Wierd brake problem

[akatao]

all the symptoms point to a master I've had bad new masters as I'm sure others have.

 

[53 stickfigure]

Ive learnt that all master cylinders and other parts are not created equal. I used to buy from the chain stores, autozone, advance, o'reilys, but not anymore. Nearly every part I've bought from these places, I've returned or had to replace. I go to places like napa and parts plus stores now. You pay a little more, sometimes quite a bit more, but the quality is alot better.

 

[DozerII]

So yesterday I took the original master cyl apart to look inside. Other than a little crud in the very bottom it was spotless, not a mark in the bore, all the rubber cups look good and the bypass ports clean.
I spent about two hours watching youtube videos about prop valve and I'm really leaning that way. The one I have is off a 75 Caprice that sat for at least 10 years, I'm thinking something in there is moving just enough at certain times to cause the problem. I'm going to order one and I have three new hoses coming Monday.

 

[oldmanb]

What are you using for residual valves? I had the same problem and ended up using a 2lb and a 10lb.

 

[DozerII]

What are you using for residual valves? I had the same problem and ended up using a 2lb and a 10lb.

I'm not using any residual valves. [S

 

[oldmanb]

I'm not using any residual valves. [S

If your master cylinder is below or equal to your caliper bleeders you will definitely need them. They hold a slight pressure in the lines, and will keep your pedal up. I ran into the same problem on my 31' a few years back. You being a Westerner , should able to find a pair from an old Venture or Uplander van,(no rust) , us easterners have to buy Wilwood

 

[DozerII]

If your master cylinder is below or equal to your caliper bleeders you will definitely need them. They hold a slight pressure in the lines, and will keep your pedal up. I ran into the same problem on my 31' a few years back. You being a Westerner , should able to find a pair from an old Venture or Uplander van,(no rust) , us easterners have to buy Wilwood

The master is about 8 inches above the caliper bleeders.

 

[oldmanb]

I went back and checked out that post on your build, and saw your mc location before I posted . ALL masters c's that are below need them, (Horton Hot Rods info) ,most above don't, but I'd say that's your problem. All GM minivans that I've worked on have them from the factory, their mc is high.

 

[MercuryMac]

I've given this occasional braking problem some thought. I like detective work. Sometimes brake fluid leaks out of your system and causes your pedal to go to the floor. It's not going out onto the road, you say, so where is it going? It has to be going back into your master cylinder reservoir. So you don't actually loose it but it squirts out of the pressured part of the system, [between the master piston and the wheel pistons]. I kept coming back to, a foreign object [a flake of rust, a piece of wire, or a wild oat hull] in your fluid, and once in a while it gets between the master cylinder and the piston rubber and causes a leak back into the reservoir. But, now I thought of something else. Have you got any part of an anti-skid braking system on your brake lines? Maybe it's letting off pressure. I've never tracked this kind of brake line so I don't know if the pressure release is in the master cylinder or somewhere else. Food for thought, anyhow. Good luck.

 

[DozerII]

I went back and checked out that post on your build, and saw your mc location before I posted . ALL masters c's that are below need them, (Horton Hot Rods info) ,most above don't, but I'd say that's your problem. All GM minivans that I've worked on have them from the factory, their mc is high.

Thanks for the great info oldmanb, one I get the new prop valve and hoses on we will see what happens, if there is still a problem I guess it's residual valves.

I've given this occasional braking problem some thought. I like detective work. Sometimes brake fluid leaks out of your system and causes your pedal to go to the floor. It's not going out onto the road, you say, so where is it going? It has to be going back into your master cylinder reservoir. So you don't actually loose it but it squirts out of the pressured part of the system, [between the master piston and the wheel pistons]. I kept coming back to, a foreign object [a flake of rust, a piece of wire, or a wild oat hull] in your fluid, and once in a while it gets between the master cylinder and the piston rubber and causes a leak back into the reservoir. But, now I thought of something else. Have you got any part of an anti-skid braking system on your brake lines? Maybe it's letting off pressure. I've never tracked this kind of brake line so I don't know if the pressure release is in the master cylinder or somewhere else. Food for thought, anyhow. Good luck.

Thanks Mac I'm thinking because everything is new except for the hoses and the prop valve one or all of them is contaminating the system. All brake parts are pre 1980 design so no ABS items.
I drove it yesterday 220 kilometers and never had an issue ( but I was on edge every time it came to braking)

 

[DozerII]

99% sure the problem is solved, everyone has told me it is the master cyl, after two new ones and still having the same problem I was doubtful. But my experience with brakes was saying the same thing. I was about to pull trigger on a third on and deicided to try the replacement Prop valve that Summit sent me. After the install and carefully bleeding all the air out I went for a test drive and I can't believe the difference in the brakes. I though they were good before despite the problem now they are awesome.[cl[cl