1994 GMC Sonoma

Brakes problem
1994 GMC Sonoma 6 cyl Two Wheel Drive Manual 250000 miles

have abs on this vehicle have replaced master cylinder no leaking anywhere. When the brake pedal is pushed down there is a hissing sound like the fluid is bypassing or not enough force to completely compress the master cylinder piston. I am not familiar with how the abs module works. Could something be stuck in the abs module or do I have a booster problem?
John singleton
August 12, 2010.

If certain none of the lines are leaking than check for vacuum leak near the brake booster.

Sep 30, 2010.