Low line pressure with manual brake setup

Could use some help from those who have chosen to tread down this path. I've got the dual master cylinder setup on my car, and I love everything about it so far...except my clamping force.

With the stock brakes, I ran a 3/4" and 5/8" F & R MC, respectively. Clamping force was spot on.

I upgraded my front brakes to 996TT calipers and 13" (330mm) rotors, and in the process upgraded my MC's to 13/16" F&R. Had very poor clamping ability, so the (very helpful) folks at Tilton suggested I decrease my front cylinder to 3/4".


I haven't yet had a chance to take the latest changes out on track, but on the street things feel a little better. But something big concerns me: my line pressure. I'm logging pressure on my front MC only, and am only able to get a max of ~600psi. I would have expected about double that.


Did anyone else experience this? What gives? With this disparity it suggests I will need a 3/8" bore (or something)...and there's no way the model is THAT wrong.

Please forgive the question, but by maximum pressure do you mean maximum pressure with your foot as hard on the brakes as is possible with the car not moving or as much pressure as you can build up before the brakes lock while moving?

Colin,

I will respond here instead of via PM.

My setup was similar, but used Integra 6Rs on front and 951 rears. I ended up running 3/4 front and 13/16 rear master cylinders.

It is a misconception that the “big” calipers that we deal with are any bigger that Turbo brakes from a fluid volume perspective. Even most of the 6 pots.

So, assuming you had a 87-up 951, you went from 36 & 40mm pistons and 318-322 x 28mm rotor to  36/44mm 330/32mm? and left the rear alone? ’88 Turbo S is the same caliper volume you have now. Without doing a bunch of math, and from only a fluid transfer perspective, I would have guessed the 13/16-5/8 combo would have been about right if you really liked your 951 setup. Probably could have left them alone and been just fine, especially if you have a Turbo S. You can probably dial in more rear bias ether way. (now)

From a one stop perspective, the only real thing you changed was pad/rotor contact area. (Front friction)

Thanks for the quick feedback guys.

Had a chance to chat with our friends at Tilton, and it sounds like my line pressure is exactly what it should be. Interesting that I had it in my head the pressure should be higher.

I'm going to head to the track and see if it's any better. I need to clean the glazing off the brakes before I go; unfortunately the lower pressure last time out with the larger cylinder seemed to cause some glazing.