930 master cylinder in SC? - need help now

jakermc · 12-08-2003, 02:43 PM

To make a long story short, my '83 has a brake pedal that feels like mush. The brake lines are fine, and the master cylinder was replaced recently as a first attempt to solve the problem. The car is at the wrench now and there are two things we want to try.

The first is a quick adjustment to the brake pedal itself. I had it lowered a while back thinking it would help heal/toe efforts and now think I may be running out of pedal. I am using a 'winged' accelerator now so the height is less of a factor anyway.

The second thought is that the new master cylinder may have been bad. If I am replacing the master cylinder again, would it make any sense to put in a 930 m/c? The thought is it would make the pedal much firmer than it is today and also allow for an upgrade to 930 calipers/rotors down the road. Would making the m/c change work without finishing the upgrade or will it screw up the balance of the brakes and make it hard to modulate the braking and achieve threshold braking?

I am currently running with stock SC calipers and rotors with a cooling kit installed. The car is nearly 100% dedicated to the track, so I am OK with it the high effort it will take to brake.

I'd like to give direction to the shop tomorrow morning, thanks in advance for the help....

Bill Verburg · Bill Verburg's Garage

Not a good idea. 25% more effort will be required for a given stop, that is a very large increase in neeeded effort. Assuming that the new m/c was installed and bled properly(bench bled prior to install), a mushy pedal will be caused by one or more of the following

the brakes weren't bled properly
a defective rubber line
a leaky caliper seal

MuffinMan · 12-08-2003, 07:25 PM

I've also heard that if the brake pedal is adjusted incorrectly so as not to allow the plunger in the master cylinder to fully retract, you will never get a good bleed. I battled with the same situation. When you push the brake pedal with your hand, is there any "travel" before you can feel it engaging the plunger in the master cylinder? I have mine set so that it barely moves before I feel resistance from the MC...it kind of "clicks" as the slack in the linkage is taken up.

Bill, if the pedal is adjusted so that the MC plunger is partially "engaged" when the brake pedal is at it's resting point, can this cause poor bleeding, as described above?

Schrup · Schrup's Garage

You can use the turbo M/C. I didn't find that out untill after I already bought a new OEM M/C. If I had to do it again I would go with the bigger one. I would bet dollars to donuts that yor problem has nothing to do with the M/C being bad. Good luck, let us know what works.

jakermc · 12-09-2003, 08:44 AM

Over the phone, they are telling me that the adjustment to the brake pedal has things feeling normal again, though I have not tested it myself yet. They have offered, and I agreed, to swap the 930 m/c for the 911 m/c to see what my preference is. They will also swap back to the the 911 m/c if I don't like it. No charge for any of this, so its worth the test. I should have a chance to drive it tomorrow with the 930 m/c and report back.

jakermc · 12-10-2003, 11:03 AM

Just drove the car with the 930 m/c, pedal effort is a non-issue so I am keeping it in. I wouldn't even describe the braking as 'high effort', it just felt 'firm'. If you ever need to replace you m/c, I would recommend the 930 over the stock 911. Now if I can just find a cheap set of 930 calipers ....

RoninLB · 12-10-2003, 11:09 AM

Quote:

Originally posted by jakermc
Just drove the car with the 930 m/c, pedal effort is a non-issue so I am keeping it in. I wouldn't even describe the braking as 'high effort', it just felt 'firm'.

allow me to ask,
about how much Less pedal travel for a given stop, generally speaking ?

Bill Verburg · Bill Verburg's Garage

Ron, its a 25% increase in effort and the same decrease in travel.

RoninLB · 12-10-2003, 11:57 AM

thanks Bill

jakermc · 12-10-2003, 01:59 PM

Bill -

How do you go about measuring 'effort'?

From my experience, it didn't 'feel' like 25% more effort, but then again percentages can be deceiving when the baseline is low. If I had $1 I wouldn't 'feel' 100% richer if I found another $1.

Bill Verburg · Bill Verburg's Garage

stock SC(20.64mm) the slave to master ratio is ~17.6 i.e. 1# at the actuaing pin becomes 17.6# at the calipers

w/ the 930(23.8mm) m/c the slave to master ratio is ~13.2 i.e. 1# at the actuating pin becomes ~13.2# at the calipers

This isn't quite true because the booster multiplies all the outputs by ~2.2 and there are several other complicating issues, but the results are close enough

Bill Verburg · Bill Verburg's Garage

What is the part # of the new m/c?

jakermc · 12-11-2003, 05:41 AM

Bill -

Sounds like a good rule of thumb, thanks.

I think it also helps explain why it doesn't feel like a big deal. I'm not sure how many pounds you need at the caliper, but lets use 500 as an example. A 911 m/c would require 28.4 pounds at the actuating pin and a 930 would require 37.9 pounds. Ignoring the pedal itself (which should act as a lever and reduce the required effort?) the difference is only 9.5 lbs. For leg muscles, some of the largest and strongest muscles in the body, this isn't very taxing. Its like adding a bucket of sand to the beach.

I'll get the part number when I pick up the car tomorrow.

beepbeep · 12-11-2003, 06:03 AM

Here's comparision pic:

We are going the same route (with Big Reds though)

Formerly Steve Wilkinson · 12-11-2003, 06:33 AM

I have the 930 m/c in my '83 SC and really don't notice any more braking effort. For all I know, it's actually there, but it sure doesn't bother me. And for what it's worth, the excellent local independent shop--Advanced Auto, in Vail's Gate, New York--routinely replaces bad SC m/cs with the Turbo unit.

stephan