The Stovebolt Site Forums The Parking Lot Big Bolts Taking out the Hydrovac

Hi guys, I have the new axle in and driveline hooked up. I have disc brakes on it and have added a 2 lb residual valve. Now I still have drum brakes in the front and going to add a 10 lb residual valve going up there. I have read this helps with a soft pedal and good braking response. Given that, is the hydrovac really a necessary item in the braking system with the residual valves in place?

Another thing I looked at, for this winter when I swap spindles to a disc brake for up front, is getting a new Master Cylinder with 2 chambers and a builtin booster. I have seen the unit that Brothers Truck parts sells and wondered if it actually fits the big trucks. They have a MC listed that looks like the one I have. And they have a bracket to bolt in where the existing MC is located under the floor. So would like to upgrade to a completely modern brake system when done.

Or I looked at this ECI MC that does not require a booster, but provides much power for braking. Thoughts on this unit?
http://www.ecihotrodbrakes.com/universal_brake_assist.html

Looking forward to hearing your comments.

Fred

What size truck? If it needed a hydrovac to begin with, I'm betting you're building a rolling deathtrap without it.
Jerry

I am looking at the same situation. I just purchased a dual chamber MC with a booster. The merchant said you would no longer need the hydro-vac unit. You are actually putting in power brakes? I am wondering if I can just leave all that stuff in place (hydro unit)? It looks like the MC bracket will bolt right to where the old MC went. The hardest part appears to be routing the brake lines. I think I might try to just "flip" them over and run the long line, to the rear wheels down the LH side of the truck, instead of the right side? I have a 1955 -series 2, 6400. I would love to hear what others think?

Fred,

The Hydrovac is a Power Brake Booster, it adds an additional 900 pounds of system pressure which means the human doesn't have to push as hard to stop the vehicle.

When swapping from a single pot to a dual pot MC you should stay with the same bore size (your bus could be 1-1/4" or 1-1/2" bore). It takes a lot of fluid to expand the wheel cylinders, so if you use a smaller bore MC the pedal has to travel further to get the correct amount of fluid to the wheels. This translates to the pedal being closer to the floor with less safety factor. The MC conversion kits sold by the vendors are for the 1/2 ton trucks, the bracket may work if you find a bigger MC.

My trucks all have stock brakes and I don't have any residual valves, I just keep the shoes adjusted.

You may need to install an adjustable proportioning valve to the rear disc brakes until you get the front drums replaced to keep the rears from locking up before the front.

The ECI bore may not be large enough for the big trucks.

Mike B

I would figure out how much your bus is going to weigh with the number of people your going to have riding in it. Then you can research how much boost your going to need to stop your bus. I have no doubt that one of those small boosters will not be enough.

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

I will be releasing the first round of orders in a week or two.... Hopefully!

Well, it sounds like taking out the Hydrovac is a bad idea. so I'll not do that until I get the modern MC and booster. Charlamagne65, where did you get your booster and MC from? And what part numbers did you get?

Thanks again for all the great feedback.

Randy, do you have any pics of the stuff you're making? I'd like to see the bracket for the MC and the Booster. Which MC and Booster are you using for the new set up?

The seller was: nelcohotrods on Ebay. He said this unit was designed for trucks over 1 ton. He has several to choose from. Also the best price I could find. See description below.

1955-59 Chevrolet & GMC truck frame mount power brake unit pedal assembly with 8" dual diaphragm booster with dual well master cylinder. This system bolts directly to the frame and will power any drum drum or disc drum brake system. This system is for larger trucks where ground clearance is not an issue.

From what I understand, The hydrovac works with only the single pot MC. Some motorhomes had two hydrovacs. One for the front and one for the rear. I have my COE converted over to 4 wheel disks from a P30 chassis for the front and a chevy 3500 rear axle. The calipers are 3 3/8 inch bore. So I need to figure out what MC I need. Im thinking of using the same one as was in a chevy 3500 truck.
Still doing the research

There are plenty of pictures in the for sale add. You can also find a link in that thread to more pictures. I had 11,000-12,000 pounds on the back of my truck a few months ago. It was harder to stop for sure, but I still felt as comfortable as anyone would driving a 70 year old truck weighing nearly 20,000 lbs in stop and go traffic.

If you splurge on any portion of your truck let it be the brakes. It took me 3 years to work out the disc brakes on mine. It really is easy to drive in town even with some weight on the back.

