Rear brakes not releasing after hard braking

A little background. My '65 Riviera has been converted to front disc brakes with a dual master cylinder. The proportioning valve was leaking out of the reset pin indicating a seal issue; so, I decided to replace it with the exact same valve.

No issues with normal braking with the new valve; however, when I brake really hard (pressing on the peddle as hard as I can), the rear brakes don't release. I have to let the car sit and eventually the rear brakes release and the car drives and brakes normally; until I brake hard again. I generally don't need to brake this hard and the car stops great. No issues with the front brakes.

I am thinking one of the following:

1. Still air in the system. I replaced all the lines running to the proportioning valve at the same time, as I did not like how they were run previously.
2. Faulty new proportioning valve.
3. The previous valve was mounted on a bracket under the master cylinder. Even though, the valve, master cylinder and booster came as a kit designed for the car and disc swap, the proportioning valve was always too close to the inner wheel well. I actually needed to grind the corner off the valve to get it to fit. When I installed the new valve, I oriented the new valve to be at an angle to the master cylinder and parallel to the wheel well. Now that I am having an issue, I am wondering...does the valve need to be level to the ground (parallel to the master cylinder)?

Any thoughts?

Thanks
Doug

 

Check the rear brake hose. Don't think a valve not sitting level could have that effect.

 

Check all the flex hoses. sometimes they can collapse internally causing pressure to hold.

 

^^^ This.

Check the rubber flex hose on the line between the body and the rear axle.

 

The valve doesn't care about orientation.

 

Stuck wheel cylinders will also do this. You're probably due to rehab the rear brakes

 

Thanks all

I’ll check the parts mentioned. If it was something mentioned it will be really disappointing as all the parts have less than 2,000 miles; including the leaky proportioning valve that sent me down this path to begin with.

 

Just because they have less that 2000 miles on them doesn't mean anything . They could me hanging up from NON USE .

 

Adjust your rear brake shoes tighter to the drum, possibly they are too loose over extending and getting hung up ,not releasing.

 

You may not have the proper length on the pushrod between the booster and the master cylinder.

If that is longer than it should be it will hold pressure it the system until it slowly bleeds off.

You would need to check the depth in the piston at the rear of the new MC vs the old one.

 

Another thing you could try :

Remove the 2 nuts &washers holding the MC to the front of the booster. Do not disconnect the steel brake lines.
Pull the MC forward until it clears the booster / MC studs. Install 4 flat washers onto the studs - 2 on each - then re-install the MC and tighten it down.

Try your brakes in the manner you had prior that caused the issue.
If it works correctly then there is the problem.
Then remove 1 washer from each stud and try it again.
This will tell you how much needs to be removed from the pushrod between the MC & booster.

I have found thru the years that most pushrods are longer than needed.

 

A couple things I've recently come across:
1) We found a leaking rear axle seal, there was some oil within our brake drum.
2) Apparently on some drum brake setups the two shoes can be mixed up and installed backwards, they look "almost" identical but aren't. And this might sometimes cause a scenario like this. (Not sure if that applies to '65 Riv but since you recently re-did the brakes this is something to check)
-Bob C.

 

Finally had a chance to diagnose this issue.

I started by loosening the master cylinder at the booster and pulling it away from the booster, when the rear brakes were locked. It made no difference, the brakes remained locked; so I am ruling out the pushrod being too long.

I then cracked open the brake line fittings starting at the back of the car making my way up to the master cylinder to see if the brakes would release. Anytime I cracked a fitting open the brakes would unlock, all the way up to where the line plugged into the proportioning valve. I then cracked open the line which goes from the master cylinder to the proportioning valve for the rear brakes. This time the brakes remained locked.

By process of elimination I am thinking there is something wrong with the replacement proportioning valve I purchased. I can hear the faintest spring like sound when the braked finally release. It sounds like it is coming from the proportioning valve. It makes sense that this would be the issue as it is the only part I replaced when the problem started.