1998 Porsche 996 Brake Problem

Help!! I have a 1998 Porsche 996 with a problem with the brakes, they all come on while driving, When I am driving I can feel all the brakes coming on until the car come to a halt, I have worked out that if I detach the booster vaccum hose and reconnect it the brakes are release until they come on again in a mile or two. I have had it in and out of the garage without success, they changed the Master cylinder and brake booster last week but still they problem exists. Now the brake fluid warning light is on and the brake fluid is at max follow the change last week. They are now thinking that the vaccum hose needs to be replaced but it doesn't appear to be leaking. If anyone can help it will be much appreciated because this is costing my loads and I can't see an end to it. Cheers.

Have you checked the flexible hoses to the calipers?

No I don't think they have but would this affect all 4 brakes.

I suspect a fault on the ABS system. MC

Are you sure that all four brake calipers are affected? I had just this problem on an Italian car and by releasing a bleed nipple I was able to sort out which caliper was affected. Worth a try and no cost! PS I replaced all eight flexible pipes with SS braided ones made to order from Think Automotive.....all eight cost the price of two from the Concessionaires!

Also check that the vent hole in the master cylinder cap isn't blocked. MC

Tell the garage to do a steering angle sensor calibration with a system tester and see if it helps .

Hi thanks for all the advice. I have disconnected the ABS pump then driven the car and the same problem occurs, so I ruled out the ABS pump for the time being. Can anyone confirm if the vacuum hose contains valves maybe that could be a cause. cheers.

This^^^^^ + TC The easiest and best way to eliminate the above is to remove all fuse that control the TC & ABS.

OK I'll try that next. Cheers.

Just been out in the car again tonight with the ABS fuse removed and alas the same problem, after a couple of miles the brake pedal stiffens, then shortly after the car starts to drag and then the further I drive the worst it gets. The only way to resolve this is to disconnect and reconnect the vacuum hose. So new brake fluid, new booster, new master cylinders and no joy. All advice and help welcome... thanks.

Did you check the master cylinder cap vent hole? MC

I've read through this again .. my thoughts but not an actual fix ... You disconnect the vacuum pipe to release the brakes .. i feel this is important somehow .. The servo cant over vacuum , there is a check valve in the pipe that connects to the servo but this is there to keep vacuum IN the servo . PSM / TC .. a steering angle sensor out of adjustment .. it thinks the car is cornering and so applies the brakes .. this should release though without the need to release the vacuum . hence a steering angle calibration to rule it out . it sounds like the master cyl rod is applying too much pressure .. not a huge amount but enough to apply a slight pressure on the master cyl . I know these servos are modified .. not a cheap replacement and the adjustment of the rod can be incorrect .. but that doesn't answer the question as to why you had this fault and the servo was replaced .. lets say incorrectly adjusted and is applying too much pressure on the master cyl . There is a fault on the 981 with the bushes on the brake pedal sticking with heat .. big recall to replace these bushes .. I'm just wondering if something similar might be happening .. could the pedal be sticking a bit and applying pressure to the master cyl causing this ? perhaps when it locks the brakes have a look to see if the brake light is on before releasing the vacuum . For me at work .. i would be checking the linkage between the pedal and the master cyl .. is there enough free play . As MC has said .. cap air vent , slacken the cap a bit and try it .. that will allow air in to prove or disprove this as a fault . I've had a fault where the customer was a bit .. er .. enthusiastic on cornering .. psm kicked in a lot of the time and bent a rod in the master cyl .. this is similar but if the fault was the same before and after a master cyl replacement then i have to say it's not that . Please confirm you drive the car normally when testing for this fault .. any roundabouts , hard cornering .. does it seem ok in a straight line but starts to drag after a few corners , steering wheel turns basically . Random thoughts of mine basically as i don't have an answer but someone might read this and have an answer ... or it might be the pedal bushes .. :dont know: EDIT .. I would also have it on a ramp and make sure its all wheels locking and not just a single one .. rather an important point to make .. 1 wheel or 4 wheels are totally different faults .

Bear with me here, it's a long shot based on little knowledge. Is it possible to fit the servo hose the wrong way round and for the one way valve to have a minor leak? Or for the one way valve to be faulty so that a larger than normal vacuum builds in the servo? So large that the brakes end up being applied and never fully release?

As deMort says, check whether it is only one wheel or all four that is locking. That is why I suggested cracking open a bleed valve to establish this. Try the quick cheap options first.