Self-Applying Brakes - HELP!!

[Kentucky]

So, I am stumped - and so is my 88 year old mechanic dad.  Can you help? 

I have a 1976 Eldorado and we were changing the calipers all around.  We were bleeding the last one and no fluid at the relief valve on the last caliper so we wonder about the master cylinder and bleed the master cylinder with the car running.  While bleeding the master cylinder, the brake pedal goes to the floor on its own and will not release.  Turn the engine off and the pedal returns.   The cause of the no fluid is a collapsed brake hose leading into the caliper; we replace it; we have fluid; we bleed that brake (engine off); and we have good pedal.  We start the car and the pedal automatically goes to the floor and stays there as before without touching the brake pad.  Turn the engine off and the pedal returns.  Although the engine has run only a minute or two, we have very warm power steering fluid in the high pressure line into the hydrobooster.  So, we assume it is a stuck valve in the Hydrobooster.  But what caused it and how to fix it?  Is there someway to unstick the valve without replacing the hydrobooster?  Is the master cylinder causing the spool valve somehow to be off where it should be?  Is there something else going on? 

[TJ Hopland]

I would guess it was just old age and operating it without the engine running was just too much for it.  You can buy a rebuild kit for them if its just soft parts that are worn but they do also wear in the bore at which point they need to be machined or sleeved.

As far as I know there are only a few styles of rebuild kits that fit most every one made and they are still used today on the heavier light trucks so it should not be too difficult to find someone to rebuild it.  Diesel shops often do it or know where to have it done.       

[Kentucky]

I appreciate the thoughts.  Somehow, though, I think it is connected to bleeding the master cylinder with the engine running.  We bled the other three calipers with the engine running with no issues of the self-applying brake pedal.  We tried to bleed the last one with the engine running and got no fluid.  That's when we went to bleed the master cylinder and that is when the brakes began self-applying every time we started the car.  I am wondering if somehow we caused the master cylinder not to return to its proper position which is somehow affecting the hydrobooster...??

[TJ Hopland]

This looks like a pretty decent explanation.  Several pop ups at first but then it appears to let you read with more ads stuck between the text.

https://www.brakeandfrontend.com/operation-diagnosis-and-repair-of-hydro-boost-power-assist-systems/

[Kentucky]

I appreciate the reference article and the help.  I had already come across the article and read it - it's seems to be about the only thing I can find on it.  It doesn't really answer the question as to what is likely going on and how to unstick the valve if that is in fact what is going on.   There seems to be very little in it on self-applying brakes.  If I need to replace the hydrobooster, so be it but if the problem is in the master cylinder, I do not know that replacing or rebuilding the hydrobooster is going to fix it.   

[wheikkila]

Is it a front or rear brake that doesn't have fluid. I would look at the proportioning valve. That controls the brake fluid.
Thanks Wayne 

[Kentucky]

Thanks for the response.  All the calipers now have fluid.  The problem on the one caliper was a collapsed brake hose.  That is fixed.  What is not fixed is the brake pedal self-applying as soon as the engine is started. 

[The Tassie Devil(le)]

The only way that a brake pedal could move on its' own as soon as the engine is started is from some misaligned internals that over-ride the operators foot.

Bruce.

PS.   I know nothing of the operation of the Hydro Boost systems, and don't really want to learn either.

[V63]

Typically, brakes are bled with engine off. I would verify the brake system first and then follow with hydaboost. The problem is likely the rear calipers and the automatic adjustment for emergency brake. I would assume the emergency brake is OFF? Verify the cables are not corroded and move freely. There is a ratchet design in the rear calipers that must reset by pumping the brakes to allow the rear pads to 'seat' to the rotor. This problem would translate to Low pedal height. With the engine off you should get a, hard, high brake pedal. Once you have that, then graduate to starting the car and review remaining  issues.

  Since you have a 'collapsed' hose you might also have contaminated brake fluid that also damaged the entire hydraulic system to include the master cylinder. If power steering fluid finds its way into the master cylinder the whole system is damaged. Some people think ps fluid and brake fluid are interchangeable and it spells disaster

[TJ Hopland]

If you think the master cylinder is causing that issue with the booster you can just loosen the bolts between the booster and master. Usually those bolts are long enough you can get like a 1/4" of gap there.

[Kentucky]

So, we checked the steering pump return lines, no blockage.  Then, we pulled the master cylinder off.  I appeared to be operating perfectly fine.  Turned the car over without stepping on the brakes and the rod from the hydrobooster went flying out.  Put the rod back in and then pressed the brake pedal with car off and the rod stuck sometimes and sometimes not.  So, either rebuilding or replacing the hydrobooster and, hopefully, that will resolve the self-applying brakes.  Any thoughts on rebuilding or replacing? 

[V63]

What does the PS fluid look like? Maybe flush the system with new fluid before diving into the hydra boost?

[Kentucky]

So, we have taken the master cylinder off.  It is operating fine.  We have taken the hydrobooster off and apart.  The spool valve was stuck - tight.  Apparently, when the master cylinder was bled, it pushed the spool valve up just a bit further than it normally goes and got it stuck leaving the booster applying whenever the car was turned on and the fluid flowing.  The steering fluid turns out to be really dirty and even containing bits of grit.   The place where the spool valve got stuck had a build up of this type of gritty gunk.  So, we will completely flush the steering fluid.  I have sent off for the rebuild kit for the hydrobooster and although it is likely we could just put it back in and it would work as we have completely cleaned the spool valve and piston (and it moves freely), we are going to take the opportunity to replace the seals now that it is out.   Will post once we have it back in to confirm whether this was in fact the issue.  Lesson learned so far:  before bleeding master cylinder with car on, make triple sure that the steering fluid is clean or, better yet, flush the steering fluid.  Also, may want to consider just bleeding master cylinder if necessary without car on and hydrobooster potentially in play.  Many thanks for everyone's help and suggestions.

[The Tassie Devil(le)]

G'day Matt,

Thanks for the report on the rectification of the problem.

Sure will save a lot of angst from other Members going through the same, or similar problems.

Bruce.