Brake Upgrade - Need Late Model Mast. Cylinder?

[richemj]

I'm in the process up upgrading my '71 stock brakes with S alloys up front and NOS M fronts in the rear. Is the stock early-model master cylinder big enough for the job. I hear guys going Carrera and 930 brake on early models need the larger master cylinder, just wondering about this setup. System was fine with alloys up front and A rears in back, should using the A fronts in back make that much more volume to render the cylinder not functional?

Thanks!

[Bill Verburg]

Originally Posted by richemj

I'm in the process up upgrading my '71 stock brakes with S alloys up front and NOS M fronts in the rear. Is the stock early-model master cylinder big enough for the job. I hear guys going Carrera and 930 brake on early models need the larger master cylinder, just wondering about this setup. System was fine with alloys up front and A rears in back, should using the A fronts in back make that much more volume to render the cylinder not functional?

Thanks!

That's a very odd setup, S front w/ either A or front M in back gives you 1:1 f/r bias. Very unusual to say the least.

Anyway if that's truely what you want, yes, you need a 23.8mm ATE m/c. By the way these are not the 23.8 m/c used on the 930s or any other Porsche for that matter.

JMO but you have a bigger problem in your choice of rear calipers that ought to be adressed first rather than the m/c

[richemj]

Originally Posted by Bill Verburg

That's a very odd setup, S front w/ either A or front M in back gives you 1:1 f/r bias. Very unusual to say the least.

Anyway if that's truely what you want, yes, you need a 23.8mm ATE m/c. By the way these are not the 23.8 m/c used on the 930s or any other Porsche for that matter.

JMO but you have a bigger problem in your choice of rear calipers that ought to be adressed first rather than the m/c

Thanks for the comments. I had a feeling that the bias would be close, but not that close.

Think I'll just throw the 'ol rear As on and look into other options down the road.

[Bill Verburg]

Originally Posted by richemont

Thanks for the comments. I had a feeling that the bias would be close, but not that close.

Think I'll just throw the 'ol rear As on and look into other options down the road.

I don't know if you are familiar enough w/ the calipers to correctly identify them but using A calipers on the back is not a good solution.

Iron A calipers have 3.5" mounting ears as do the alloy S calipers, these 2 types of caliper can even use the pads which are 77w x 70h x15t(19 for S)all mm. Both A & S use 2x48mm pistons

Iron M calipers all have 3" mounting ears. Doesn't matter if they were fronts or back and they all can use the same pads which are smaller than the A or S. M pads are 62w x 56.5h x 15h(or 18) again in mm. front M use 2x 48mm pistons while rear M use 2x 38mm pistons


Now what a '71 911 wants is A or S front and rear M in back and a stock 19.05mm m/c

[richemj]

Originally Posted by Bill Verburg

I don't know if you are familiar enough w/ the calipers to correctly identify them but using A calipers on the back is not a good solution.

Iron A calipers have 3.5" mounting ears as do the alloy S calipers, these 2 types of caliper can even use the pads which are 77w x 70h x15t(19 for S)all mm. Both A & S use 2x48mm pistons

Iron M calipers all have 3" mounting ears. Doesn't matter if they were fronts or back and they all can use the same pads which are smaller than the A or S. M pads are 62w x 56.5h x 15h(or 18) again in mm. front M use 2x 48mm pistons while rear M use 2x 38mm pistons


Now what a '71 911 wants is A or S front and rear M in back and a stock 19.05mm m/c

Gotcha- Getting my head clear from all of the brake fluid that has seemed to have seeped through my skin and fuzzied my thinkning.... I was running alloy S's in front (3.5") and M rears (3") in back. In switching to the alloys up front (in combo with 3.5" biltsein strut) I had the M steel fronts (3") extra. Thought I would play the game of moving the M fronts (3") to the rear, thus the thought of needing larger MC to move the added volume. Decided to go back to ol faithful (M rears) in back and call it good. Seeing the piston sizes of A/S/Ms fronts, I can see how moving M fronts to the back would make the bias is 1:1. For info sake... what would the braking character with a 1:1 be like? I would assume that the rears would lock early due to the weight moving forward and equal pressure applied to all calipers. Not good for locked rear would want to swing around quickly, Right?

Thanks for the quick (smack against the head) brake lesson. =)

[dbanazek]

The rears would lock up before the front and you would spin nearly every time you hit the brakes hard. I did this by mistake with some race pads in the back and some street pads in the front once.

[richemj]

Originally Posted by dbanazek

The rears would lock up before the front and you would spin nearly every time you hit the brakes hard. I did this by mistake with some race pads in the back and some street pads in the front once.

Thanks for the confirmation. I threw the M rears back on last night. Brakes hard and smooth... lesson... don't mess with Factory S setup... its pretty darn good

[Todd Simpson]

Mike-

I used Carrera front and SC rear for a few years in a car very similar to yours.

You get fatter rotors up front and the same bias as stock. Master cylinder remains stock as well.

If your problem is heat at the track it's a cheap and easy solution. Just for the street you're fine with what you have.