figuring out master cylinder size for caliper question.

[bryant]

so ive been searching all over the net for a clear formula or calculator for figuring out master cylinder size and cant find anything definitive.
i find plenty of stuff about pedal ratio and bore size pressure equations but none factor in what the caliper piston sizes are.
what i have right now are these specs
a pedal ratio of 6 to 1
a master cylinder of 15/16"
wilwood 4 piston calipers on all 4 corners
wilwood lists the front calipers as having a piston area of 4.8"
and the rear calipers having an area of 3.0"

so what ive figured is that i have 960 psi with 150 foot pressure force according to these calculators http://www.wallaceracing.com/brake-master-cyl-calc1.php

im experiencing a lot of pedal travel with a squishy feel that seems like air is in the system but i have bleed it thoroughly and am confident i dont have air. every thing indicates that i have to small of master cylinder. the brakes stop fine and i can even lock them up if i push the pedal practically to the floor and when i get caliper kick back during the autocross, the brake pedal needs lots of travel before its effective.

i cant find any into that tells me with my caliper piston size what size master cylinder i need to go to.

i know right now from the way the brakes feel and perform i need a bigger master cylinder. ive seen lots of references to using a 1" master cylinder on mavericks with brake setups really close to mine. its just such a small step op from my current 15/16"s mc that going to a 1 1/8" seems like it should get me closer to what im expecting for pedal feel and performance.

so the question is what size master cylinder should use?

[65 drop top]

Assuming you are talking about manual brakes, there is no way you would need an 1 1/8" master. The pedal would be way too hard. I use a 7/8" bore wilwood master on my manual baer setup and the pedal pressure is moderate without excessive travel. Still sounds like air in your lines.

[manOwar]

Call Wilwood Tech.

They recommended 3/4" for the front and 7/8" for the rear on my manual set up which is 6:1 pedal, 14" front with 4.04" piston area and 13" rear with a 1.98" piston area.

[andrewb70]

Bryant,

I know you said that you have no air, but I suspect you do. I used to run a 15/16" MC with C6 Z06 fronts and LS1 rears. I swapped to a 7/8" MC and the pedal is softer, but I don't consider it to have excessive travel.

Andrew

[cheapthrillz]

See if this helps
http://www.jakelatham.com/radical/info/brake_calculators.shtml

[bryant]

wow now thats a serious brake calculator! ill need to go get more specs from my car!
ill give it another shot at bleeding. i vacuum bleed it when i installed the master cylinder. ill go old school and do the pump the pedal open the bleeder method.
thank you for the help and advice.

[Apogee]

wow now thats a serious brake calculator! ill need to go get more specs from my car!
ill give it another shot at bleeding. i vacuum bleed it when i installed the master cylinder. ill go old school and do the pump the pedal open the bleeder method.
thank you for the help and advice.

You can't really beat a good manual bleed IMO (unless you have a reverse bleeder), but make sure your master cylinder if fully bench bleed as well. It's possibly that you don't have any air in your lines or calipers, but still have air trapped in the MC creating the symptoms you've described. The plugged port bench bleeding method is more definitive since it will hydraulically lock when there's no more air, so that's what I'd do first, then reconnect one brake circuit at a time and fully bleed before connecting the other circuit to the system.

Tobin
KORE3