Author Topic: New Master Cylinders / Brake Bias  (Read 36 times)

Offline Rosebud

New Master Cylinders / Brake Bias
« on: June 11, 2019, 12:15:17 AM »
(reposted from another forum)

Some of you may recall, I chased a front/rear brake bias issue for years [https://www.facebook.com/notes/poser-motorsports/front-rear-brake-bias/2285826984823298/]. The rears would lock-up well before the fronts. Using dual master cylinders w/ a KAD bias bar wasn't enough to keep the rears from locking. So I installed a Tilton bias valve. Still not enough. I swapped out the 3/4" wheel cylinders for 5/8" cylinders and after much tinkering, got the front/rear bias where I wanted it. Fast-forward to last week when the brake pedal went to the floor while sitting at a red light. It happened several more times on the way home. I should mention that the entire brake system was replaced during a not-so-long-ago restoration.

About a year ago I noticed that a fine, black particulate material would gather at the bottom of the front master cylinder. I'd use an eyedropper to pull the black junk out and top off the fluid. I even flushed the entire system a couple of times, but the black crud would reappear. After the pedal went to the floor, it occurred to me that the black stuff was probably a deteriorating O-ring. Since a rebuild kit was half the price of a new master cylinder, I dumped the Girlings and popped for 2 new Welwood masters.

Well, the bias problem returned in a big way. I had the bias bar and the bias valve maxed. I had the rear brake cylinders backed off so far my hand brake barely worked. I combed YouTube for clues and found a video by Tilton that offhandedly mentioned, "Of course, on a dual system you'll want to install the largest bore cylinder on the rear circuit." Wait! What?

A call to Tilton's tech-ref confirmed; the larger bore cylinder goes on the rear circuit, the smaller on the front. I've always had it the other way around. It just seemed to make sense—biggest brakes, biggest cylinder. Once I swapped the cylinders and readjusted the bias bar, valve, and brake cylinders to a more neutral position, my front/rear bias is spot on. My hand-brake even works again. Who knew?
Rosebud
...the sled, not the flower
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