Just when I thought I had the worn master cylinder solved, it came back. If your car is leaking brake fluid or the pedal feel changes, pull over and check the fluid level and get to a shop as soon as possible. Ignoring a leak is putting your safety and the safety of others at risk.

The car had all the classic symptoms of a leaking master cylinder. The deciding factor was the brake fluid in the vacuum supply line. The owner said it was the third master cylinder in three years. It was on a modified car with a lumpy cam and little if no vacuum at idle, but this was not the problem.

The real failure was the booster. Chronic master cylinder failure due to fluid leaking past secondary seal in the master cylinder bore is typically not the fault of the master cylinder. What could be at fault is the pushrod seal in the power brake booster allowing vacuum to draw fluid past secondary seal of the master cylinder.

What solves the problem is making sure the booster is not applying vacuum to the back of the master cylinder. If vacuum is detected, replace both master cylinder and power brake unit. If you get it right the first time, the repair does not suck!