stevesp98

This is easily both the scariest and strangest thing that has ever happened to me in a Porsche. I was driving my 1988 928 yesterday on I-15 when the brakes completely failed on me. By failed, I mean that the pedal went to the floor with no resistance and I got 0 braking effect whatsoever. With a combination of downshifting and the parking brake I was able to safely exit and get it parked.

I thought for sure a brake line or fitting had failed and there would be brake fluid all over. But when I got under the car everything seemed fine and I couldn't find anything obviously wrong or leaking. Then after about 30 min after getting it stopped while I was on the phone with AAA, I put my foot on the brake pedal which was still on the floorboard and it popped right back up. The pedal was now firm so I started the car and tried to drive it. Brakes worked PERFECTLY. Still had it towed though.

So my question to the 928 braintrust is this: What the hell happened? I had driven the car for about 30 minutes prior to this in stop-and-go SoCal rush hour traffic and the brakes functioned normally. The engine was running hot - right at the top white line on the temp gauge. But that was the only "not normal" thing I noticed.

I don't see how a braking system can completely fail and then heal itself? But then again, these are amazing cars.....

James Bailey

I would suspect that a dragging caliper may have boiled the brake fluid....

Speedtoys

But no pressure to all 4?

Would have created a drag and pulled the car too, and leaft a nasty ugly looking caliper.

M/C failure...seal is about gone..may have fluid under the carpet.

Adamant1971

Yep, and do not drive it again until the MC is replaced.

James Bailey

I bought and drove home an 86 1/2 which was dragging the brakes and after a few miles there was almost nothing there !! Brake booster was applying the brakes all the time. Pulling the vacuum line to the booster would stop the dragging but left it with only manual brakes. It got a replacement booster and master cylinder too !! It had leaked brake fluid into the booster where it was sucked into the engine.

stevesp98

Absolutely nooooo driving this car. I could go the rest of my life without that ever happening again....

Ok so then we think that the failure mode here is the seal on the M/C was going and repeated use / temperature / bad luck caused the failure which after giving it time to cool / rest / etc was then able to seal up and make pressure again?

soontobered84

When I picked up an '88 from Santa Barbara a few year back, there were a couple things I wanted sorted before I left for my trip back to Texas.
One was the fuel line between the fuel rails at the front of the engine. I would almost swear I could see through the original one.

Roger was kind enough to overnight one to me.

I also needed an alignment. The local Santa Barbara Sears was kind enough to accommodate my requirements (No lifting the car, dammit!!)

I stopped by the local auto parts store and picked up some oil for the trip and a bottle of Castrol LDS brake fluid and started out down the coast and into LA.

With all the goings on, I got a bit of a late start and ended up in Rush Hour(s) traffic. Nightmare. 8 lanes headed east, I think, and all filled with bumper to bumper stop and go traffic. I was in the next to left lane when I determined that I had absolutely no brake pedal and no brakes at all. Panic erupted for a split second, but then I saw an exit a mile away. I signaled, and began to move over using my handbrake and low gear.

When I finally exited, there was a Shell station and I pulled into it. Popped the hood, and brake fluid was exiting the top of the brake reservoir in a stream through the vent hole in the reservoir cap. I cleaned the fluid off the paint and removed the reservoir cap.

I dallied about for about 15 minutes (filled the car with fuel) and then refilled the reservoir with the Castrol I had purchased earlier. My brake pedal had returned and all braking functions seemed to be normal at that point.

I had no further problems the rest of the trip, aside from the Texas LEO that pulled me over outside Sweetwater and asked if I knew I didn't have any license plates on the car.

stevesp98

There was nothing leaking from the reservoir in my case and anywhere else that I could find.

So do you think they just overfilled it? Did you end up doing anything when you got back to TX?

soontobered84

No, it wasn't overfilled. The old brake fluid in the lines had absorbed enough moisture that the stop and go driving made it boil, and the boiling fluid in the lines pushed the fluid back into the reservoir and out the vent hole (think geyser)

I flushed the brake lines and reservoir with new brake fluid. I never had any more issues.

FredR

When the brake fluid boils the system goes from perfect to useless in the blik of an eye. Happened to me at Dubai Autodrome, was braking very late as in at the turn in point and trail braking to go past a 911 and then squat! Not much fun doing just under a ton- spun the car through 360 and managed a recovery. Seems I had skills I did not know even existed.

Now you need to find what caused the boil up - could easily be old fluid having absorbed too much moisture. In my case stock fluid with my semi track pads and lots of rubber to use it on proved too much compounded by heat some heat soak I could not avoid.

stevesp98

Documentation on the car says the brake fluid was last replaced in Feb 2015 so it's old by Porsche OEM standards but not way out of bounds. The car has lived it's entire life in Southern California and now spends most of its time in my very dry garage in San Diego. So it's difficult for me to see ambient moisture being the source, if it is a moisture contamination issue.

Imo000

Pull the calipers and look for the stuck piston.

dr bob

Drive the car a little, and use your handy IR temperature gun to compare rotor and calipers temps immediately on stopping. If you have pads dragging, you'll have a hot rotor/caliper.

Boiling fluid almost always includes ejecting some fluid from the reservoir cap/vent. Any evidence of such leaking? Might be a "stain" on the MC or the paint on the booster and other metal underneath.

stevesp98

There definitely wasn't any fluid leaking when I stopped the car and went under the hood. Will check again for stains but it all looked pretty clean and dry to me.

Adamant1971

Any strange odor from the brakes when you stopped? If they were dragging you should have smelled the burning pad or pads. If there were no odd smells or overly hot wheels my guess would still be the MC in this case. Even a large air bubble in the MC could have done this. Right after stopping did you try to pull the pedal back up or did you not touch it again until 30 min later?

If you did fiddle with the pedal immediately after stopping and if it still stayed down but then came back up in 30 min, that points to the fluid as others have said.

Personally I would not drive it period, instead jack it up and spin the wheels to see if they rotate freely. If something is dragging you will hear/feel it.