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Brakes lock up after 10-15 minutes of driving

Another thing to check is the large vacuum line (actually a fuel line) that runs from the manifold to the brake booster, as well as the check valve (which the large hose runs into). Mine was cracking and leaking apparently, and not letting the booster get enough oomph. All the parts needed can be had at Autozone (check valve is on the shelves, HELP! brand). My car now stops amazingly well
 
This is my first post ....
And I am sitting here in my 740 waiting for the brakes to unlock (again) .
Thought I might have sorted it by cleaning front calipers and changing 1 front brake hose , but alas no . Here we go again .
Was fascinated to read all of the suggestions on this forum and to discover someone else has the exact same problem .
 
Time to replace the calipers. Sticking pads against rotors then boiling brake fluid.

Is one wheel is hotter than the other? Unusual wear on either side?
 
One front brake hose was replaced and I had difficulty undoing the union for the other so left it .
Removed and cleaned the front calipers which did seem to be an improvement .
Even if the car is not being driven but left idling , after 10 minutes the pedal will become hard and the brakes will start to hold on .
It is really only stupid people who pick up turds in my experience . So I am safe .
 
Adrian B said:
Even if the car is not being driven but left idling , after 10 minutes the pedal will become hard and the brakes will start to hold on .
Click to expand...
Disconnect and plug the big vacuum hose going to the brake booster (basically disabling it), then let it idle for the same 10 minutes and see if anything feels different.
 
^^

Adrian B said:
Well , all wheels are locked up that is for sure .
Can it really be as simple as replacing the calipers ?
I hope so ...
Click to expand...

All wheels locked up? As in you get squirted on cracking a bleeder? Can you expand on that? I'd get the booster out of the picture too if that ever happened to me.




Adrian B said:
It is really only stupid people who pick up turds in my experience . So I am safe .
Click to expand...

If that's true, I wish we had more stupid dog owners in my neighborhood.
 
Adrian B said:
Even if the car is not being driven but left idling , after 10 minutes the pedal will become hard and the brakes will start to hold on .
.
Click to expand...

This sounds highly unusual. You're sure about that?

MasterBlaster said:
Disconnect and plug the big vacuum hose going to the brake booster (basically disabling it), then let it idle for the same 10 minutes and see if anything feels different.
Click to expand...

This, then report back. I've never heard of a self pressurizing brake system and can't figure how it would even happen.
 
i've worked on a car from a collegue which had similar problems. In that case the return hole in the master brake cylinder had clogged.
The car braked fine but after 5-10min of driving the brakes got hot and the car stalled. Apparently it was possible to push fluid INto the lines but it was not possible for the fluid to flow back from the cylinder into the reservoir.
i took off the master cylinder and disassembled it, cleaned it thoroughly and discovered the clogged hole, pierced it, did some more cleaning, used a rebuild kit to reassemble the master cylinder, put brandnew DOT4 in, bled the entire system and that fixed it. Car good to go.
 
Janspeed said:
i've worked on a car from a collegue which had similar problems. In that case the return hole in the master brake cylinder had clogged.
The car braked fine but after 5-10min of driving the brakes got hot and the car stalled. Apparently it was possible to push fluid INto the lines but it was not possible for the fluid to flow back from the cylinder into the reservoir.
i took off the master cylinder and disassembled it, cleaned it thoroughly and discovered the clogged hole, pierced it, did some more cleaning, used a rebuild kit to reassemble the master cylinder, put brandnew DOT4 in, bled the entire system and that fixed it. Car good to go.
Click to expand...


I used to have a 69 164 that had this problem. However, it was only happening in one circuit of the dual master cylinder. The left front brakes even after I changed the calipers and added stainless braided lines still locked up the front left after about 10-20 min of driving. The next owner replaced the master cylinder and it was repaired.
 
