Brake problem auto brakes

[Eric B]

Brake mystery. So, my first attempt at posting is a request for help. My 1999 boxster has decided it knows more about when to use the brakes than I do. Here’s what happened. I had been out cruising for about half an hour when my car seemed to be slowing down even though I had not let up on the gas peddle. In fact , I gave it more gas and it still decelerated. I down shifted, more gas and still we slowed. I pulled onto the shoulder and came to a dead stop without using the brakes. Sitting there I pressed on the brake peddle and it was solid as a rock. Put the car into 1st gear and was unable to budge the car. Did I mention the smoke coming from the rear calipers? Only the rear brakes where hot. The front rotors where cool. I took out my trusty 11mm wrench , reached between the rear wheel spokes and opened a caliper bleeder. A small squirt of break fluid ejected all over the inside of my nice clean wheels. I hopped backing the car and drove 20 miles home thru traffic. Brakes worked fine. I thought that possibly moisture in the brake fluid caused the problem. I have completely changed the fluid with new ATE super blue DOT 4. Also new rear pads. Rotors look fine. Have taken the car out for several drives since and no problems until yesterday. My Boxster went into “auto break” mode again. Where is the pressure in the brake hydraulic system coming from? I have standard ABS brakes, no TC or PSM. Does the hydraulic unit/ control module create “pressure” or just “pulse” the pressure created by stepping on the brakes via the master cylinder? The ABS warning light has not come on. I need help to solve this mystery.

[randy_k]

Well, Let me tell ya a story. I bought a 1999 Boxster about two months ago. The first colder than normal day we had. My brakes exhibited the same problem you have. So, kinda like you. I took it to my mechanic and had the brake fluid flushed and replaced thinking there was moisture in the lines that was freezing. Picked it up and all seemed good. Until the next day of below 20f temps. Then it recurred. After doing some research. It turns out that the brake boosters that where used from 1997-2000 or there abouts. Allowed moisture to get into them from the bottom mounting hole and turn to rust inside and the moisture just lays in there. So when It gets below freezing. The moisture freezes and the booster will not release.
I checked this as the next time it did this. I took a heat gun to the booster for about 15 minutes off and on and the problem disappeared. That I'm not sure of the temps where you are at. This may or not be your problem as well. There is a write up on this on the Renntech forum. Go to the 986 section and the the forum titled "Suspension, brakes and wheels. There is a guy overseas that had the same problem and did a DIY on it.
Hope that helps, Randy

[Eric B]

So hopeful this was an answer but alas, temp has been above freezing and car is stored in a heated garage. I have ordered a new master cylinder (can't hurt) and will carefully inspect booster when I change out the master cylender. Thanks for your info.

[randy_k]

No porblem. Was hoping it would solve your problem as well. Maybe the next guy will find it helpful though. Let me know what you find out. Good luck, Randy

[mattatk]

Happened to my 01 Boxster S a few times. It had PSM & I thought it was the PSM locking up the brakes for me! I had done some work on the clutch hydraulics previously(bleeding & clutch master cylinder replacement)...don't know if that caused any problems.
I also had a leaky brake fluid reservoir, so I replaced it, checking out the booster in the process ( torch & mirror to look inside & it looked brand new, no sign of water ingress ). Then I did a complete fluid flush & bleed. The old fluid definitely had water in it....anyway, appears to have fixed it real good....7000kms later running beautifully.
Various people suggested the booster to me, but it looked OK, others said that I had water in the lines. When the brake fluid heated up, the water expanded (or boiled?) & created enough pressure to lock on the brakes. I'm not saying that this was the cause of my problem, but it was fixed with a flush & bleed (maybe the reservoir change helped too!?).
Cheers

[Wayne 962]

Interesting problem. My gut instinct is that it is not the booster, as I would think that the booster problem would equally affect all four brake calipers uniformly. On Eric's car, there is definitely something odd, but only with the rear brakes. I also think it's odd, because I believe that the Boxster's braking system is like most other cars, where there is a disconnect between the two in the rear (they should not be controlled by the same chamber in the master cylinder, I believe). I'm not sure of this though.

I realize that you cracked open the brake hydraulic line and pressure came out, but the rear brakes also have the parking brake mechanism, so some thought should be given as to whether there is an issue with the cables being pulled or damaged, or something like that. I doubt it, but it's worth a thought regardless.

The issue that you saw when you cracked open your brake line (fluid shooting out), most likely means that there is some type of blockage in the line. Older cars, like the 356s would have this problem when the rubber inside the flexible brake lines corroded. The lines would get very thin (like a clogged artery), and would expand when the brake pedal was stepped on. Then, when let go, the pressure would drop, the line would contract, and in some cases, it would completely seal up, holding the pressure in the caliper. This was a symptom of failed flexible rubber brake lines, and is very similar to what you have seen. The Boxster really isn't that old to have this problem though, so it seems a bit odd to be caused by this. Still, I would probably change out my flexible hoses anyway.

For future reference, here's the link to my brake line article: Pelican Technical Article: Boxster Brake Line Replacement - 986 / 987

Also, here's the link to my article on the Master Cylinder replacement: Pelican Technical Article: Boxster Master Cylinder Replacement - 986 / 987

And finally, the one on bleeding your brakes: Pelican Technical Article: Bleeding Boxster Brakes - 986 / 987

Hope this helps,

Wayne

[Eric B]

According to the diagram in Bentley Service Manual my Boxster has an ABS 5.3 (3-channel system). I believe the rear brakes share a common hydraulic line. I bled one side and the pressure was immediately removed from all rear wheel calipers.

Th mechanical parking brake is working fine. The flexible hoses are in great shape. I agree that it's not the booster.

A "Pelican" will deliver a new master cylinder Monday. After that the only possibility remaining seem to be either the proportioning valve or the ABS hydraulic unit.

There is going to be an increase in "super-blue" sales.

Thanks for all your excellent advice.

Eric B

[green03]

Eric, I experienced the exact same brake problem you describe today in my '03 Boxster. Rear brakes slowly applied themselves as I drove along. Stopped (had no choice) and let cool for 30 minutes. They freed themselves but after a couple miles it happened again. Did you solve your problem? Thanks

[3stewartS]

Reviving an old thread because I have a similar problem but with the front brakes

Front brake lock up

Any help would be appreciated