Brakes locking on intermittently - any thoughts?

[David Rooke]

I'm being tormented by an intermittent (of course!) issue with the brakes on my '66 912.

It's the single master cylinder system with a 5 year old master cylinder and refurbished brake calipers all round.

The problem is that the pedal goes firm and brakes bind on for a relatively short period of time and the pedal returns to normal and brake released. The binding can be variable from not noticeable other than brake pedal change to slowing the car. It has resolved on one occasion by reversing backwards but I'm not sure if this was coincidence.

Prior to dismantling the car I had only driven it once, 450 miles from London to home and the pedal feel was lousy but not sticking.

The issue has appeared after a wee resto in which the pedal box, master cylinder, fluid reservoir have been out of the car. I have bled all 4 wheels several times and for the most part the brakes work fine and pass the MOT testers requirements.

Thanks for your thoughts?

David.

[Nige]

Have you changed the flexi hoses? Are the pads sticking in the caliper? Is the piston sticking in the caliper? Can't be much else.

[KS]

Check that your flexible brake hoses haven't collapsed internally, or a hard line hasn't been crimped somehow.

[PeterK]

Does the pedal return to normal and brakes release whilst still driving, or do you need to allow the car to rest / cool ?

I had a similar issue (non Porsche) following a master cylinder rebuild. A combination of a sticky pedal pivot and maybe an actuation rod being a shade too long, caused the master cylinder to not fully vent back into the reservoir when the brake pedal was released. As the brakes were applied, the brake fluid warmed and expanded, gradually locking on the brakes. If I stopped and allowed the brakes to cool, the pedal feel returned to normal. A short term fix was to shorten the actuation rod (I could have added washers under the mounting lugs), but in my case the proper fix was to rebuild my pedal box.
Peter

[David Rooke]

Does the pedal return to normal and brakes release whilst still driving, or do you need to allow the car to rest / cool ?

I had a similar issue (non Porsche) following a master cylinder rebuild. A combination of a sticky pedal pivot and maybe an actuation rod being a shade too long, caused the master cylinder to not fully vent back into the reservoir when the brake pedal was released. As the brakes were applied, the brake fluid warmed and expanded, gradually locking on the brakes. If I stopped and allowed the brakes to cool, the pedal feel returned to normal. A short term fix was to shorten the actuation rod (I could have added washers under the mounting lugs), but in my case the proper fix was to rebuild my pedal box.
Peter

This sounds very similar but without the heat component. The brakes can return mid drive but the floor has been repaired under the pedal box and the pedal box has been modified at some point so it may be the actuator sticking...Thanks

Regards the other suggestions, thanks too. Would kinked line, or collapsed flexi hoses be so intermittent in everyone's experience? I had discounted this on account of the brakes being good most of the time.

[KS]

Not the kinked line, but the collapsed hose can cause intermittent problems (ask me how I know...) if the inside wall is starting to break up. Normally this manifests itself as a change in colour in the brake fluid (darker) when you bleed the brakes, though.

[911hillclimber]

My bet is on Peters thoughts.
Had this on my 911 about 3 years ago.
No rubber hoses on my car etc for20 years.
I relieved the brake pressure when locked on by cracking the nipple on the rear calliper.
Check the pedal and cylinder rod clearances maybe adding 3mm to the pedal travel before pressure is felt on the disc.

[fetuhoe]

The 'piston' attached to the pedal on the single cylinder system is adjustable for length.

If it has been 'restored' it may have been re-assembled with too little free play between the end of the piston rod and the cylinder and as the brakes warm up it will cause the brakes to bind.

In extreme circumstances it can cause the brakes to lock completely and the only way to 'unlock' them is to release the pressure are re-bleed.

If this part has become mixed up with a piston from a dual circuit car then it will almost certainly bind up the brakes.

[smallspeed]

I believe tim (impmad2000) had this exact issue after a spirited drive not long back!

[David Rooke]

Thanks everyone.

I will add rod length adjustment to my lengthy to do list

David.