4 pot caliper piston travel

Hi all, simple question, I have 4 pot wilwood calipers with 1.5" pistons at the front, I am wondering how much the pistons will move under braking so I can calculate brake fluid displacement to calculate the smallest MC diameter to start testing with. pedal ratio is 1:5 , I have about halve an inch mc travel.(I don't like too much pedal) I have no booster so don't want to end up with a mc that is too big, thanks ( P.S. total piston area at the front is 14.14 sq.inch)

Ron Earp

Not much at all. They’ll be around 0.050” to 0.075” off the rotor, probably not even that much, and will only travel that distance under braking.
Not much at all. They’ll be around 0.050” to 0.075” off the rotor, probably not even that much, and will only travel that distance under braking.
Might want to move that decimal point one more place to the right-like ).0.005'' to 0.0075''.



Don't try to go too far undersize on your master cylinder. You'll need enough volume to account for thermal expansion, caliper flex, tiny leaks, hose expansion, etc. You don't want to run out of brake pedal!
thanks guys, information is much appreciated, can't wait to start on the brakes as it is the last thing to do (was not happy with the esprit front brakes I used before but the car is legally on the road now so I can change them for someting better)
looks like a 0.625 MC is already giving enough volume, I have a 0.625 a 0.7 and a 0.75 MC laying around so will start with the smaller one and see how it goes. thank you all.

Terry Oxandale

Skinny Man
I use 3/4" on my MCs, but I've got larger pistons (1.75"/1/82") on my calipers. For my needs, it's a great balance between effort and travel.

Howard Jones

Here's some basic piston area to master cylinder ratios that I know work pretty well. I have this one on my GTD and am very happy.

Front calipers are 1.75 X 4 and rears are 1.375 X 4, master cylinders are both 5/8 (.625)

Front total caliper area (both) =19.28 Sq inches, Rear total = 11.84. For a front caliper to master ratio of 30 to1 and a rear ratio of 18.9 to one or the fronts are about 58% bigger than the rears.

However I have a adjustable proportioning valve in the rear line and I believe it is reducing the rear pressure by about 10-15%. My best estimate is the front to rear adjusted ratio is very close to 66% on the fronts and about 33% on the rears.

I used this model to do my SLC and with the same ratios I am very happy with it's balance.

There are other variables such as pad friction, pad area, peddle ratios and rotor diameter. All of which have varying effects of overall performance, but if you start off with good caliper and master cylinder diameters sizes and appropriate ratios between the two it will help you start off close to a optimum setup that can be refined with pad selection and balance bar adjustment.

As to your car. I think that you may fine that the 1.5 fronts may be a bit small, may be difficult to match to rears, and will require a heavy foot. But then again you may like it that way. Anyway I hope this helps.

Bill Kearley

GT40s Supporter
My VW Vanagon master and booster work well with the 4 piston Willwood, F&R. The large booster works well with the lowish vacuum I have.