RE: 1990 Chevy 1500 Cheyenne Pickup
I had the same problem on both my 1990 and my 1995 C1500s. On my 1995, I ended up replacing the vacuum booster and my master cylinder, and honestly, I didn't notice much of a difference. I never bothered replacing either on my 1990, since it cost me over $800 just to have the work done. It's probably the master cylinder though, thats what usually causes a squishy brake pedal. Take a look at your rear brakes to, sometimes the auto-adjusters seize up. To fix this, just drive in reverse and slam of the brakes. The pads could be worn on the back brakes as well, you can check this by removing the drums, and if they don't come off, you'll have to manually crank the adjusters with a screw driverso that you can get inside. If that is the case, the pads are worn.