Awesome !! so much traffic here and so encouraging . After a week off from trying this and that I am now going to try some of the suggestions such as 'cracking a bleeder' when everything is locked up to see whether it is in fact hydraulic . Vacuum hose first I think and then pending the outcome of that maybe master cylinder which I tentatively did first in the vain hope it was the legendary blocked bypass port . This time will dismantle and clean .
Have tried to cover most inputs without responding to all individually as being a newbie and basically crap at forum etiquette .
 
kyote said:
This sounds highly unusual. You're sure about that?



This, then report back. I've never heard of a self pressurizing brake system and can't figure how it would even happen.
Click to expand...

Well , this happened when I drove the short distance to the train station and left the car running for 5 or so mins before driving home again . On the shirt journey home the brakes totally seized and this has never happened before of short trips in and out of town .

So I tried this again the other day after arriving home after the short trip into town with the brakes still working fine . Left the car idling for five mins and the brakes locked up .
 
Adrian B said:
Hmmm ..... this just left me with no brakes as the booster seems to work only with the assistance of the vacuum hose .
Click to expand...
Normally they should still work, you just need to press HARD on the pedal.
I was hoping that if the brakes locked up without even driving (put it in neutral and see if it rolls), disconnecting the hose should take the booster out of the picture.

The booster has 2 chambers separated by a rubber diaphragm and a central moving "piston" with internal air valves in it. Normally, the rear valve is closed, and the front valve is open, keeping the same vacuum front and back. When you press the pedal, the movement closes the front valve (sealing the front chamber from the rear chamber), then opens the rear valve, allowing air to enter the rear chamber. The pressure difference moves the "piston" assembly forward and against the master cylinder.

It's possible (though rare) that the internal valves could mess up, opening or closing when they shouldn't, letting the "piston" move and apply the brakes without any help from your foot.

If you can catch it the next time they lock up, try pulling the hose AND the check valve off the booster, and see if the brakes release with no vacuum signal at all.
 
Booster Hose

MasterBlaster said:
Normally they should still work, you just need to press HARD on the pedal.
I was hoping that if the brakes locked up without even driving (put it in neutral and see if it rolls), disconnecting the hose should take the booster out of the picture.

The booster has 2 chambers separated by a rubber diaphragm and a central moving "piston" with internal air valves in it. Normally, the rear valve is closed, and the front valve is open, keeping the same vacuum front and back. When you press the pedal, the movement closes the front valve (sealing the front chamber from the rear chamber), then opens the rear valve, allowing air to enter the rear chamber. The pressure difference moves the "piston" assembly forward and against the master cylinder.

It's possible (though rare) that the internal valves could mess up, opening or closing when they shouldn't, letting the "piston" move and apply the brakes without any help from your foot.

If you can catch it the next time they lock up, try pulling the hose AND the check valve off the booster, and see if the brakes release with no vacuum signal at all.
Click to expand...

Ok , I will try that again because it sounds interesting . It was suggested to me that the tolerance between the Master Cylinder Piston and the Booster rod was to tight and maybe needed backing off for when things get hot . Today I did back it off when I was refitting the old master cylinder after having fitted a second hand Master cylinder that seemed turned out not be any good .
Alas upon latest drive the same problem was there . I am exhausted having bled the system so many times , although I must be getting closer . Going to source another Master cylinder to rule that out or in .
 
Master Cylinder

R-Pow3R3d said:
When I had this on my 740, I went through and replaced everything before finally finding the M/C was to blame. Not sure if that will be relevant to your case, but our symptoms seem remarkably similar.
https://forums.turbobricks.com/showthread.php?t=349161
Click to expand...

Yes , I am going to persist with that option to either rule it out or in . Yesterday I did buy a second hand one and fitted it and bled the MC and then the system but at the end of all that the pedal went straight to the floor .Soooooo .... confused . Have to try it again as it is possible because the MC had been at the wreckers for who knows how long the seals may have perished .
So today I refitted the original one and am back at square one but have ruled out it being the tolerance between the booster rod and MC piston .
 
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