Brakes. long pedal with slow application

Hi All. I have a 2001 Buick Park Ave 4dr sedan. 50,000+ miles. If you apply pressure to the brake pedal slowley it will move quite a bit. If you apply pressure to the pedal fairly quickly, the pedal firms up right away an only move down a little. What might cause this, and could I fix it my self? Show I have a mechanic do a test? Thanks in advance.

Here’s a few maybes: Low fluid level in brake fluid reservoir: air in the system or a faulty master cylinder.

I’ll go with the master cylinder. I suspect that applying the brakes slowly is allowing the fluid to pass by the piston seals at a slow rate that isn’t being felt with a fast, hard brake application.

I agree that it is the master cylinder. Brake fluid is leaking past the master cylinder piston at a slow rate. As the pedal is slowly pushed, the leak is enough to prevent the buildup of pressure and allows the master cylinder piston to continue to travel down the cylinder. When pushed quickly, it will firm up because the rate of leak doesn’t change and quick movement of the piston rapidly creates pressure sufficient to operate the brakes. If you then keep your foot on the brake pedal and keep pushing, the pedal will eventually travel to the floor and you will slowly lose brake function. The master cylinder needs to be replaced.

My GM truck is like that from day one. Long travel scares the heck out of me for normal braking,however if i slam on the brakes the stroke is much shorter,and stops on a dime.
At least for trucks the issue lies in the rear brake proportioning valve. In other words the brake system is unbalanced. Thats what i was told anyhow.

Master cylinder, internal leak

Update: 6-24-08. I took it to the mechanic. They were going to put on a new master cylinder. Upon opening the the brake fluid cap, they noticed the rubber seal was a worn out, and there appeared to be a foreign substance in the brake fluid. The mechanic thinks someone used transmission fluid instead of brake fluid, which can harm the seals and rubber in the braking system. They flushed the entire system and filled with brake fluid. Hopefully not too much damage was done, and if so it was only to the master cylinder. Thanks to everyone for their advice. Jason.

“there appeared to be a foreign substance in the brake fluid. The mechanic thinks someone used transmission fluid instead of brake fluid”

I am going to guess that you had your car “serviced” at a quick lube place. Am I correct? If so, then you have learned just one of the many hazards of using a quick lube place. They give new meaning to the term, “being serviced”.