Brake Residual Valves

[Merle Coggins]

My brakes seem to have excessive movement in the pedal before I get good pressure. Once pressure builds the brakes work good. I've also noticed that when I'm driving it, the pedal seems to feel better after a couple of applications. I'm beginning to wonder if the residual valve in my Napa replacement master cylinder may be faulty or non-existent.

I also noticed, this past weekend, a brake fluid leak at my left rear wheel. Although I haven't pulled the hub for inspection yet, I am suspecting a leaking wheel cylinder. I remember reading somewhere that the residual valve in a drum brake system helps keep pressure against the cylinder cups so that they seal better against the bore. Without that residual pressure they could relax and leak fluid. This again makes me suspect a lack of residual pressure.

Although I won't have time for it this week, before I take the truck out to Pennsylvania for the WPC meet, I'm considering pulling out the M/C to check for a valve. I'm also considering purchasing an inline 10 psi residual valve to install instead of one in the M/C.

I don't believe there's air in the system. I've bled it many times during the rebuild process. And I've checked for free play. There seems to be an adequate amount of free play before the push rod contacts the M/C plunger.

The brakes work well and stopping isn't a problem, but I'd like to lessen the pedal travel. Am I on the right track with my thoughts on the residual valve?

Merle

[Don Coatney]

Merle;

Lack of pedal is an adjustment issue. Soft (spongy) pedal is a bleeding issue. Sounds like you have both.

[grey beard]

Merle,

I agree with Don, especially if you say the pedal improves after a few strokes. Doesn't sound like an air issue to me. By the bye, residual pressure on drum master cylinders in on the order of two or three psi, not ten. That much would cause a constant drag to my thinking.

Much as you may hate to do it, I suggest you go back and re-do your major and minor adjustments on each wheel once again. Those two bolts that hold the steering arms onto the spindles up front are also brake cam eccentric adjusters, and they are a little bit of a pain to get right. Pull the cotter pins, back off the nuts and keep trying the drum on until you are satsfied you have it right. I wish I had taken the opportunity to grab an extra drum last time I had a chance to get one for a half ton. It could have been sacrificed to make a brake adjusting tool by cutting windows in the sides outside the lug holes.

Why do you need a drum with windows to adjsut these front brakes? The weight of the steering linkage arms on the two adjuster bolts makes them almost impossible to turn freely enough to "feel" the adjustment point. I finally got mine right by repeatedly pulling the drums and adjusting them untill they just fit over the shoes. Then once you have the major adjustments made, the minor ones up on the sides is easy.

I'll bet you a milk shake that if you readjust those four wheels again your pedal will be there when you need it. JMHO

Let us know how it works out.

[Rich Gengo]

Just a note on residule valves. 2 psi is for disc. and 10 psi is for drums in the hot rod world. On normal brakes usually if you have a better pedal on the second pump it's usually a wheel out of adjustment from the others

[Merle Coggins]

Well, yesterday I pulled the left rear drum to locate the leak. It appears to have stopped. However this again confirms that possibly a lack of residual pressure allowed the cylinder cups to relax and leak a little while it was sitting. Now that I'm using it again they have sealed up. No fluid on the shoes, so I washed down the backing plate with Brakeclean and reassembled it. I then readjusted all 4 brakes. They were a little off, but not by much. This didn't help the pedal feel much, but maybe a little. Then I adjusted the M/C push rod. Once I got the return spring off I realized that there was more play there than I thought. I adjusted it up and now the pedal feels great. I also hooked up the pressure bleeder and rebled the system. I didn't see any air, but let it flush a bit from each corner.

The brakes work MUCH better now.

Merle

[tctrkca1]

Inside each wheel cyl there is a brass expander that helps to keep pressure on the cup lips.The info I have read is that it is required anywhere that the operating temperature is below 32F.I have also seen older parts books that say US whl cyl didn't use them.The lack of them will allow seepage while sitting.Low temp here is -45 so we need them.Jim C

[Paul Beard]

Merle I was told that the odler M/C's had a one way valve to let fluid out but not back into the M/C. The man said that the valve kept the fluidd in the lines because all whl. cyls. are below the M/C. I have dics on the front and drum on the back of my truck with orig. M/C and I have no problems with my brakes. I only put a proportioning valve to the rear brakes.