The Stovebolt Site Forums The Parking Lot Big Bolts Brake System Help

If no one has gotten tired of my stupid questions yet, I was wondering if anyone could educate me on the brake systems on big bolts? I am just getting my feet we in mine so I have a lot to learn. I got to looking at the brake system yesterday and there is a lot going on under there that I have never seen before so any help would be greatly appreciated!!

It is confusing for us small truck guys.

I have a 54/55 2-ton GMC and I suppose it is, in general, like your system.

Most likely your system is a vacuum-assist system, using a hydrovac unit.

http://chevy.oldcarmanualproject.com/shop/1954truck/54ctsm0525.html

One thing to keep in mind is to adjust and bleed the system as described in the Shop Manual.

1. The hydrovac bleeding procedure.
2. The wheel cylinder bleeding procedure.

I decided I'd spend the money on a new master cylinder and new wheel cylinders and new brake shoes (all from Rockauto.com, I think). You might not have to do that.

More big-truck-knowledgeable people will post wiser/better opinions/advice.

Well mine has sat for almost 40 years so it is going to need basically everyeverything. I swapped both axles so aside from wheel cylinders ot may not need much on the hubs but all the hydraulic parts and probably all the vacuum parts will Need replaced. I am hoping to learn alot about the system and start to understand how it works before I got to tear it apart.

Tim got it right, but I'll add this...

When stepping on the brake pedal you put pressure on the entire brake system via the Master Cylinder. This pressure also activates the Hydrovac unit which uses engine vacuum to amplify the pressure in the brake line to assist in stopping. If memory serves me the Hydrovac unit can put an additional 900+ pounds of pressure in the brake lines.

The rest of the system is just normal brake stuff (steel and rubber lines, wheel cylinders and brake shoes).

The Hydrovac will be a large canister hanging on the frame. Inside the canister is a piston that is attached to a small hydraulic cylinder. When the brake peddal is pushed, line pressure triggers a control valve on top of the Hydrovac to open engine vacuum to the large canister allowing the outside air to push on the back of the piston to assist in adding pressure to the brake lines. So, your Hydrovac will have a large fresh air hose (could be connected inside the cab to a filter), a vacuum hose from the engine (may be tied to a vacuum tank as well) and the hydraulic brake line off the hydraulic cylinder.

Hope this helps,

Mike B

Yes this info helps very much, I can at least identify what parts are what now, I was curious about both the booster and the reservoir tank. Can the booters be replacer or rebuilt? I am sure mine is no good as long as it has set, and I am assuming the tank is pretty straight forward as far as repairs, thanks everyone for the help!

Yes, rebuilt Hydrovac's are available, check with your local NAPA or BIG truck parts house.

There is also a tech tip on a newer style Hydrovac rebuild here on the Stovebolt...check it out.

Mike B

Mister, I have been developing and have worked out all the kinks of a disk brake set up that will fit your truck. Not sure what direction your heading with your truck, but I drive mine weekly in stop and go traffic with plenty of weight on the back.

Randy, I have thought about going to disk brakes but I have not made and kind of decision yet. Is it just basic hydraulics on your setup?

In my experience here in St. Louis, you will have a hard time (a very hard time) finding a "kit" to rebuild the hydrovac yourself. My guess is that the kits are all snatched up by the rebuilders. Fortunately, the GM hydrovacs can be rebuilt, as mentioned, quite routinely, by dealers. There should be a tag on your hydrovac, similar to a carburetor tag, with a number that a rebuilder uses to identify the hydrovac. Ask at NAPA, or check with your local "big rig" service center. They will no-doubt have a resource for this, and given the number, should be able to quote you a price for a rebuilt unit.

It's usually a core-swap deal, so you give your old one in, and they send a newly rebuilt one back. By-the-way, be sure to take all the fittings, such as elbows, flare adaptors, etc. off the hydrovac before sending it in. The rebuilt ones don't usually include those.

Hope this helps,

-Michael

The kit is very basic hydraulics. Uses a standard master and vacuum booster.

Thanks,

Thank you everyone for your help, it has been great! I have run into a few issues with my truck this week so progress has slowed way down, I be looking into getting the hydravac parts soon, right now I am having a heck of a time finding just the main master cylinder any where, would it be the same as say a 3/4 ton truck?

Hy mystro, a 1954 Chevy 6700 would use Raybestos p/n MC9220 as a master cylinder, that part is available from RockAuto $85.79, hope that helps.

Yes! That is very helpful, I went ahead and ordered it, that should get me moving on the brakes. My plan is to replace the master cylinder and front wheel cylinders and plumb them directly to it temporarily while I am working on the hydravac that way when and if i get it running I can at least drive it around the yard.

If you want to yard drive it,you're better off making sure that your parking /emergency brake is functioning properly. You'll have to STOMP,and HOLD that brake pedal,and it will take about 30 feet,at 10 M.P.H.,to stop,without the hydrovac! And that's on LEVEL ground!

So then is it the wheel cylinders themselves that require so much pressure or is it the springs in the brakes? I just assumed the hydravac was for extra stopping power under load, I am gonna get this all figure out eventually I swear!

Make sure to replace all of the steel and rubber lines while your going through everything. The lines can look good on the outside, but they fail from the inside...nows the time to do it!

Mike B

The Bendix Hydrovacs are named by what 'series' they were... literally 'series one, series two, series three, etc'. Yes they had part numbers, but the evolution of the 'vacs were classed as the 'series'.

Oddly, series one (the oldest) and series three and four 'vacs can be rebuilt quite readily. The second series 'vac (early to mid 40's) are the ones that are hard to find rebuild kits for. I got two, both from England.

I have rebuild manuals for the series one, two and three 'vacs, but parts are a little hard to find.

Midwest Power Products (google them) is the go to place for 'vacs... dude knows his stuff.

As far as brake upgrades, its more important to run dual circuit brakes than disc. But there are few guides on splitting the single into a dual circuit... most who want dual circuits run dual 'vacs (like mobile homes did).

Thank you, I emailed midwest about my situation, so hopefully they van take it from there. Is the vacuum reservoir (at least im assuming thats what this bigger canister is) anything more than a seal container for storing vacuum pressure? I havent gotten a good look at it yet.

Its a big can with one or more pistons (depending on your vac model). The piston seals are not the hard part.. the slave cylinder seals and the vac diaphragm are the more 'unique' parts.

Biggest issue with 'vac cans is water pitting the bottom of the cylinder. I can be repaired, but usually its better to move up to a Series Three or Four 'vac for parts availability.

Another option is to use or replace the master cyl. W/ a dual unit, put a hydroboost unit on the back of whichever master one chooses and do away with the whole hydrovac system, especially useful if going to a diesel engine w/ no vacuum, need to installs power steering pump for the hydroboost; that is what I am going to do.

Well I received my master cylinder this weekend also got a hold of midwest, they said rebuilding my unit would not be a problem, so now I just need to figure out all of the vacuum lines and whatnot.

Almost all gasoline engines are still using the hydrovac principles.. engine vacuum on a large diaphragm to increase hydraulic pressure to the brakes. The only real difference is that the vacuum chamber is part of the master cylinder, not remotely mounted.

Coupled with dual brake circuits its a very safe setup.