Hello,
I have been working on the brake system for my dad's 76 Eldorado. The problem we are having is that the brakes slowly lock up after driving for a while. It has been sitting for about 20 years. The brake master cylinder was bad so we replaced that. We had the problem with the brakes locking up, and research indicated the problem was the hydro boost. We replaced the hydro boost with another used one, and then took it out. we drove somewhere between 10 and 20 miles, and then had the same problem again.

The brakes seem to apply evenly when they lock up.
We limped it home by loosening the nuts holding the master cylinder to the hydro boost. This released the brakes.

Hoping someone knows how to fix this problem. Thanks in advance for any help.

- Max

 

Nearly always, the problem is that the master cylinder primary piston cannot return to it's proper "at rest" position. This blocks the "vent" port between the actual cylinder, and the reservoir that sits on top of the cylinder. Any heating of the fluid causes it to expand--and with the vent blocked, the expanded fluid applies the brakes. Which heats the fluid more, which applies the brakes harder. Cycle repeats. In short, the system has to "breathe" into the reservoir due to temperature changes.

Loosen the nuts, and the piston can fully return. Which then vents the brake pressure, and the brakes release. I think you've absolutely proven that this is the cause of the brakes self-applying.

There are multiple potential reasons for the master cylinder piston to not return to the "at rest" position--incorrect master cylinder, misadjusted pushrod between master and booster, incorrect adjustment of the pedal-to-booster pushrod, incorrect adjustment of the brake-light switch. Not all vehicles have all these adjustments. At one time--for certain vehicles--GM said to remove the non-adjustable booster-to-master cylinder pushrod, and replace with the special "adjustable" pushrod that has almost certainly been discontinued for the last three decades. Other vehicles were built with an adjustable pushrod.

It would be worth your time to locate a Genuine GM service manual for your vehicle. There are also "DVD" copies of the Genuine GM manuals. They're searchable, which is nice. The .pdf files tend to be FREAKIN' HUGE, which makes scrolling a pain, though.

Dear Old Dad had a '74 ElDo, the way I remember it, the thing had a vacuum booster.

 

I think that the problem is that the booster is applying itself. We are going to replace the return line from the booster, as we think it may be clogged, causing pressure to build up in the booster. I will look for a service manual for the car.

- Max