Brake experts - Please Help!

[michael_J_brown]

Brake experts, here's your chance to help me (and others) diagnose significant brake issues. I'm a long-time Porsche owner with a 928s, 914 and 911, this being my first Boxster.

Car:
'99 Boxster
Purchased 3 weeks ago with 50k miles
Always garaged and in near mint condition

The car drove perfectly for the first week of ownership then....

Original Issue:
Driving at 45 mph car developed major drag, causing the car to almost stall. I immediately thought the clutch had gone out but depressed the brakes and the drag stopped. Over the next few days, the problem persisted and got worse.

Attempted Resolution No. 1:
Replaced brake fluid using Pentosin Dot 4 with my pressure bleeder at 20 PSI. Bled each caliper using Porsche recommend order until no bubbles were present in the line. My thinking was that the car sat for long periods of time and the brake fluid became contaminated.

Result:
No change, calipers still sticking.

Attempted Resolution No. 2:
Replaced master cylinder. I bench bled the new master cylinder with Pentosin Dot 4, installed and bled the brakes using my pressure bleeder at 20 PSI until no air bubbles were present.

Result:
Car was experiencing significantly more drag than the original problem! I raised the car to discover that all 4 calipers appeared to be engaged and causing major drag. I opened the bleeder nipple on all 4 wheels which allowed the calipers to disengage. I then tested the booster using the pump and start test, the brake pedal went down as expected. Pumping the brakes with the car off does not seem to build up pressure.

I then started the car and let it idle for 2 minutes, then applied brakes while on the lift. This action caused the calipers to engage and NOT release. I was suspecting a booster failure so I disconnected the vacuum line from the booster, fully expecting the calipers to release and pressure to escape, but neither happened.

I have messed with this for 2 weeks now. I hate to give up and have it towed to Porsche but I;m running out of options. Any Ideas?

[JFP in PA]

Yes. Pull the vacuum line off the booster and either drop a bore scope down into it, or get a piece of material thin but long enough to reach the bottom and look for the presence of either water or a lot of rust. If you find either, you need a new booster unit.

[Lapister]

Maybe there's some type of check valve within the booster that's not releasing? I'm just shooting from the hip as I don't really know the boxster system

[bwdz]

Don't give up, you sound like a very competent mechanic and are going through the right steps to figure it out. I am assuming that you checked the actual calipers but the chance of all 4 acting up is slim to none. Sounds like the problem is in the pedal or the booster. Did you check the pedal and rod? There is another thread on here with the booster failing and he gets a hard pedal but many people had their booster rust out on the inside. I would at least pull the booster off and the brake rod and have a look, you already have much time invested and this won't take too long.

[Steve Tinker]

I'm with JFP here - if the front compartment drains are (or have been) blocked, rain water collects around and then into the booster causing the internals to corrode......

[michael_J_brown]

Update on this issue. The brake booster had failed, which according to Porsche is very rare. I had it replaced and the car brakes just fine. I was going to replace the booster myself but was concerned about having to replace the linkage so I took it to Porsche.