71 240z with triple webers (low vacuum source) and a refreshed stock brake booster.

The master cylinder was changed to a new one (7/8" dia) and the calipers were changed to 4 piston Toyota's.

The pedal pressure is fairly hard which I think is due to the low vacuum and larger caliper piston area.

I am thinking that finding 3/4" master and maybe increasing the booster to a 280z 8.5" will help.

Any suggestions, comments?

Where can I source a 3/4" master?

Few things:

(1) Does your manifold have a balance tube, a'la Cannon?

(2) Does brake effort decrease as RPMs increase?

Holding master cylinder piston diameter constant, increasing caliper piston area should decrease effort but increase travel, per Pascal's Law. In other words, if you switch to bigger calipers, pedal effort should decrease.

Blue, given the larger volume of fluid that now needs to be pushed to your calipers, I don't think you'd want to decrease the size of your MC. Everything I've read says you need to go the other way to avoid increasing the travel of your brake pedal. I could see how increasing the diameter of the booster would help. '73's started with the larger boosters. If you swap the booster make sure you measure the length of the pushrod that actuates the MC in relation to the booster so that you do not throw another variable into the mix. Also keep in mind that the later MC's had the front and rear reservoirs reversed from the earlier ones, so if you change you might need to swap the hard lines to the MC.

Edited March 9, 201312 yr by tlorber

Thanks guys, this helps.

In parallel I decided to crunch the numbers. Now I see what is going on.

I think I have to focus on the brake booster and vacuum source w/o changing the master cylinder (as you both mention above).

Remember that your booster will see and hold the max vacuum your engine has produced since the last brake pedal application. So when you release the throttle to begin braking, unless you pop the trans into neutral, there should be plenty of vacuum available. If the check valve or booster are leaking then all bets are off.

Steve