Bad Master Cylinder and Booster?

[djmcmath]

Before I spend $$$ replacing parts, I'd like to have a warm-fuzzy feeling that I'm doing the right thing. I just finished replacing rubber brake lines and rebuilding calipers as part of the "while I'm in there" projects for my suspension rework. Now I have a soft brake pedal.


Recent action taken:
Bled brakes - ~2L of ATE and Motive Pressure Bleeder at ~10-15psi. I did not tap the calipers with a wooden dowel, though I'm hesitant to dump another 2L of fluid into the system if I have to replace the MC and Booster anyway.


Symptoms:
1 - Clearish fluid in boot, around smugglers box. Doesn't appear to be at all fresh, though fluid appears to peel paint. Level in reservoir is not lowering; cannot confirm that fluid is not from a spill in the process of bleeding, or even a spill from the last time I refilled windshield washer fluid.
2 - There is visible rust on the booster; that would concur with the idea of spilled fluid, as the rust doesn't appear to have any real source, but is merely general in location.
3 - There is no fluid in the driver's footwell, that I can find. Bright lights and reaching around with fingers has yielded nothing.
4 - Testing the booster, per the Bentley's: 10 pumps while engine off, then starting engine yields a very slight give. It's not imperceptible, but it's definitely a small enough give to make me wonder.
5 - Testing the M/C per the Bentley's: 10 pumps and hold yields constant pressure. The pedal doesn't sag, reduce pressure, etc. on holding.
6 - When driving, pumping 2-3 times lightly on the pedal before actually braking seems to help marginally. Pumping more than 3 times yields no additional benefit. The brakes are still more than enough to lock up the tires, even on the first pump, but just seem mushy.

So the question: Would poorly bled brakes look like this? Am I liable to find success by doing the post-caliper-rebuild adjustment of seals and a serious bleed, or do I need to replace MC and/or booster? What further testing can I do to confirm that the MC is good or bad?

Thanks in advance,

Dan

[djmcmath]

Oh yeah -- the car is an '86 911. Everything in question is stock, to the best of my knowledge.


Dan

[djmcmath]

Bump -- am I deluding myself in hoping that I could just bleed the brakes? Am I living in denial of a bad master cylinder?

TIA,

Dan

[Bill Verburg]

The boosted m/c's tend to leak into the vacuum canister and from there all over the trunk. Check to see if there is fluid in the vac. canister

[djmcmath]

Great, thanks! ... just one question -- which one's the vacuum canister?

Thanks a heap!

Dan

[djmcmath]

Is the vacuum canister the same as the vacuum booster? After digging around in my boot/trunk for a while, and hunting through the manual and this board, that's my best guess.

If so, the vacuum canister looks like dog-puckey. It looks like something corrosive got spilled all down the front of it, like the reservoir overflowed once, real good. Paint flecks, rust, lots of general nastines -- having inspected the whole area thoroughly, my new plan is to just pull the whole assembly for inspection. If it's bad, so be it -- I'll replace what I have to. If it's still ok, then I understand my brakes a bit better, at the cost of spending another $20 on brake fluid to rebleed everything.

Thanks for all the help,

Dan

[Jim Smolka]

Silly question, are the bleeders on the top of the caliper or on the bottom? They should be on the top. (Don't ask how I learned about this years ago).

Also, there are bleeder check valves for the calipers available that only allow fluid to go out of the caliper.

[Bill Verburg]

Is the vacuum canister the same as the vacuum booster?

Yes

[djmcmath]

Jim -- the bleeders are, in fact, at the top. I do recall reading the thread where someone (I'm sure you wouldn't be that foolish, it must have been someone else) installed calipers on the wrong sides. Good stuff.

Bill -- thanks for the confirmation. Yeah, the booster looks like crud. I can't tell if it got spilled on or if it's leaking from inside, or maybe both. Either way, I'm just gonna pull the whole assembly for inspection.


Dan

[JeremyD]

If it's all cruddy - then it is probably leaking in there.

