BRAKES LOCKING UP AFTER BRAKE BOOSTER AND MASTER CYLINDER REPLACEMENT

1999 911 Carrera Mileage: 62 000BrakesClinton, MissouriHelp please, there are many threads on Rennlist etc... many inconclusive. After failed brake booster due to water intrusion I have replaced both booster and master cylinder, complete flush and bleed, new Brembo DOT 5.1 fluid as opposed to DOT 4 (INTERCHANGEABLE FLUID HIGHER BOILING POINT), retrofitted new style booster with linkage from existing system (old style for 1998-1999 models) adjusted pedal length at linkage to match clutch pedal height. Brakes firm and responsive with cold start up, after 5 to 10 minutes of normal driving, brake pedal becomes hard as a rock, all four brakes engage gradually until system is compeltely pressurized and callipers are in a closed position ( Complete brake lockup). Parts are correct, the brake pedal is set with some play by not being topped out and everything looks correct. There is no adjustment on the connecting rod leading from the booster bellows to the master cylinder that can be adjusted to provide slack. Symptoms indicate that booster is providing enough pressure to the master to engage the brakes. As I see it, possible issues include. 1. Bad Booster or Master 2. An adjustment that I am missing. Thanks in advance !

Are you sure you got the correct master cylinder? Is it a new Porsche master cylinder? Sounds to me like the master cylinder piston is not releasing the brake fluid pressure when you release the brake pedal. Either the piston is seized in the bore, it is incorrect p/n, or is completely faulty. The brake booster is simply for assist when applying the brakes and I think has nothing to do with this problem. See brake system components with p/n's here http://www.autoatlanta.com/porsche-p...section=604-00

Verified with Pelican, yes the right parts are installed as per their database. Parts are TRW, which are OEM suppliers to Porsche.

Strat,. Have you replaced your flexible brake lines that connect to calipers?

I hate to compare our 996's to a minivan but.....I had exactly this problem with my 2008 honda odyssey: replaced master cylinder and then all 4 brakes would lock up completely after a several presses of the brake pedal. Root cause was incorrect adjustment of the pushrod between the brake booster and the master cylinder. Issue was diagnosed by loosening the bolts holding the master cylinder to the brake booster. As soon as they were loosened, the brakes released. There was a nut to adjust the height of the pushrod base inside brake booster and tolerance needed to be pretty precise. Adjustment was also different for different brands of master cylinders. I'm not sure if the same adjustment is needed or even possible on the 996.1. I did a few internet searches and the only relevant thread I could find is here: https://www.renntech.org/topic/28489...hrod-986-my02/ Let us know what you figure out!

I had exactly the same problem in my 1999 C2 last summer. The problem came suddenly without warning. I overhauled the brake calipers, changed the flexible brake lines, ..etc without fixing the problem. Then Ahsai suggested it may be a booster problem. I took my booster out, it was nice and shiny, without a speck of rust even though it was 20 yrs old (California car). But when I measured the pushrod of the brake booster against the recess of the master cylinder, there was only about 0.5mm of clearance, which probably became zero clearance after the brake fluid heats up. My brakes would drag and get hot only after a few miles of driving. I ordered a brand new TRW one from amazon, the new booster's pushrod recessed about 1mm more than the old booster, that was enough to fix my problem. I also had to adapt the new booster to the old-style linkage like you did. By brake still drag a tiny bit (jack up the car and the wheels can be rotated by hand, but they stop spinning immediately instead of keep spinning like on my other cars with single piston calipers). I don't know if that is normal, but the wheels are no longer hot after a drive so the dragging isn't too bad, and I get about 20mpg on the highway which is normal for me. Measure carefully your master cylinder and your booster to make sure there is some clearance when the brake pedal is not pushed. The pushrod isn't adjustable like on other cars, so I don't know what you can do if there isn't enough clearance. Good luck

I have just ordered another Brake Booster, I have come to the conclusion that the clearance is not adequate with the bellows in its neutral position, and its ever so lightly engaging the master cylinder. As you have noted there is no adjustment to the rod entering the master cylinder....... waiting for it to come in the mail, once I have switched them out I will update the forum. Thanks for input.

It is your brake booster. I had this happen to me. You only replace the booster with a factory original booster. Don't buy any rebuilt stuff. The rebuilt one had the rod half a millimeter too far out and it did exactly like yours. Starts fine, then locks up after a mile of driving. I changed back to a brand new booster from the factory and it went away.

yelcab, that makes sense.