Tight brakes on 72 Chevy Pickup

[Peterbylt]

I just finished restoration of a 72 Chevy Pickup. It has front discs and rear drum brakes.
As i drive along, the brakes seem to build pressure and apply themselves. The brakes and wheels build heat, worsening the issue.
When I crack the bleeders, I get a small squirt of fluid out and then the brakes are free again. This has happened several times.
The calipers are original except that the mounting/floating sliders have been replaced.
Master cylinder is new.
Proportioning valve is original.
New lines and hoses.
Synthetic (dot 5) brake fluid.
Seems like there is a check valve that won't let the brake pressure fully relieve itself after braking.
Pete

[maxwedge]

The oe proportioning valve will not work with disc brakes now will the oe master.The parts are different for disc brake set ups, for starters. Not enough pedal free play can also cause this, but your components still must be changed, if I understand your post correctly. Maybe I am off base here, were the disc brakes oe equipped on this truck, if so recheck the pedal free play as I said.

[Peterbylt]

The front disks were OE on this truck. The master cylinder was a rebuild, and the proportioning valve was on the truck.
It is possible that they gave me the wrong master cyl, I'd have to check.
I have good pedal. 2-3" travel with the brakes activating about 1/3-1/2 down. As I drive on, the travel on the pedal reduces.

[MagicRat]

Originally Posted by Peterbylt

The master cylinder was a rebuild,.

I bet you a box of Oreo's the master cylinder is defective.
Often, rebuilt master cylinders do not work properly. This is a common problem.

Likely, the piston is not retracting in the bore all the way and is covering the pressure relief hole for the front brake circuit. The front brakes never fully release their pressure from the brake pedal application, so the front brakes drag.

Replace the master with another one, and probably your problem will go away.

[Peterbylt]

Thanks. I talked to a brake system engineer and he echoed your same thoughts, that the piston is not retracting far enough and uncovering the relief ports.
I'll take the master cylinder back and exchange it again.
Thanks for the input, I'll post a reply when the master cylinder is replaced.
Pete

[maxwedge]

Next time this happens, and I've seen this. Disconnect the vacuum hose to the booster, if the brakes release, the booster is bad.

[Peterbylt]

Going on the theory that the piston is not retracting fully, i looked at several things:
1 - Free play in the pedal/linkage. This was OK but I added a little for good measure. Did not fix the problem.
2 - Checked the linkage with and without the vacuum line attached to the booster. NO change
3 - Pulled the master cylinder and added a washer between it and the booster. Checked seals for proper vacuum and re-bled system.
Pressure build up gone.
Drove it around with of lots of starts and stops over the weekend. No problem other than how baked the rotors and pads got during the problem. Little pulsing and squeaking. Gonna leave it until I'm sure the problem is fixed.

Final solution is to remove the washers and grind the pin between the booster and master cylinder piston. Turn rotors and replace the pads if necessary.

[maxwedge]

Good work, just enough piston movement to block the bypass port.

[bobss396]

Good job fixing the freeplay problem. Rebuilt parts sometimes are not quite exactly the same as OEM issue parts for many reasons, even an accumulation of manufacturing tolerances between component parts can do it to you.

The older cars had an adjustable rod that went into the back of the master. I found this out on my '66 Chevy many years ago, the fix was simple once I learned about freeplay.

Bob