[djmcmath]

Yeah, that's kind of my assumption, at this point. I'm already thinking ahead to replacement parts -- I could maybe just replace the seals, but that's considered a poor choice by many. Most say that it's wise to just replace the whole MC (and booster, assuming it's also bad). Then I said "Why spend $170 on a stock MC when I could get a shiny new 930 23.81mm MC from VCI for only $185?"

It's a slippery slope. I hope to pull everything tomorrow (Vet's Day) and order parts. Any "gotchas" I need to be aware of in upgrading to a 930 MC? Aside from the stuff mentioned in Wayne's book and the Bentleys, any big problems I'm going to hit?

Thanks again, all,

Dan

[JeremyD]

Why going to the turbo MC? You won't need it unless you upgraded you calipers to 4 piston 944t/964 or 993 or Turbo calipers. Otherwise the stock carrera MC will work best.

- Bench bleed the replacement MC - it's messy - but worth while

- Get some cheap dot 3 brake fluid - lots of it. It took me a gallon to bleed my brakes after replacing lines, new 993 calipers, new MC, etc.

- Don't forget the bolt in the foot well and the one on the outside side of the MC -

[djmcmath]

I was looking at the 930 MC 'coz it's supposed to be a worthwhile upgrade -- better pedal feel, less movement from the pedal while braking, that sort of thing. If I have to replace the thing anyway, it makes sense to spend the little extra to get a better part, no? Further, if I decide, at some time in the future, that I want bigger calipers, I already have the MC ready, no? The ultimate plan for this car is to gradually migrate it from the mostly stock status to something that's capable on the track (my budget won't allow for a true track car), so it seems sensible. Am I wrong?

Roger the tips; cheap DOT 3 fluid would have made a lot more sense for the first phase of this project, too. (sigh) Oh, well.

BTW -- if I'm replacing the MC 'coz it's leaking into the booster, should I also replace the booster, or is the booster something I can clean up, replace some rubber bits, repaint, and restore to service?


Thanks for all the help,

Dan

[JeremyD]

I'll defer to Bill Verburg about this - he is tha master when it comes to all things brakes - but I was under the impression that you really need a "matched" system when it comes to brakes. Bigger MC when you have larger piston area (less brake pedal travel) - I would assume the other way would be true too - less travel, higher brake pedal with a Turbo MC with stock brakes. I would think it would be difficult to modulate.

Even with a turbo MC - with 993 brakes front and rear my car has much less brake pedal travel - up high and hard as a rock - can't imagine with stock calipers -

[djmcmath]

Yeah, I've read a lot of Bill's posts on the topic of brakes -- the stuff he knows off the top of his head is amazing. In fact, before I started this thread, I did a search for his name, and just read his threads.

Maybe I'm wrong, but I was under the impression that brake pedal travel was (generally) a bad thing. In a perfect world, the pedal wouldn't move at all, and you could modulate the brakes by pressure, not position. That simplifies heel-and-toe operations and reduces one variable out of the modulation control equation.

But again, it's entirely possible that I'm completely wrong, as I'm just a guy who reads too much. You say the 930 MC is plenty big, even with bigger pistons? I'd be curious to see if other people have any data -- is there anyone else out there who's done a 930 MC with stock calipers?


Thanks again,

Dan

[Bill Verburg]

You can use the 930 m/c w/ stock brakes but I wouldn't.

Yes , travel would be way down, but effort, even w/ the boost, would be way up.

Best to stay stock unless some big piston calipers are in sight.

[djmcmath]

Thanks for the input, Bill -- that definitely seals the deal, then. Stock replacement, it is!

The other question -- assuming the MC is leaking into the booster, and from there out to the rest of the trunk, do I need to replace the booster, or can I clean it up and re-install it?

Thanks again,

Dan

[Bill Verburg]

Depends, brake fluid and moisture are a nasty combination. Inspect and evaluate is all I can say.

[djmcmath]

Thanks, Bill. I'll pull it out and inspect. //Disappointed, wishing there was a black and white answer so I could order the stinking parts already, goshdernit...// I appreciate your expertise.

Dan