Brakes Locking Up

[David Hillman]

I've had a problem with brakes locking up and have finally been able to reproduce the problem.  Start the car and let it run for a bit without driving.  The brakes gradually lock up without driving nor activating the brake pedal. Let the car sit for several hours and I'll find that the brakes have released.  Brake booster ?  Check valve ?  69 GT350 with original brake booster rebuilt a few years ago by Booster Dewey.

Thank you in advance for any thoughts ? 

[jimhyc]

Check the clearance between the master cylinder and the booster rod. 

[Bob Gaines]

Have the brakes gone from working normal to brakes locking up or what?

[roddster]

As in any recent repairs?

[pbf777]

Check the clearance between the master cylinder and the booster rod. 

    And to add to the above:  Be sure the piston in the master cylinder is allowed to return and rest at the rear retainer stop, as otherwise it might be possible that the fluid communication port in the bottom of the reservoir is not exposed and this blocking the fluid from returning (that which is ahead of the piston seals) to the reservoir and being retained in the high pressure circuit of the system.  Though this in time would cause a "sinking" pedal position, when the vehicle is started-up from ambient temperature and the running engine warms-up this results in the heating and expansion of the brake fluid in the exposed proximity, which results in the pistons in the wheel cylinders and calipers to be moved outward in providing the additional area needed due to this expansion and results in applying the braking effect. 

    Just a "wild-azz-guess"!   

    Scott.

[geraldt52]

Start the car and let it run for a bit without driving.  The brakes gradually lock up without driving nor activating the brake pedal

That is an odd but not super uncommon failure mode of a power booster.  More common is a hissing sound when the brakes are applied along with increased pedal pressure.  I had one once that as soon as the engine was started the brakes would apply themselves, and the car couldn't be moved...turn it off, and they'd release.  It's wierd when it happens, but I'd bet a lot that you need a new booster.  You're lucky it's a small block car...

[Road Reptile]

Hi,
Think we need to start with a good visual inspection on a hoist. Look for the simple things like leaks or hoses that show damage. Determine if all wheels lock up or if it is just fronts/rears. What condition is the brake fluid? Would be a good time to Flush out and replace fluid if it is suspect....this would also let you bleed and check for dirty or muddy fluid. This is the basic place to start and involves mostly time-not money. It also gives you an idea of the condition of parts. Does this car sit for long periods?
Do you have a record of all brake history? Just my way of approaching it without wasting money on guessing what will correct the fault. Hope this helps.
R.R.

[gt350shelb]

booster is bad just went thru this  issue

[Road Reptile]

Hi again,
This is a good example of repairing a fault without determining the root cause of failure. Most boosters fail on cars that sit and a HUGE possibility is failure from brake fluid leaking from the master cylinder into the booster. A proper inspection should find this type of symptom-low fluid with no external leaks visible.
You need to remember base brakes need to be operating properly on all 4 wheels.
A thorough inspection will verify all components are up to spec.It is important to not overlook the simple!
R.R.

[FL SAAC]

best thing to do is take it to a local mechanic and have him or her do a visual inspection and resolve your issue immediately instead of people guessing at your issue


be safe out there