My friend,Billy W.,installed one of those dual-reservoir/booster units,on his '59 Viking. It DOES NOT WORK!! Some vendors will tell you anything to sell their product,but hesitate to back their claims. With the drums adjusted,the left front and rear grab,while the right wheels spin freely. The bore is 1-1/8,when it should be 1-1/2. Please do yourself a favor,and find a later model 2-ton set-up,with the correct bore/2 hydrovacs. This will save you a bunch of headache,as well as hours of wasted time. Just my 2 cents.

51Chris, getting a MC from the truck you've used the brake components from sounds like a good idea. I'm thinking of researching some of that info now. I will have the same set up on my bus. P30 front discs and HD3500 rear with discs. I just the rearend all finished up today with the brakes installed and put it back on the ground.

So lots of noodling this information over in my head and a bit more searching for the right parts I think.

It can, and is, being done. I am in the process of finishing up my brakes this week or so.

I will end up with a frame mounted 7" dual diaphram booster and dual pot master cylinder feeding the original drum brakes on my '46 2 ton.

This will work for my truck, which should weigh in around 6500 lbs. I cannot speak for those who are actually using their truck as a true work horse. Mine is more of a hot rod.

Puppy, where did you get your parts? I'd like to look at them to see if they are something I'd go for.

Thanks.

Fred

I used a 7" dual diaphram booster from MBM. Sold nearly everywhere as the house brand. I also used one of their frame mounted pedal kits. Picked up the master cylinder from Autozone.

All new 1/4" brake lines and rubber hoses, rebuilt master cylinders, existing pads, and stock drums.

I can get all of the part numbers I used later.

Should have it all assembled and tested in a week or so.

I'll get some pics too.

Hope it works for ya,Puppy! work horse or hot rod,they all gotta stop,and the dual pots that are "readily available",are mostly for the 1/2-1-ton trucks,with a smaller bore than the 1-1/2-2-tons. Pedal goes halfway to the floor,and the brakes work minimally at best. Everything (Brake wise), on that bus has been replaced/tested,and the result is searching for a different M.C./Booster set-up.Live and learn. Wasted time and $$$$!!

I also have to figure out the size master cylinder I need. I will most likely mimic what was in a mid nineties chevy 3500 of a p30 motorhome. Some motorhomes used a dual hydrovac(what I've heard) or hydroboost which runs off of a power steering pump. or master cylinder. Remember, all this stuff was engineered to work for a specific application. I would not be very good at re-engineering which parts work with others to achieve proper braking.

I've seen a couple of videos,of the hydroboost swap. It looks quite complicated,especially for a "backyard mechanic",or novice. I drove an I.H.C. 1600,with a V-8 diesel,back in the mid '80's,and the brakes were electric assist. You touched the brake pedal,and the (brake assist),motor would automatically apply the brakes. If you needed 'em quicker,stomp the pedal,and the system would over-ride. I'm pretty sure the truck had a dual hydrovac set-up as well as a reservoir tank. If I can find a completer unit in a junkyard,for cheap enough,I'm thinking about going that route. It all depends on the bore,though.Without the volume,the cylinders aren't going to push enough fluid to stop a "Big Bolt". Not even at slow speeds on gravel,where you'd expect a wheel or two to grab and "lock-up".

You need a 'vac or booster. Big trucks need high pressures. The vac on my 2.5 ton puts out 900psi.

I'm not sure there is a benefit to disks.... you gotta invent a new brake system, hack parts together then claim its more reliable the the single loop drum system that been on the truck for 60+ years??? Drums don't warp like rotors. Drum fade can easily be reduced with better friction materials and driving technique.

Think of it.. even tractor trailers built in 2014 still use drum brakes and they go twice as fast with 10x the weight.

If using modern axles, by all means, get the MS that came with that truck, why try to reinvent the wheel. Many trucks have power steering, so the addition of Hydroboost brakes is simple. Hydroboost can be used w/o power steering if one doesn't need or have it.

Fred I still got the 76 m/h full brake system on that. Probably going to junk if you can help get some things out of it that I need it might be up for picking. J

My intention here was not to take the HV completely out, or any means of an additional booster making it manual brakes. Rather just replacing the HV with a modern booster. Looking at the set up I am going to get a proportioning valve to add to the system, bleed the brakes in its current configuration, change the front axle to disc brakes later on this year and possibly just keep the HV and update the MC. That may just be the best way to go. I did find an interesting resource that is a catalog from Cardone with all the different MCs available through them. It lists them by part number first and also by bore size in ascending order. There are lots of options for big bore MCs. One would just need to figure out the attachment of the pedal and to the truck. I plan on looking into this further. I think with my 1 1/8" bore I could stand to bump up to 1 3/8" or larger.

As always, thank you for all your input and ideas. And I welcome further thoughts on the above.

Fred