The Stovebolt Site Forums The Parking Lot Big Bolts Dual Master Cylinder plumbing

Hello... This might sound like an ignorant question...

'52 - 2 ton Chevy - 6500. She's in for all winter major restoration and new dual master cylinder has been bought.

My question is - should the plumbing come out of each reservoir then simply "T" into the main line that runs to the Hydrovac? It would seem that might defeat the purpose of the dual cylinder, no? Any hints on how best to get the plumbing out of the new master would be helpful. Or a point to where this topic has surely been covered....

Many thanks, Pinch.

Hy Pinch, on a truck as large as yours you would definitely want power brakes, especially if you were going to carry any loads with it. You are correct when you say feeding the two outlet ports of a dual master cylinder into one line to feed the wheel cylinders defeats the safety aspect of the dual cylinder. I think the easiest way for you to achieve dual system power brakes would be to buy a power booster to mount in front of your new dual master cylinder under the floor in the stock location. There are many suppliers of such kits, one note of caution is that these kits are designed for small trucks and the master cylinder they supply may not have the capacity to operate the large wheel cylinders in the rear of your truck. When you purchased your master cylinder did you specify it was going to operate drum brakes? The only other way to do it would be to install a second Hydrovac and possibly get two of a smaller size since the one on your truck now was designed to activate all the brakes on your truck, hope that helps.

The only effective way to get dual system capability on a rig such as yours is to install two medium-sized hydrovacs and plumb one chamber of the master cylinder to each one. Unless you adapt a huge booster to the pedal linkage and eliminate the hydrovac, the pressure developed by the primary master cylinder won't give you enough boost. Some motorhomes I've worked on had the double hydrovac setup, and they worked pretty well. If you're going to stay with the single big boost unit, there's no point in going to a dual chamber master cylinder.
Jerry

Thanks much... what I was afraid of... Hmm....get a new single master and risk full failure - or spend $$$ putting a dual system together...

Thanks for your input fellas...

Brake master cylinders provide little (well, its the volume of fluid too) pressure to the brake lines. The master cylinder is simply a fluid reservoir and control for the hydrovac control valve. So no, nothing you bolt on the output of the master cylinder will work.. there simply is no system pressure there.

All system pressure is generated by the hydraulic cylinder at the hydrovac. The pointy end.. that the brake line plug's in to.

My manuals show the largest Series 3 vac only having a 3.5cu/in hydraulic cylinder displacement.. that's total displacement... a well adjusted brake system will not use nearly all of that volume at between 800-1200 psi depending on model.

If you have your vac PN I might be able to get the displacement and line pressure out of my manuals (as long as its a Series I, II or III made by Bendix) and with that info you could do as Lincoln suggested... install two vacs each dedicated to the front or rear. The master cylinder plumbing would be simple, and the hardest part would be finding a spot on the frame rail for a second vac and the plumbing up to the intake.

So you could control two vacs with a single master cylinder, but for a true, dual circuit setup you'd want a dual port master cylinder to truly isolate each hydraulic loop.

At my talent level, sounds like I'm best to simply get a new single master - re-connect to the current hydrovac (which is working fine) - and re-kit with new lines, slaves, etc. for a "new" system throughout minus the Bendix, and hope for the best.

What are you other heavy big bolters doing out there? Is it either all in with crazy amounts of time and $$ to retrofit in a newer brake system on the truck or are there any other simple solutions? I'll try to get under there and see what my Hydrovac #'s are at some point today. It does look like a vac that has been replaced not too long ago. So...

These big rigs hauled millions of tons of cargo millions of miles without too many disasters not caused by the driver or the other fools on the road. A well-maintained original brake system is going to be safe and reliable, particularly since few if any of the restored oldtimers will be worked anywhere near how they were used when they were new. Just don't take any dangerous shortcuts rebuilding the system.
Jerry

The hydrovac is still used today in thousands of applications. Most passenger cars use a diaphragm rather than a piston but the mechanics are exactly the same.

The mandate to switch from single to dual circuits was 1968 IIRC by the Government.

As long as you keep the leather and felts oiled it should last many, many years (piston vacs need oil about every 6000 miles). Most rebuilt vacs will use silicon grease which is maintenance free.

This seems like a good point for my latest dumb question. How does one tell or what are the warning signs of a failing hydrovac system? I have fixed a couple large vacuum leaks in lines leading to the hydrovac. It is working, and the truck now stops with a noticeable "whooshing" sound from the air unit in the cab. Then there is a continued hissing noise while the brakes are held down. Is this normal? Or do I still have a leak somewhere? Right now, I am just testing the truck unloaded, but need to know that it can stop with when loaded.

First, if your brake fluid disappears without an obvious wet spot on the rims the most likely place is inside the vac cylinder. There is only one seal that keeps the fluid from entering the vacuum cylinder via the hydraulic piston.

The test for a working vac while on the vehicle is simple. Start the motor and let it warm up. At idle, press the brake pedal a few times to evacuate the cylinder or air and hold the vacuum (you want to do this at idle as you should have 15-20in vacuum available.

Turn off the motor.

Now, if everything is working the vac canister should hold the vacuum. With the motor off, you should get AT LEAST 2-3 hard pedal brake applications before the brake pedal gets soft. Each time you hit the brakes you are letting in atmospheric air and depleting the stored vacuum.

Any other result means time to bleed the lines or take off the vac for testing. Don't forget to test/inspect the vacuum check valve (usually mounted on the firewall or near the intake manifold vacuum port.

I have a manual that shows SERIES I and II vacs and testing/troubleshooting. Its HERE. I have several SERIES III vac manuals but not online.. the pluming and operation is the same... just the internals are slightly different.

pf,

Thank you! I am going to take a copy of your instructions to know that I get it right the next time I have the truck running. I pulled out the seat for repair yesterday, so it will be a while before I get back to starting the truck. Before this, I was driving it around to take off racks and old wood (that came off the racks) piled on the bed. After fixing the vacuum leaks, the engine ran much better and the brakes seemed to hold while stopped. I have already replaced the check valves (they are plastic and I broke one taking it off). (My 66 GMC has two check valves for separate lines to the hydrovac and, if I remember correctly, to the two-speed rear end.)