AP Brakes Master Cylinder Size

[Bobski]

I have the AP Brake upgrade offered from RF with 6 piston front calipers and 4 piston rear calipers. I'm looking at what size master cylinders others are using on the three cylinder floor mounter pedal box. Reason I'm asking is that my brakes take a lot of effort to stop and this may improve once they're bedded in. Don't know what size I'm running and I'll be checking tomorrow.
Thanks in advance.

Bob.

[Ross Nicol]

Hi Bob
I've spent a lot of time on this and my M/C sizes should work for you as I have the same calipers and although I have a Tilton balance bar that wouldn't make any difference.I started with 2 - 3/4 M/C , had to have the balance bar biased all the way to the front and the pedal was hard requiring a lot of effort.I've now got an 11/16" M/C front and 1" M/C rear with balance bar centred.With the smaller front M/C pedal pressure is much lighter and the travel is fine.
Ross

[Bobski]

Thanks Ross, I'll check my sizes and if different, I'll try them. Thanks again.

Bob.

[JulianBW]

My car has two 3/4 AP master cyls and as you say the bias is fully to the front, with a fairly (very) heavy pedel with little feeling!

I have had conflicting advice to go to smaller master cylinders 0.7" front and .625" rear?

Is anyone using servos and where can they be mounted?

J.

[Ross Nicol]

Julian it's very unusual to have a smaller Master cyl for the rear.The reason is the rear brakes must not lock up before the front under hard braking.You need to check the caliper piston area and work from there.There is a very good article on the Tilton website on selecting M/C size.The reason I'm confident my sizes will work for Bob is that he has the same calipers and similar sized discs.His pedal ratio could be different but I assume it to be 5 or 6:1. If your pedal is hard and lacks feel a drop down in M/C size for the front is a good start, and going to .7 from 3/4 is pretty right as my 11/16" is pretty close to .7. Is your balance bar biased toward the front at the moment? if so going smaller with the rear M/C is definately wrong.
Ross

[Julian West]

Ross,

Just curious, but surely a smaller MC to the rear would mean the rears are *less* likely to lock first?? A smaller MC would shift less fluid & hence less piston/pad movement??

WDYT?

[Julian West]

BTW, just noticed there are two 'Julians' in this thread... could get confusing?!?

[Jim Pearson]

Hi Julian W,

My undersanding is as follows:-

You apply a force of, say, 100lb through your right Gucci loafer to your 5:1 pedal, thus conveying 500lb to the MC. A MC piston of .8" diameter has a surface area of approximately 0.5sq" and so you will generate 1000psi in the brake line. A MC of .7" dia, having a surface area of approx .38sq", will generate some 1315psi in the line.

So the smaller MC generates more pressure, albeit at the expense of requiring greater pedal travel.

I think ....

Jim

[Frank Catt]

Jim, you are right, smaller bore/larger force applied/slightly longer pedal movement.

[Julian West]

Thanks Jim,

I've just had a read & what you say is also confirmed by Carrol Smith! Appreciate the info,

Regds,

[Straight 8 Racing]

Hi Bob,
Can I ask what size rotors you have on your RF, we have done our calcs and have chosen the MS setup we plan to start with but "real time" feedback is worth a month of calcs !

Ross, What size rotors are you running, do you have the 6 pot / 4 pot combination in your car

Iain

[Ross Nicol]

Nice wavy slot pattern there Iain.I have 330mm by 31mm rotors on the front with 6 spot caliper and 304mm by 31mm on the rear with 4 spot caliper, the standard AP Racing calipers that RF fitted although I have spaced the rear Caliper to take the 31mm rotor.
Ross

[Ross Nicol]

One should check before posting.
Front rotors 330mm by 32mm
Rear rotors 304mm by 28mm
Ross

[Bobski]

The size of my rotors are 330mm x 32mm front and 286mm x 28mm back with six pot AP calipers up front and 4 pot AP calipers at back with a PBR handbrake caliper which works during breaking as well.
Bob

[Bobski]

I haven't tried Ross's suggestion as yet, but when I do, I'll provide feedback. Thanks Ross.
Bob

[Ross Nicol]

Interesting Bob the stock rotors I took off my car were 330mm by 27mm ( Were machined once or twice so probably 28mm or 30mm originally) on the front and 280mm by 20mm on the rear. As I said I had to fit a spacer to the rear calipers to take the 28mm rotor.
Ross

[Bobski]

I will double check my measurements as the car is away from me and I had my mechanic measure them yesterday. I'll take more measurements tomorrow.

[Jim Cowden]

Bob
From memory you used a 5/8 front master and a 3/4 rear.

If this is correct I would suspect your first thought of not bedded in or the compound is to hard.
(check your compound)

Jim