Why are my brakes so sensitive?

Question:

I have a 1987 GMC 20 series Vandura with a brake problem. The brakes are extremely sensitive—it takes very little effort on the brake pedal to lock the wheels. This is especially true of the rear wheels. It has power brakes with discs up front and drums on the rear. On a wet road or loose surface road the rear brakes will lock up with very little stopping pressure on the pedal.

Can you give me any idea why the brakes would be so sensitive? This is a conversion van. I am sure it was bought as a work van and converted so the power brakes may not have been a factory installation. However, I have no way of knowing this for sure.

Answer:

Several things can cause the brakes to grab and be hypersensitive. You may have the wrong master cylinder. A master cylinder designed for drum brakes all around will have a residual valve that maintains pressure on the front discs when it shouldn’t.

A more likely problem, however, is a bad or missing proportioning valve. As your vehicle brakes while moving forward the force of its mass is transferred to the front wheels. The harder the braking action the more the weight transfer to the front. As the weight moves to the front it moves away from the rear and with diminishing weight over the rear axle the rear wheels become more prone to locking up.

With disc brakes up front and drum brakes in the rear you need a proportioning valve specifically designed to operate with this brake system configuration. You may have the wrong valve, a broken valve or no valve at all.

Something else that can make a braking system too sensitive is bad brake pedal linkage geometry.

Most vehicles have two mounting points for the rod that connects the brake pedal lever to the master cylinder or booster and which point you are supposed to use is determined by whether you have manual or power brakes. You should utilize the lower mount hole for power brakes and the upper one for manual brakes. The upper mount hole considerably increases the force of pedal application, which is needed with manual brakes, but a problem with power brakes. It’s quite possible that the mount hole was not changed when your van was converted to power brakes, if, in fact, it was.