weak braking 1988 GMC 1500

[d1940]

I bought a 1988 GMC 1500 4 wheel drive. The brakes seem weak. The pedal is solid and I have new linings. The vacuum booster holds vacuum after the engine has been shut off. If I hold the brake down when I start the engine the pedal go down about an inch when the engine runs. Any idea why I have to push very hard to stop.

[N2TRUX]

There's a couple things to look at on this year model truck. One is the front rubber brake lines. They have a tendacy to expand or ballon with pedal pressure. This uses line presure, and the front brakes don't work as well. Consider replacing them.

Another area of concern is the rear "anit-lock" brake control. There is a valve at the rear of the truck that is supposed to regulate line pressure to prevent the rear brakes from locking with no load. This valve is prone to sticking and preventing line pressure on the rear brakes.

Regardless of these issues, the 88 had really crappy brakes. Take a look at these items and see where you are...

[d1940]

Those two items I had not checked. thanks

[d1940]

The only valve I could find is connected to the master cylinder support bracket.The brake lines from the master cylinder go in the top and the lines to the wheel cylinders out the bottom. There are two opening on each end of the valve. When I push on brakes out of front opening comes a rod. The rear opening nothing happens. Thanks for any help