So I've got a 1970 c50 truck sitting in my driveway that I need to actually get around to enjoying. Well, actually since picking up a real racecar I want to make it the dedicated tow rig.
Long story long; when I got the truck the brakes "worked" but the M/C leaked like a sieve and same with half of the wheel cylinders. I, of course, unable to refrain from "upgrading" the brakes decided that the weird brake system it had could be simplified with modern equipment. I replaced all the wheel cylinders, relined the shoes, and swapped the undercab booster for a regular hydroboost unit from a one ton. The brake sucked. Didn't even stop the truck. I, in my limited diagnostic skill, went overkill and grabbed a m/c and booster from a topkick/kodiak. Now the truck stops but the pedal doesn't return. No pedal return spring. Replaced that and it halfheartedly returns. Doing research I don't think the newer master has a residual pressure valve and if it does, it's at a much lower pressure because those trucks got disc front instead of drum at all 4 corners. I grabbed some wilwood 10lb valves from summit but this just leads to my question.
Where should I install these? Down on the framerail? At the m/c?
Picture to drive conversation;
RPVs hold a little bit of pressure in the circuit to help overcome return spring force and/or to keep all the fluid from draining out of the corner brakes when the MC lives below the floor. Is the truck drums all around? For passenger cars, On a drum circuit 10 psi RPV is normal, and on a disc circuit 2 psi is typical. IDK if a medium duty would be different.
I would install them close to MC outlet if it's convenient / accessible.
Yeah, It's 4wd drum with two wheel cylinders per drum.
I figured closer to MC was better but wasn't sure.
I ran 10psi RPVs on the International I just converted to modern master and booster. They are both installed at the master. I also ran them on SanFord and mounted them at the master.
I don't think they will help you with the pedal return issue though. If anything they may make the problem worse by keeping the pressure in the lines instead of using that pressure to push the pedal back.
Toyman! said:
I don't think they will help you with the pedal return issue though. If anything they may make the problem worse by keeping the pressure in the lines.
i had the same thought. when you say pedal doesn't return, is that with engine running?
Toyman! said:
I don't think they will help you with the pedal return issue though. If anything they may make the problem worse by keeping the pressure in the lines instead of using that pressure to push the pedal back.
Ruh Roh.
In reply to AngryCorvair (Forum Supporter) :
Yeah, engine running. I drove it from Seattle to Tacoma (roughly 50 miles) after putting it back together having to lift the brake pedal with my toes after each press.
Totally out of my wheelhouse, but I bet ya get good answers here. 
I did want to say : "I am in LOVE with your truck!!!"
I would replace all brake system with something more modern. But I don't know if that is best plan.
Actually, I would set whole cab on modern chassis!!!
Again, not saying that is the best way...
SanFord and the International I converted both use a largish spring for pedal return.
RPVs prevent fluid from draining back to the master when the master is at or below the level of the wheels. Think like a Model T with the master mounted to the frame rails. Even if your master is below the floor, I doubt it could possibly be below the cylinders. RPVs don't help with returning the pedal.
Just be cautious here. You can't just put part A with cylinder B. I know you know that, but this isn't a time for "let's try this."
Fluid pressure isn't what returns the pedal. The springs in the master and/or the springs on the pedal do that job.
03Panther said:
I did want to say : "I am in LOVE with your truck!!!"
I would replace all brake system with something more modern. But I don't know if that is best plan.
Actually, I would set whole cab on modern chassis!!!
Again, not saying that is the best way...
Hey thanks. I've always been enamoured with the old big trucks. Anything snub nose is just cool. Yeah, I was actually trying to find a newer beam axle assembly locally but that seems to be pretty difficult. My work has been contemplating retiring their fleet of cateye diesel one tons duallies and that would be a perfect chassis but I just don't have any gurantee when that might be. And I'm too cheap to go out and buy one.
Curtis73 (Forum Supporter) said:
RPVs prevent fluid from draining back to the master when the master is at or below the level of the wheels. Think like a Model T with the master mounted to the frame rails. Even if your master is below the floor, I doubt it could possibly be below the cylinders. RPVs don't help with returning the pedal.
Just be cautious here. You can't just put part A with cylinder B. I know you know that, but this isn't a time for "let's try this."
Fluid pressure isn't what returns the pedal. The springs in the master and/or the springs on the pedal do that job.
You're definitely right. Brakes aren't anything to halfass. I think the pedal-no-return just confused me too much. With both the hydroboost and maxboost, or whatever GM calls their medium duty 2" m/c setup, the pedal stopped returning. Funky. I was just looking to what may be missing from the system that was there before and presumed that since the newere m/c's were setup for disc that they wouldn't have a pressure valve equipped for the olds drums.
I see complete one to 2 1/2 ton complete chassis,( former trucks, step vans etc.) on Craig's or fb for as little as $250 - $500 time to time, but that's in SE; and sometimes they ask 2 grand for 'em. Hard to figure.
