23mm master cylinder with unassisted brakes - Need advice!

[pjo046]

Hi

I have a 1972 911 on which I have replaced the master cylinder when renovating the car. I put in a 23mm master cylinder because I thought that would give me better braking power. Other than that the brakes are stock, but with new pads and discs, renovated calipers and new lines/hoses. The pads/discs have been run in though.
I bought the pads from this site, Porsche 911 & 930 Brake Components | Front & Rear Pads UK , the standard type they carry. Not sure of the brand as I don't have the packaging anymore.

As I read more on the topic I now see that when going with such a large diameter master cylinder I should ideally have a brake booster to help push the increased volume of brake fluid. Beeing a 1972, my car of course doesn't have a booster...

I have bled my brakes many times the past year, thinking it had to be trapped air in the system causing the lack of braking power...

So, can I do anything to remedy the situation, or do I have to either change back to a 19mm master cylinder or live with the fact that I am unable to lock my wheels when braking from any higher speeds than 20mph? As it is now, the brakes needs a lot of leg effort to use and I find them performing less than adequately.

If possible I would like not having to change the MC again as it was a real hassle to get those plastic lines into the rubber grommets underneath the car...

[Flieger]

Change the master cylinder back to 19mm. Or you can install 930 Turbo brakes as those are matched for the 23mm master cylinder. The effort will be reduced by having bigger caliper pistons.

[pjo046]

Do these bolt right on? Do they require different discs? Do I have to change both front and rear or are the rear calipers on the 930 the same as the ones I now have?

Installing different pads with more agressive friction won't do the trick alone?

[Bill Verburg]

Originally Posted by pjo046

Do these bolt right on? Do they require different discs? Do I have to change both front and rear or are the rear calipers on the 930 the same as the ones I now have?

Installing different pads with more agressive friction won't do the trick alone?

W/ stock 911 brakes use the stock m/c

the effort is proportional to the associated pistons slave/master ratio

for a '69-76 911 w/ unboosted 19.05mm m/c that ratio is 41.311

for your setup, stock '69-76 brakes w/ 23.8mm m/c the ratio goes to 26.467

the lower the ratio the better feel/control but also the more effort is required

A 911 like yours w/ 930 calipers and unboosted 23.8mm m/c has a ratio of 33.077

Anything under 35 or so will usually want a booster for most people most of the time

there used to be an unboosted 20.5 mm m/c that would be excellent to use, ATE part # ATE # 3.2120 - 3901.3, Porsche part # 901.355.012.04, w/ this m/c the ratio would be 35.674, I haven't seen one of these in several decades though.

930 brakes are an expensive mod that most people don't need, the front is a bolt on as long as you can get the '78-80 930 flat hats and rotors or aftermarket equivalents, the rear rotors are a bolt on that requires discarding the backing plates and the rear calipers are a bolt on once they have been machined for 911 trailing arms.

[pjo046]

In other words, I have to get a 19mm MC... Darn...........

[yundar]

I tried several times to get the plastic hoses into the rubber grommets while underneath the car without luck. I finally removed the plastic hoses from the reservoir and put them on the master cylinder outside of the car. Then I brought the whole assembly under the car and fed the hoses back up through the body and reattached them to the reservoir. Sounds like a pain but it wasn't too bad. Way easier than trying to push them in while under the car. Also remove the front sway bar to gain some clearance.

[pjo046]

Ok. Thanks for the advice! :-)

Last time I changed MC I tried disconecting the plastich hoses, but they were quite stuck so I gave up. Guess this time I'll try harder to remove them!! Last night I ordered a new MC from *********************, as they had an OE Quality Aftermarket unit which were half the price of the OEM. I hope that wasn't a bad move? 911 / 356 Porsche Parts | Porsche, Wurth and Fuchs Wheels | Sierra Madre Collection

[yundar]

I am not sure where my master cylinder came from. It was a Christmas present! (Santa knows all I want are Porsche parts.) It was a URO brand and seemed to be of good quality but it was missing the two little washers that go under the rubber grommets for the reservoir lines. Be sure and check for those.

Andy