valves in the discharge of each reservoir, then go to a common header to the Hydrovac. You would have a single hydraulic signal going to the hydroboost. If you lost 1 reservoir, the check valve would prevent backflow into that side. The chance of loosing one side with a short line, no rubber hose would be nill. You'd still lose all breaks except parking brake if the hydrovac or downstream lines were lost. If me, I'd replace, repair the hydravac and old master. Good luck. Jim

Yeah,I TOLD the "experienced" brake mechanic,(Billy W's '59 Viking bus),that the system would need a proportioning valve,in order for the system to function properly. I also told him about the possibility of the rubber lines collapsing,causing the "no rear brakes @ 1,000 # pressure". He looked at me like I was crazy,and told Billy that he couldn't figure why the brakes don't work. I didn't think about the check valves,but another great idea! More to come,as soon as weather permits...

I think to convert a big truck brake system to a dual master cylinder along with an updated booster will be more expensive than rebuilding the original system. A person needs to take into account the size of the wheel cylinders and the size of the shoes/drums that are on the truck and what it takes to operate it. It takes more thought and planning than on a light pickup where you can add a dual reservoir master cylinder, proportioning valve and some residual valves.

Some early 1970's motorhomes used a dual system with two medium-sized hydrovacs- - - -one each for front and rear. That would be the only way I can think of to have a dual system with sufficient boost to stop effectively. Jerry

I'm not sure if my tucks are plumed the same way yours is, but the master cylinder does not provide any pressure to the brakes (well, if the 'vac in inoperative it does provide a little pressure via the control lines... but not enough to stop rapidly).

The 'vac provides all the pressure. Any modification of (well, my truck, at least) would not involve any changes to the master cyl, just a proportioning valve at the end of the vac hydraulic cylinder.

This is for a Series I, II or III vac.

Any system involving a remote vacuum assist the master cylinder is just a control valve and a fluid reservoir. The vac does all the heavy lifting (typically 1000-1500 psi for a series III vac).

Do you have another number for the hydrovac guy? I tried 815-943-0550 and it is no longer in service...

Never mind!!! I found a good number for Midwest Power Products 877-966-0550

Sorry, my fault. It should have been 942 instead of 943.
Guess he now has a toll free number also.

I drove mine in stop-and-go traffic,during "fair-weather" months. Looking back-I WAS PlLAYING WITH FIRE!! The speed limit is usually 35-45 M.P.H. I had no problem stopping the truck,even with the drum brakes. I could "lock" the rears,(if I had to),since the brakes worked that well. I NEED to go through everything,and replace the lines/hoses. "Why fix what ain't broke",doesn't apply to 59 year-old ? brakes. Now,"Tater"'s parked,under going an electrical harness transplant,as well as engine-bay detailing. Brakes are on the list,before the truck leaves the drive-way. I've got too much invested,to trust the brakes,when the parts,(rebuilt M.C.,hydrovac,lines/fittings/hoses,rear wheel cylinders,(Front lines /hoses, cylinders are new),don't cost much more than a new electrical harness. "Let's be careful out there".

Rather than starting a new thread, I will ask here,,It seems I have an issue as well with the 42 Chevy Firetruck I am working on,, I have found that the hydrovac unit is seized up, cannot press the pedal, engine not running, the brake system is all stock, single master, hydrovac unit, dually 7-20 tires out back on a 2 ton brush truck, it will be driven with a new 292 and 700R4, has been sitting awhile before i got to it, found brake master cylinder has no fluid, its wet inside but rusty, looks like it needs a rebuild. Sounds like I need to rebuild master before i can tell if i need to replace the hydrovac unit? BTW: Jim Carter has rebuilt units available too.
Thanks, John

Yeah, but I noticed Carter's hydrovac is $600. About 6 months ago, Midwest charged me $275. Picture looks like the same unit.
BTW. I do not have any connection to Midwest at all. Just that he was very nice to deal with and cheaper for the same item.

I was recently at the U-pull yard. The GM Van units look really easy to pull.

Looks like if you pull 4 bolts, you can get the pedal, hydro unit, and master cylinder. Hack the hydro lines and walk away.

I pulled the brake mc off today, it was rusty inside the reservoir, and inside the cylinder, needs a kit, now to find my brake hone, Also used Knocker-loose and PB blaster to 'un-stick' the hydrovac unit, hopefully it will be usable as is, wont know till after the mc is rebuilt and reinstalled, might take a few days to get the parts.

Welcome,John 53!Your brake pedal is connected directly to your M.C. It's your M.C. that froze-up,not the hydrovac. more than likely,though,it sounds like your hydrovac is bleeding fluid from your M.C.,(as long as you don't see any "wet" tires,or leaks/puddles. My M.C. is sucked dry,if sitting for 3-6 months. from other posts,I gathered that the seals in the hydrovac caused the leak. After filling the reservoir,and pumping the brakes,they work great,until I let the truck sit for extended periods. But now it's time to re-build/repair/replace all of the components. I REALLY want to drive '"Tater" to K.C.,this September,and make it there and home again,in one (un-damaged) piece.

I disconnected the brake rod from the M.C. First,,it was then I Found the M.C. was not stuck, and I was able to determine that the hydrovac rod was stuck, accumulation of rust on the steel rod caused it to seize up, after getting it unstuck, I hooked the brake pedal back up and the hydrovac unit was moving, the brake pedal going thru the normal motion,, so hoping after the M.C. is rebuilt I will have brakes,, time will tell.

Wow!! Guess I stand corrected. I'm saving up to send my (hydrovac) core to Midwest Power.For the price,it sounds like a new unit doesn't cost much more than a rebuild kit. Will post vid's/stories as work/weather permits.

Just had rich at Miwest Power Products send me a new Hydrovac, and other brake parts, for my 1956 GMC 630 great guy and he knew what I needed makes the job so much easier.

There was about a 100 dollar differnce for new over rebuilding, I just went with the new.