930: Master cylinder oddities

[Craig 930 RS]

Soft brakes no matter what bleeding etc - BTDT on all the techniques.

Question: If a 930 MC is going south, do they always leak externally? Or is the softness due to an internal problem that does not present itself in the form of a leak? Mine does not leak...

[sww914]

They don't usually leak externally during failure, the O rings around the piston leak and the fluid flows past them back towards the reservoir a little.
If you just put new pads and rotors the pedal can often feel a little softer until everything is seated properly, from my experience.

[JP911]

I know you said BTDT on techniques, but did you try tapping the calipers with a rubber mallet while bleeding? We had the same issue with my buddy's SC (after a track day) and he was getting really frustrated. He was ready to replace the MC. We used the rubber mallet method and LOADS of bubbles released. After 2 rounds of bleeding we had things better than ever.

[Bill Verburg]

b/f will leak into the vacuum can.

[Craig 930 RS]

It's been driving me nuts for a year.
I tapped, bled 6 ways to Sunday - did it all.

Pedal is always there on track, but it is just too soft.
Oozes closer to the floor if I give it med/high pressure at a standstill

[sww914]

If it creeps down, you need a new master cyl.
I rebuilt mine, but I've never been happy with it.

[Craig 930 RS]

What is odd - it doesn't mush to the floor - it just goes down via what feels like compressive use - and feels the identical way the very next brake application. Fluid level never decreases.

I'm fully aware that a 930 MC with big reds tends to feel a bit softer - but this is way softer than other BR/930 MC 930s I have driven.

I actually like the progressiveness of this vs. the on/off rock hard pedal - but more feel would be nice.

Bill?

[Bill Verburg]

You get better feel from a bigger m/c. Unfortunately a dual master is the only reasonable way to do that on you car.

If you have access to brake dynomometer data for the pads you use, look for an early spike followed by a gradual decrease in brake torque, that also makes the brakes feel better.

[Craig 930 RS]

But what might account for the difference in feel from my BR brakes + 930 MC vs. others with the same setup?

Only difference is the reservoir shares fluid witht the clutch for the G50/50 transmission. Ideas?

[sww914]

I would assume that you have braided stainless lines and the others do as well, if not, that could account for the difference in feel. ither than that, I'm about out of ideas.

[Bill Verburg]

But what might account for the difference in feel from my BR brakes + 930 MC vs. others with the same setup?

Any flex anywhere in the system can be interpreted as a soft pedal, if the seals aren't leaking maybe they are kust worn or not seated or even stiff so that they don't act like a new set., flex in the firewall, strut, ball joints, play at the pedal pivots could be anything

[ianc]

I have rebuilt my entire braking system and bled the crap out of it, but my pedal still seems soft to me as well. Quite a bit softer than my SC.

Dunno why.

Someone postulated that if, when you are installing the master cylinder, you get air in the lines there, bleeding will not get it out since it would have to travel downhill to get to the calipers to be bled.

Interesting theory...

ianc

[sus911]

quote:
Someone postulated that if, when you are installing the master cylinder, you get air in the lines there, bleeding will not get it out since it would have to travel downhill to get to the calipers to be bled.

Interesting theory...

ianc ianc I have rebuilt my entire braking system and bled the crap out of it, but my pedal still seems soft to me as well. Quite a bit softer than my SC.

Wouldn't pressure bleeding take care of this?

[ianc]

Hey Sus,

I have bled using a pressure bleeder and the good old two-person pump 'n hold method. I see no air coming out of the lines. I have walloped the calipers with a rubber mallet during the bleeding operation. I have 'exercised' the caliper pistons in and out numerous times with a set of brake pads with the lining material removed, but pedal still soft.

The brake system has new: master cylinder, flex lines, pads, rotors, and all calipers have been rebuilt. I have about 4K mi. on this work with a small improvement over time, but still pretty soft.

Sorry Craig and don't mean to hijack, but I'm sure you're experiencing similar problems.

ianc

[Craig 930 RS]

I sure am. Driving me bananas........!

[don911]

Craig,
I know changing the MC is a PITA but if you want to test, I have a good 23mm MC that I removed about a month ago. MC is about 3 yrs old. I thought I had a bad MC but found out, after replacing the MC, that I had a leaky fitting on the RR caliper.

[sus911]

Craig and Ian.
I have rebuilt my brakes recently and have a similar issue.
I had my mate who is head wrench at Benz service shop help me bleed the brakes. First we used vacuum with an a/c- pedal but got air. One pump to full pedal.
Next day we used a professional pressure bleeder and pumped about 3 bottles of brake fluid thru that sucker and still had the same pedal but a little better.
He advised me put put pressure on the brake pedal with a chock against the seat and leave it like that for a few days.
Lo and behold- a big improvement! However, I still to have just a little bit of pedal missing on first pump but it appears to be improving over time. We did a manual bleed with no improvement.
Now I do remember seeing a post, very recently about how to remedy this and I believe it has to do with the settling of the piston seals in the calipers, not air. I wish I book-marked it because it had a fix to try.
I'll go hunting for it and post if I find it.

[Randy Blaylock]

You're simply stroking more fluid volume than with the original 930 calipers, as has been mentioned. Your front calipers have 44/36mm pistons and the rears are 1986 951 front calipers custom machined to fit with 38/36mm pistons, which are larger than any O.E. type configuration, even the RS cars, and particularly vs. the 930 calipers at 38/38mm and 30/30mm. I set it up intentionally with larger than normal rear piston sizes, and a PRV in that circuit because I liked being able to tune the bias for track conditions and particularly to be able rotate the car on trailing brake. Just a personal preference thing.

It's simply a master cylinder bore volume/combined piston bore volume relationship issue, and shifting the ratio more to the master cylinder is the only way to get a rock hard pedal feel. Or, actually, if you get in the habit of lightly stabbing the pedal with your left foot as you're approaching braking zones, which is a good technique anyway to ensure the brakes are optimal, in effect you'll "pump up" the pedal.

As Bill mentioned, a double M/C is the only way to do it on your car. The only other thing you could do is install a rear caliper with smaller pistons, and get used to a understeering car on trailing brake.

Or, just make sure they're properly bled and enjoy the superior feel.

[Craig 930 RS]

As an important reference point, a friend has the identical calipers F&R + the 930 MC and the feel is much different. Actually the pedal is too hard. Nearly impossible to modulate. Wasn't much fun locking up the brakes into turn 8, and going off track in someone else's car. Thus my post.

So I'd like something in between.
It seems from the different experiences listed on this post that at times things are 'just that way' for a hard / impossible to define reason.

I like the subheading under your name & the location.

[Bill Verburg]

Actually the pedal is too hard. Nearly impossible to modulate. Wasn't much fun locking up the brakes into turn 8, and going off track in someone else's car. Thus my post.

You have some other issue. As a matter of ergonomics the harder the pedal the easier it is to modulate. A human leg foot can control applied pressure at a fixed pount far better than at varying points.

The issue w/ bigger m/c which provide the high hard pedal is generating line pressure, hence they are always boosted.

The ratio of slave to master(all other things being equal) tells you what kind of pedal you will have.

using the #s that Randy provided, which are significant
your slave:master ratio is ~21 which is at the soft end of acceptable about as unsporting as can be used in a fully functioning system.

Stock 930 is ~16.5
993RS or Cup is ~16.8
964RS is is ~16.4

You have 2 problems,
1) the slave/master is too small giving you a pedal that is hard to modulate
2) you have way too much rear brake, I would put an adjustable p/v on the back

*dual master would solve both problems