996 Brake come on by them self after 3-5 miles

pwadds · 02-27-2023, 09:03 AM

1999 996 3.4 all stock.
Took the car to a track event, car ran fine. A week later took it for a drive and with in 4 to 5 miles the brakes started to drag and pedal got rock hard which at this point the car would not move. Brake lock up and very hot. At this point I decided to pull the brake booster line which I got a swish sound and the brakes released. Got home looked over the system and nothing was jumping out with me.
Since then I've changed the ABS unit, master cylinder, and 2 brake boosters, one at a time and tested. Still having the same issue.
What am I missing? HELP!

Mark44 · Mark44's Garage

Have you looked at the calipers and rotors? They are the components that got hot, so they would be the first things to look at. I would get the car up in the air and see which wheels are affected. If the brakes got very hot, as you said, it can affect the calipers.

Also, if the brake fluid hasn't been regularly purged, it's possible that one or more of the pistons in the calipers is gummed up or corroded (due to water in the brake fluid) and not returning to its rest position.

You mentioned changing two brake boosters. As far as I'm aware, there's only one on your car.

tonythetarga · 03-01-2023, 04:04 AM

Have you changed the tire size from the track day? Sounds like ‘ice pedal’ which is caused by the ABS sensing the tire rotation is outside the parameters of the factory spec. I think you have only a 5% variation in total circumference before things go a bit squirrelly.
Another possibility is the ABS system needs to be reset with PIWIS software if you change certain brake components. Both issues can cause the ice pedal feel you are getting.

ilucky1 · ilucky1's Garage

Pwadds, Another long shot with early 996s is when replacing the master cylinder, this can happen. The newer 996 replacement parts don't mate exactly into the older 996s, and until some minor hand tuning is done inside, brake lock ups like you described can happen. For my 1999 996 when I had my master cylinder replaced, your type symptoms happened, and it took my mechanic a bit of research to understand what was required to tweak the new master cylinder into behaving...worked great since. My limited understanding of the problem is there is a rod inside the master cylinder with a pin/needle on one end that needs to be shortened by a very small amount, which allows the master cylinder to release the brake pressure as it normally should.