bigger brake master cylinder

[Flynn]

Thanks guys! Wondering what would be best for a GT running single piston GT4 brakes..

X2

[mech5107]

It's pretty easy...

If you running single piston/2piston sliding caliper (like levin), then find the size of the piston (say 51mm) divide by 2 (so 25.5) and then muliply by 3.14 (80.47) and then multiply again by the half diameter (25.5) so it's 2041.785.

This is the area of each piston. If you have single piston, then multiply by 2 (the caliper is moving to acomodate that there is no piston in the other side, so the master cylinder is actually like moving twice the fluid.

If you have twin pot then multiply by 4.

If you have 4 piston, multiply by 4 again.

This is the area of your pistons in the caliper.

Now say you run a 7/8" master. Multiply 7/8 by 25.4 (22.225mm), then divide by 2 (11.1125), multiply by 3.14 (34.89325) and multiply by the half diameter again (11.1125). Total is 387.75. This is the area of your master cylinder.

So if you pad to disk clearance is 1mm, then the pistons of the caliper has to move 1mm so the volume of fluid to be moved is 1 by 4083.57 (for a 51mm single).

Your master has to move by 4083.57 divided by 387.75 = 10.5314mm.

Also if you need say 10000newtons to push the pad to the pad to the disk (in order to brake), then your needed fluid pressure is Force (10000) divided by the area (2041.785 for the single 51mm). So 4.897.

To achieve that, you have to apply force to your master cylinder equal to pressure (4.897) multiplied by the area of the master cylinder (387.75 for 7/8"). So 1899N.

LONG STORY SHORT:

You have too much pedal travel for your like but good braking? => Increase master cylinder size

You need to puch too hard to achieve decent braking? => Decrease master cylinder size

You don't have much travel, you have good braking but the pedal feels soft? => Decrease booster size

You have good pedal travel, too hard pedal and not good braking? => Increase booster size

Your pedal travel is perfect, your effort is perfect but can't get good braking? => Change pads and disk size

In any case you need to take something as reference (your current setup/feeling) and decide what you like and what not. Then act accordingly.

There is no correct or wrong answer.

[gorganl2000]

well the 7/8 bmc was installed today (came out of a 1992 gt ep82)...everything fit right up with NO modifications needed...this replaces the original 13/16 bmc that came with my car came (ep91 glanza v)

for reference, my brake set up is celica/levin twin pots and astra discs

i can safely say that i am now satisfied with my brake travel and braking ---it feels firm, but not overly so

that long travel and "spongy" feeling is gone

[Amjad]

It's pretty easy...

Not what I thought when reading all that

But the summary was great, thanks!

well the 7/8 bmc was installed today (came out of a 1992 gt ep82)...everything fit right up with NO modifications needed...this replaces the original 13/16 bmc that came with my car came (ep91 glanza v)

for reference, my brake set up is celica/levin twin pots and astra discs

i can safely say that i am now satisfied with my brake travel and braking ---it feels firm, but not overly so

that long travel and "spongy" feeling is gone

Good to hear! If anyone else changes theirs do let us know how you get on.

I've been told the MS out of a '94 Mitubishi GTO or FTO (V6) will fit our cars, not sure on the size though

[gorganl2000]

my recommendation in cases like this...check your existing bmc size...then upgrade by 1/16, i know it doesn't sound like a lot, but in bmc sizes its significant

ps- remember to always bleed your system properly and make sure everything is properly connected and in satisfactory working order, afterall these are brakes we are talking about----very important stuff