23mm Master Cylinder on Early 911?

[Roger 911]

I'm contemplating putting a 23mm master cylinder on my '72 911 with S brakes. Over the past 8 yrs I have replaced or rebuilt the entire braking system (MC, calipers, flex lines, hard lines). I am still not satisfied with the pedal feel. There seems to be too much initial pedal travel (2 inches) before the pedal firm up. It has been like this for years. I bleed them, both power and pedal pump, and nothing really changes. I made sure the pistons weren't retracted too far, etc. The master cylinder was replaced about 8 yrs ago with another 19mm Ate. This is now the oldest component in the system, however, this car sees very little usage during the year. I do about a half dozen track days and an occasional trip to Dairy Queen with my son.

I see that Automotion sells a 23mm master cylinder that is a direct replacement for the early non power assisted cars. Does anyone have any experience with this? Will a 23mm MC over power a car with S brakes? I guess the only issue I can imagine would be going to the other extreme of what I have now, i.e., not enough pedal travel and too difficult to modulate (especially on the track). Any thoughts out there?

[onboost]

23mm has been the "standard" upgrade for many years.. Just do it, you should like it!

P-

[Bill Verburg]

w/ stock S & M calipers stay w/ the stock 19.05. The 23.8 would require King Kong's legs to operate.

there was a very rare 20.5 non-boosted that I haven't seen for 3 or 4 decades that could also be used
Porsche # 901.355.012.04
ATE # 3.2120-3901.3


the stock 19.05 is/was 901.355.012.02

[onboost]

That explains my right leg muscles! ;-)

[Pele911]

If you have an updated aluminum cross member you may have to notch it to fit a larger Master Cylinder. I went to 23mm on my 71 but I have 86 brakes and susp and it does take a little extra effort to push the pedal.

[Roger 911]

Hmmm, not a concensus yet.

Maybe another question would be whether or not a couple inches of pedal travel is "normal" for an early car with S calipers? The car always seems to stop fine. It just seems "wrong" that the pedal moves so much before you encounter real resistance.

[john walker's workshop]

Originally Posted by Roger 911

Hmmm, not a concensus yet.

Maybe another question would be whether or not a couple inches of pedal travel is "normal" for an early car with S calipers? The car always seems to stop fine. It just seems "wrong" that the pedal moves so much before you encounter real resistance.

flexy aluminum calipers add to the feel. go cast SC calipers. and i 2nd the too much leg effort on the 23mm.

[Turboo934]

Works just great on my 76 turbo with 89 turbo brakes.Also used it on my 73 with S calipers great upgrade.

[Bill Verburg]

Originally Posted by Roger 911

Hmmm, not a concensus yet.

Maybe another question would be whether or not a couple inches of pedal travel is "normal" for an early car with S calipers? The car always seems to stop fine. It just seems "wrong" that the pedal moves so much before you encounter real resistance.

If you go w/ a consensus opinion it will as likely be wrong as right. I don't know how to evaluate this "that the pedal moves so much before you encounter real resistance"

but the fact remains the 19.05 or maybe 20.5 is the correct m/c for S & M

[Bill Verburg]

Originally Posted by Turboo934

Works just great on my 76 turbo with 89 turbo brakes.Also used it on my 73 with S calipers great upgrade.

Yes, the 23.8 is the correct m/c to use w/ '78 > 930 calipers.

Also used it on my 73 with S calipers great upgrade

You must have some very unusual legs.

Even w/ bigger 964 calipers whaich are a much better match for the 23.8, the amouint of leg effort unboosted is right on the border of what a very healthy leg will handle.

given the fixed mechanical advatage of the 911 unboosted brake pedals, the relevent #s to look at are the slave/master hydraulic ratios small is higher and harder, bigger is softer w/ more travel
16.171 is right on the edge of what is considered to be acceptable for a very healthy leg w/ the 911 mechanical linkage

stock 911 boosted is 17.596
stock C 3.2 is 19.098
SC/RS is 16.539 unboosted
stock 930 is 16.539 boosted
964RS is 16.393 boosted
993RS is 16.827
stock 964 is 21.502

S & M w/ 19.05 is 20.656 unboosted
S & M w/ 20.5 is 17.837 unboosted
S & M w/ 23.8 is 13.234 unboosted

[Turboo934]

You have to change the rod from the brake peadle to master cylinder,one from a67 works.Elimenates the travel.

[onboost]

Hmmmmmm,

Legs don't seem to be that unsual.. although now that it's been mentioned, my 73 does have a Turbo brake upgrade as well.
Guess that negates the whole S caliper part for me!

Roger911.. good luck getting your issue resolved!!

[RWebb]

OEM-90135501204 CYLINDER
- This is a special-order, non-returnable, Porsche part
(availability and shipping charges may vary).
$78.75

[Roger 911]

Originally Posted by RWebb

OEM-90135501204 CYLINDER
- This is a special-order, non-returnable, Porsche part
(availability and shipping charges may vary).
$78.75

What exactly is this part?

[Roger 911]

Well, I get the feeling that the 23mm may not be the answer for my S calipers. I read on another post where someone (either Steve W or John W) said to hold your foot on the brake for two minutes and see if the pedal falls to the floor. I have only done this for a few seconds and it seemed OK. I'll see if it holds it's seal for the longer time period.

If it does, I guess I'll just have to assume that this is "normal" feel for an early car with S calipers.

Roger

[Scooter]

For the record: I had a Mercedes 23 mm master cylinder in my '70 with SC brakes and they worked great together. Not too hard or too soft...just right.

[john walker's workshop]

that's 'cuz yer legs are shorter and legs push harder at the almost fully extended position than in the just past 90° position. just my opinion of course.

[Scooter]

John, do I see a short joke in there somewhere? If so, I take offense! J/K

I see your point, maybe I just don't know my own strength with my short legs.

[RWebb]

Not just an opinion, but verified biological fact.

It can be the curse of rock climbers and tai chi practitioners...

You can also run your own experiment in any gym with a "squat machine."

_________________
p/n is the int. sized cylinder - read the entire thread.

[MJHanna]

I have a Smart Racing 23mm master kit I never used. New in box, if you go that way let me know.