M/C - who has had a master cyl. fail(ing), and what was it like?

[Craig 930 RS]

M/C failing - what was it like for you?

[velo5338]

Mine ('87 930) failed with a whimper. I removed the front calipers when renewing the front wheel bearings. Put it all back together and could not pump up any pressure when trying to bleed the brakes. Tired everything I could think of including bench bleeding - but to no avail. A new master cylinder from our host ($272) solved the problem. I guess at some point the piston was moved outside its normal travel and caused a problem with the seal.

As an aside, I have a very low brake pedal on my '86 Carrera, pressure's fine but low pedal. Could be yet another master cylinder is in my future?

[concentric]

Spongey pedal that wouldn't pump up and the brake fluid draining from the tank out of the broken M/C and onto the gravel of my alley.

[juicersr]

Pedal would work with pumping, but would eventually go to the floor. Brake fluid eventually started leaking onto my shoes in the footwell.

[JP911]

My pedal would hold pressure for about a second and would then slowly sink to the floor. No detectable fluid leaks, but disassembly revealed that the seals weren't holding pressure in the chambers. The odd thing is that the MC was the Mercedes 23mm unit and less than a year old.

[p911dad]

I have had 2 fail over the years, one like JP911 recalled, and one in an old Triumph that required a quick pump up to work(the last pump up was a little late and resulted in the front of the TR3A getting stuck on the back of a classic old Packard. The Packard needed a new tail-lite, Triumph needed a lot more)

[911pcars]

Early in its life, something wore through the metal brake line running through the tunnel. The BF was always low. Finally discovered BF was pooling inside the tunnel and underneath the chassis. Fortunately, the car was parked when the pedal finally hit the floor.

Replacing the brake line means the drivetrain comes out.

PS: You'll find that most MC failure cases immediately follow manual brake bleeding.

Sherwood

[Christien]

If there's any leakage, you'll be able to smell it. As has been said above, dripping into the footwell and onto the garage floor. Although when mine was leaking there was no appreciable brake pedal pressure loss. Of course, I didn't put that to the test, and got it replaced pretty quickly.

[rentadate]

happened in my BMW 5 series. kinda freaky when you need to brake for a light at 70MPH. kinda spongy feel. Long line of oil drips down the road. It was going out, I had to pump it to get it to work before it completely died.

[Craig 930 RS]

Is there a possibility of "internal leakage" -- ie fluid flowback within?

[911pcars]

Originally posted by Craig 930 RS
Is there a possibility of "internal leakage" -- ie fluid flowback within?

Yes. That's what happens when an internal seal leaks. The result is similar to a squirt bottle that no longer squirts - bad pump seal.

Sherwood

[Craig 930 RS]

THX Sherwood -

Basically my setup and issue is this:

Big reds up front 944T fronts in the rear.
930 MC, quite possibly the original MC from 1982.

This setup flows quite a bit of fluid, yet I drove a 930 with an absolutely identical setup, and the pedal was much, much firmer.
Not rock hard, but a big difference -

The pedal is a bit unpredictable on track and compresses easily, never feeling firm, even when pressed fairly hard while at a stop.
No external leaks or fluid loss.

[nineoneone]

Does anyone sell the rebuild kits for the m\c? I would like to rebuild mine as well as the slave for the clutch.

[JP911]

Not sure if rebuild kits are available, but I've taken one apart (after it failed) and getting it back together would require an act of God (or special tools).

[911pcars]

These are just ideas, not definite answers.

Air in lines.
You've probably bled the system already, either manually or with a pressure bleeder. After these methods, I'd suggest gravity bleeding. Attach tube to bleeder screw, open screw and allow gravity to do it's thing. Swing tube upward and the system expands past the screw. Any air in the system will rise to the top of the tube. You can exercise the brake pedal (with bleeder open) to move small amounts of fluid and air out into the hose. If you look closely, you can see air bubbles in the hose. Try this on each wheel.



Flexible flex lines.
Brake lines can flex internally. The result is pedal movement w/o a commensurate movement of caliper pistons. If the flex lines are old, this could be the source.

Thin sheet metal.
If the brake pedal chassis mount isn't sufficiently rigid, the sheet metal will distort when depressing the pedal and result in less fluid movement. However, this is a stretch in most 911s due to the mounting geometry of the brake pedal with brake booster assy.

And there's always a good chance a seal is torn inside the MC, especially if the pedal went to the floor when manually bled.

Hope this helps somewhat.

Sherwood

[911pcars]

Originally posted by JP911
Not sure if rebuild kits are available, but I've taken one apart (after it failed) and getting it back together would require an act of God (or special tools).

MC, brake booster or ? I've rebuilt several MC's. It's not difficult. Cleanliness is important. In addition, keep petroleum products away from the repair area.

Sherwood

[Craig 930 RS]

I've beld by every method possible the past 2 years - no difference.
Metal is firm, stainless jacketed lines in great shape. I'll recheck the rears.

Thanks for the help and suggestions - any more?

[rusnak]

I wonder if air is trapped behind a brake proportioning valve?

I think since the pedel "never feels firm" it is probably air in the lines somewhere, or perhaps too much clearance between the pads and the rotors.

I had a nightmare brake bleeding about 6 years ago in the 914-6 after changing the MC to a 21mm. I finally got the last of the air out by standing on the pedal, and having someone open the bleed screws one at a time to shoot the air out past the brake proportioning valve and out the caliper.

[lateapex911]

Craig, I have a similar issue. I have, however, installed a new MC. But I've bled until I've flushed the system, and have also done the gravity bleed trick.

At no point have I considered my brakes really as good as they should be. I've come to the conclusion that a better pad is probably in order. Then on Monday at the Watkins Glen, the pedals initial "bite" changed on one lap to a much lower position. ANd I felt the braking was softer...less retardation, and the pedal was less firm. Honestly, it seems to change...mostly low, but occasionally higher.

Pumping doesn't seem to help. Of course I bled them. No air. Grrrrrrr.

Then I noticed the FL wheel bearing was a tad loose. Ah HA! I tightened it, and .....no better. I would examine your bearing s and rotors...a littel warpage can lead to the rads being slapped back a bit, which can lenthen the pedal

I'm kinda stumped on my issue.

[Craig 930 RS]

Wow, that is weird Jake.
What pads are you using - stock brakes.

I know pads pretty well ;-)
www.Rennstore.com