Need brake help

[JP911]

I'm stumped here:

On my drive home after a recent track day at Laguna Seca I started to notice that my brake pedal was descending slowly to the floor when stopped at a light. The system had fresh fluid for the track, and the brakes worked great all day. As this was my first time at Laguna I didn't push it too hard and didn't work my brakes nearly as hard as I would on Thunderhill or Infineon.

I got the car up on jack stands and bled the system. I didn't find any leaks, but the pedal didn't get any better. At this point I'm thinking that the master cylinder may be the culprit, but there's no fluid leaking into the pedal box (which I thought was common of a failing M/C).

The M/C is less than a year old. It's the 23mm kit that Smart Racing sells. The brakes themselves are 930 on all four corners with new stainless braided lines. The pedal has always been an absolute rock, but now after a bleed it feels firm at first, but then slowly descends to the floor.

Any thoughts on what is happening here? My instinct is M/C, but why? It's practically new and has has only been used for light street driving and about 6 track days.

Thanks for any and all help!

[JP911]

bump.

[rusnak]

It sounds like the master cylinder is faulty.

[Steve@Rennsport]

Jon:

Its likely the MC, pal.

Put your foot on the pedal and push hard,......hold it for 2-3 minutes. It should NOT sink slowly to the floor.

Now, if it does, you are probably experiencing something we call "shuttling" where the piston seal that separate the front & rear chambers has failed and fluid is being pushed past it from the front chamber to the rear one. Sometimes, its caused by "pump & hold" bleeding.

Time for a new 23mm MC. SR has them, as do we.

[JP911]

Good info! I'm bummed that the MC is dead in less than a year, but it's good to know that my experience is not a "one of a kind". Steve, do you recommend against the "pump and hold" method? Is there a better way to operate the pedal or is a power bleeder the only way to go?

[911pcars]

I agree with Steve in that the MC seal is probably the culprit. However, seeing that the MC is only a year old with a cylinder bore in probably pristine condition, I'd personally try to find a $.05 replacement cup, if possible, for the one that's leaking. I realize there's a liability issue whenever a repair shop works on a customer's brakes, thus some reluctance to rebuilt versus replace. If it's only a year old, there might still be some life left on the SR warranty. Ask.

If you trust your mechanical skills and are scrupulously clean in the replacement, I'd go for it. The worst case is you crash against the garage wall when you try to stop from .5 mph, but you should be able to spot any impending doom before that point just as you have with your current MC.

BTW, in regards to bleeding, while many use a pressurized tank, I go the old fashion way by gravity bleeding. Do this under flat rate and you lose your shirt, but working in the privacy of your workspace, it's almost cathartic. Besides, a minimum amount of fluid is used and MC seals stay intact.

Sherwood

[rusnak]

I'm by no means a brake guru here, but I THINK the problem with bleed and hold is when the brake valve is allowed to stay open too long, and the pedal causes the master cyl to bottom out. Possible damage as a result?

Otherwise, bleed and hold is always a candidate for those times when all else fails, and you have a pesky bubble somewhere, like in the proportioning valve that doesn't open until you are near panic braking type of pedal pressure.

I use the motiv bleeder, which works about 90% of the time, but sometimes I just get a huge bottle that I keep in the back of the garage with a rubber hose on it. One end of the hose goes on the bleed screw, and the other in old brake fluid. Then I can gravity bleed the brakes with the bleed screw open, and slowly pump the pedal, making sure not to bottom it out.

If you try to blame it on the manufacturer, they may claim installation error (did you bench bleed the mc?). You can also get an Ate 23 mm MC, which is actually a Mercedes Benz part. It just bolts right in, but I think you have to plug one of the ports with a small bolt and copper washer. I installed one in my 914-6 about six years ago.

[911pcars]

At less than 12 mos. old, the cylinder should be pretty clean and with no deposits that would normally rip the cup.

Ask the retailer to disassemble and inspect. What can they say besides no?

Sherwood

[JP911]

I appreciate all of the good advice. The MC is indeed the Mercedes part with one of the ports plugged (this is what SR sells in the kit). For a Porsche part, it's actually fairly reasonable. I'll call SR on Monday AM and see what they think.

[rusnak]

good luck, JP, I hope it's just a blown o-ring