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I'm experiencing something I've never experienced on any other car. My '87 928s4 is apllying its brakes by itself! I will be at a stop light and all of a sudden car will barely move and my brake pedal is rock hard! When checking which calipers are locking, it seems all are because of the heat generated at each rotor, but of course the front rotors are the hottest. This happened to me about 8 months ago and I assumed it was a foreign matter in the brake fluid so I flushed and put in new ATE Super Blue. Now its happening on a regular basis over the last 2 days.
Is this a master cylinder problem? I've heard of brake booster problems and the brakes seem to get tighter as I idle (leading me to think it has something to do with vacuum) Did I see where there is some sort of brake booster check valve somewhere??? Could this be problem?
Any insights would be greatly appreciated because I am in an are with no Porsche mechanics and like to fix my car without them screwing it up more and charging the big bucks!!!
If you think you know what this problem is, if you could lead me to detailed instructions on how to repair, I would be forever indebted.
TIA,
mark
'87 928 s4 Gran Prix Weiss 77,000 mi ( for sale )
'98 BMW 740i Schwarz 90,000
'87 BMW 535iS Alpine weiss 260,000 mi
I had the same problem recently. It was INDEED the master cylinder. EASY fix.
Get one from your favorite of the Big 3 (should be around $175-80) and count on about an hours worth of your time.
Thanx for the QUICK response!! Wow, that was fast!!
Before i start to replace MC, is this all I need to replace? What about booster? Like I referenced in message, this occurs when idling. Maybe it is just the MC based on you experiencing the same problem, just don't want to tear into and have to go back later!!
hope it's not the booster...bigger $ and MUCH more labor!
It locks at idle, because the calipers aren't releasing fully and the friction causes everything to heat up. Heat = expansion. Sitting @ idle there's no cooling and the expansion gets going.
Thanx so much for your responses. Looks like I'll order the MC and replace to get me back on the road.
Real quick, any tricks / tips on replacing MC?? While I know some things can be straightforward, sometimes when people have gone thru the process, they now know what they would have done the "first" time had they been told what they would run into. Just want to do this quick and right and not have to do again!!!
Don Carter,
Thx for reviving this thread.
Same thing happened to Stretch's "Black Widow" the other day. Luckily he's about a mile from Craig Johnson's shop in Katy, TX.
MC should be a straight forward job. I'm hoping that Roger didn't tell me the other day that 86.5 and up MCs were NLA or on Backorder. For some reason, that's in my head...
Yes, there's not a whole bunch of late master cylinders available, right now.
The common distributors are out.
Porsche is out. (Porsche has recently superceeded the part number, BTW.)
Occasionally, you can find one on some unlikely shelf.
Chuck, glad to help. Hope Stretch can find a replacement. It seems like they should be easily rebuildable but I think there are some check values inside that are not easy to restore. Does he really need power brakes :-)
When this happened to me (on my '88) it was the calipers. Early S4's have no dust boot on the caliper pistons and they can get pitted and start sticking. I don't recall how my pedal felt though. The breaks did not lock up by themselves but when you applied them they would not un-apply. Also, it was just the one wheel, so that might be a useful diagnostic tool: figure out which wheels are locking.
Thx for the feedback y'all!
Wonder if a good used one from 928 International could get him by for a couple months?
Craig will not rebuild due to liability, he said they normally get brand new ones to install..
Interesting thread revival. My 928 was experiencing this same issue and as everyone noted it was the MC. I'd love to see a cross section cutaway that must allow fluid to travel in one direction, but not return to the reservoir and pressure to build. I've never had another car with this sort of MC failure mode, but seems to be common on the 928.
The job itself is pretty straight forward. One big question will be whether or not to replace the blue hose going to the clutch MC. Most guys will say replace it, but honestly, I think that hose is really hard to place onto the clutch MC unless you remove the MC from the car. If the hose feels pliable and not cracking, I'd be inclined to leave it. Its just gravity fed and not under pressure. I was replacing the cultch MC too, so it was easy enough to do out of the car.
Some guys have had problems accessing the 13mm nut on the outboard side of the brake MC but I was able to I was able to get to it using a regular 13mm box end wrench - you only get about 1/8 of a turn, so it takes a few minutes, but overall not too bad.
@Chuck Schreiber : If you get really hard up I have a MC that I removed from my car that (I believe) I didn't really need to. I wasn't paying attention very well after I put on some new brake lines and the pedal was going to the floor. I couldn't see anything really wrong and, after reading and asking for help here, it was suggested that I replace the MC, which I did. After I did that, I discovered that one of the brake bleeder screws was not in all the way and I had a little puddle of fluid in one of the wheels. Doh!
I kept the old MC, though, and would be happy to send it to a new home. I think it's still OK.
06-25-2004, 03:45 PM
Brakes lock up on there own!
