Topic: 1978 TA brake problems

[eric78]

Hello.  I am having the front brakes remaining applied whenever the brakes are applied.  They are not de-pressurizing.  Can stock proportioning valve (gm p/n 25509518) be causing this?  As a test, I have tapped this valve with a screw driver and the breaks gradually release and the front wheels will spin again.  The rear brakes are not effected.  When the brakes are applied again then the front brakes remain active and front wheels are not able to be turned.

Any thought on the cause?

[eroc022]

when were the hoses replaced last ? if its been some time, try replacing those, as they tend to swell up and hold pressure, slowly allowing fluid to return..

[eric78]

Thanks for the rely.  The calipers and the hoses are new.  Both calipers are locking up so I doubt it would be the new hoses or the calipers creating this issue.  Maybe one caliper could be bad but both would be unlikely.

[Angelo]

Someone recently had this problem was caused by the brake light switch putting force on the pedal, rather than letting it return. The front brakes I think engage first before the rear so that could be it. If the prop valve balance was off the brake light on the dash would light up, assuming it is hooked up. I would get a new prop valve, cheap insurance. I think a right stuff or inline tube prop valve is maybe $60-80.

[eric78]

I'll order a new proportioning valve, which will require another round of bleeding.  Look for the results next week.

Thank you

[Flyinlow]

I would look at the master cylinder not fully returning as mentioned above. Lines won't hold pressure by themselves. It's not the proportioning valve likely -more a m/c or booster rod issue. Have you recently changed either?

[Angelo]

As I hinted, you can (almost) rule out the prop valve if your brake light is hooked up and not coming on when you have the problem. At least that tells you that the pressure is balanced between the front and rear. The light though only tells you balance, there could still be something wrong with the proportioning or metering valves in the prop valve.

If the valve is working correctly, the rear brakes should engage first before the front, though just barely. This assumes the rear brakes are installed correctly and adjusted. Here is a good article how the system works: http://auto.howstuffworks.com/auto-parts/brakes/brake-types/master-brake2.htm The front disc has more stopping power than the rear pads though, so it is possible that the rear pads are also grabbing, it's just the front is where the problem is. If that is the case, then yes there's something wrong at the mc/booster or even at the brake pedal.

Just to keep things back to basics, you are using dot 3/4 fluid? Did you properly lube the caliper pins? Are you using metallic or ceramic front pads (I would not use organic for front brakes)?

I just searched google, two tips to try: check for pressure in the master cylinder by taking the cover off when the disks are locked up. Another thing to check is disconnecting the vacuum to the booster to see if that is causing the issue. (I don't suspect these tests but they are simple tests)

[JupiterBandit]

I agree with a bad master cylinder

[eric78]

I have changed the master cylinder as well as had the Boost rebuilt.  Once I changed the proportioning valve the front brake would release.  However, there is some seepage on the proportioning valve on the master cylinder lines so I need to tackle that next.   I need to continue testing before it goes on the road. 

Its been one thing after another with a unrelated engine issue that has occupied my time. 

[Angelo]

We all understand that. Seems like for every problem you try to solve with these cars there is at least 1 new problem before it's all fixed.

[JupiterBandit]

squishy pedal also could be you haven't bled it out properly.

[JupiterBandit]

also keep in mind that just because you had the booster rebuilt doesn't mean they did it right. so keep it in mind

[eric78]

I will bleed the brakes again as I will replace the MC to proportioning valve tubing.  There is a little seepage on both fittings.  The nut are in poor condition as well.  Once that is bleed I'll report back.  It will be at least a week before that happens.  Before this is ready to drive, snow will be on the ground.