Brake Master Cylinder Help Needed

Hill Bullock · #1 (**permalink**) 06-10-2002, 11:07 AM

Hey Folks,

I have a question on brakes and master cylinders. My car has separate master cylinders, one for the front brakes and one for the rear. My brakes have never been very strong (bought the car turnkey), but someone with the same kit who has the same rotors and calipers, said my brakes should be much stronger than they were.

Analyzing the setup, I noticed the front master was 7/8 and the rears were 3/4. From everything I had heard, to keep the brakes biased to the front, the front master should be smaller. While looking at the 7/8, I noticed it was leaking and that there was a flaw in the cylinder.

I went to Racer Parts Wholesale online to get a replacement master. Since the 3/4 was okay, I could either get a 7/8 and end up moving both masters, or I could get a 5/8 and put it in the front, leaving the 3/4 in the rear.

As a side note, I also ordered some braided steel brakes hoses for the front, hoping that getting rid of the rubber flex would add some brake power.

This weekend a friend of mine and I installed the 5/8 on the front with the 3/4 on the rear. We also managed to get one of the braided steel lines installed, but not the other side as the person who had built the car had tightened one of the connections down to the point where we were stripping the crap out of it trying to get it off.

Anyway, we bled the fronts and got all the air out, but it would never develop any real pressure. The pedal would go to the floor every time (with some effort). The 5/8 master is supposed to require more travel, but give more braking force, in theory. But is 5/8 too small? Is it requiring so much travel that I am getting to the floor before it firms up? There were no leaks anywhere that we found, and pumping/bleeding yielded nothing but fluid.

Suggestions?

Thanks!

Bob Putnam · #2 (**permalink**) 06-10-2002, 02:18 PM

We also use a 7/8" and 3/4" setup. The front wheel cylinder size is much larger for our front calipers than the rear - requiring more volume. Our balance is still about 70% effort to the front.

The 5/8" cylinder will require almost twice the travel as the 7/8" for the same fluid volume. If the original design was approximately right, you simply don't have enough fluid volume to bleed the brakes. Not to mention, you might as well disconnect the rear brakes. They're not doing much anymore.

Hill Bullock · #3 (**permalink**) 06-10-2002, 03:39 PM

Aha! Thanks for the info, Bob!

So you use 7/8" up front or in the rear? The reason I was trying smaller master cylinders was to account for the fact I never seemed to develop much braking force in the front. I had dinked with bias and all and couldn't seem to get any improvement. No matter how hard I stomped on the brakes, I could not get them to lock up. I have the same disc setup as someone else I know, but his brakes worked MUCH better.

Bob Putnam · #4 (**permalink**) 06-11-2002, 05:12 AM

We use 7/8" in the front, 3/4" rear. Your caliper piston sizes may not be the same size as ours, obviously.

Roscoe · #5 (**permalink**) 06-11-2002, 10:44 AM

Let me add that the proper way to bleed dual master cylinder brakes is right side/left side not front/back.

Roscoe

PERUCH · #6 (**permalink**) 06-12-2002, 01:39 AM

Do you know the pedal ratio?,pedal ratio for manual brakes is about 5 to 1,power pedal ratios are about 3to1

boxhead · #7 (**permalink**) 06-12-2002, 07:44 AM

With the smaller diameter brake master cylinder your displacing a smaller amount of fluid for the same pedal travel ... so if you need say 100cc to fill the front calipers then you will need to push a 5/8 cylinder through a longer stroke than you would a 7/8 cylinder.