Brake expert needed

[Alshefchek]

Boxster brake problem - 99 Porsche Boxster base w/ABS - approximately 48000 mi.- I have been having this problem since having the car transported about 2000 miles in freezing temps. My first indication of a problem was a feeling that the brakes were sticking at stops. I immediately took it to my mechanic and had the master cylinder and booster replaced. Upon driving several miles I started to feel the same locking of the brakes. After stopping for awhile everything seemed to be okay again. When I was taking back to my mechanic and driving 5 miles or so, I again felt brakes bogging down the forward motion. I noticed that when I put my foot under the brake pedal the binding would release and the car would accelerate. My mechanic replaced the master cylinder and booster again. Did the same thing after driving about five miles. I decided that I was going try to find the problem myself. I discovered that the rubber boot on the push rod was torn and smashed up against the firewall, and thought this might be causing it to bind going through the firewall. I replaced the bellows kit, replaced all the fluid lines to the calipers with braided stainless steel, cleaned everything up, and bled the system twice with Petosin DOT 4, however I did not bleed the ABS. When I test drove everything felt good with exception of a pretty hard brake pedal. After driving a few miles I started feeling that familiar binding of the brakes, but this time they totally locked up. I couldn’t move forward or or in reverse and couldn’t even push it in neutral. After sitting there for about 15 minutes, I pulled the vacuum off the booster with engine running to make sure it was drawing a vacuum and it was. I reinserted the vacuum hose, got in the car and brakes were unlocked and everything was fine. Drove back to my garage. Since I’m afraid this will happen again I plan on having it towed to the shop. Just wanted check to see anyone has had a similar experience. Computer, ABS module ??? Thanks

[DMadigan]

If there is water in the calipers it can boil and lock the brakes. Have you ever completely replaced all the fluid? You should after 3-5 years. Brake fluid absorbs moisture from the atmosphere and will rust brake parts from the inside.
Did the ABS ever activate whilst braking? If not then it is not likely the problem.
Try the brakes without the booster to see if it is actuating the brakes.
You can use P80 rubber lube on the boot to make it slide easier on the pushrod.
It does not sound as though replacing parts without finding the cause is the best solution.

[tonythetarga]

I think you may be on the right track with the abs. Does your shop have the PIWIS software? This may be the only way to properly bleed the abs unit and make the brakes work properly. You need to locate a independent Porsche shop with the right diagnostics software. You’ve thrown a bunch of money at it already.

[Alshefchek]

Thanks Tony…I’ll check that out before I take it in.