I have not been able to figure this out and have also asked in the 1980-96 forum. I was driving home a couple of weeks ago lightly braking down a hill and the break light came on the the pedal started feeling a little soft or like it wanted to travel down farther.

I thought I had a leak but the fluid level in the resevior did not drop. I have replaced the master cylinder and booster and no change in problem. Someone suggested maybe the vacuum pump. I put my finger over the hose and it felt like a decent vacuum.

If I shut the truck off or unplug the vacuum from the booster, the pedal will only travel so far then stop, but if the vacuum is hooked to it the pedal stops but if I keep pushing, it feels like it will continue to depress. It feels like it takes about the same amount of pedal pressure to stop the truck whether the vacuum is hooked up or not, but with it hooked up the pedal wants to continue to depress. The brake light won't stay on all the time but will come back on if I keep depressing the pedal.

Maybe there is air in the line but this problem started before I took anything apart and I think I have bled most of the air out anyways.

Does the brake light only come on if there is a loss of pressure or low fluid? Any way to check the vacuum pump or lines?

Sorry for the book and thanks for any help.

 

Slowly falling brake pedal is 'normal' for these trucks. It happens more when stopped at a light but light braking down hill might be about the same. there was a fix but I gotta find it. Maybe someone will jump in here and help.

Have you adjusted the rear brakes lately? You need to keep them very close to being too tight for the pedal to feel good.

BTW: You vacuum booster sounds OK. If it were bad the pedal would be rock hard, not soft. A good indication that the pump is going bad is when you can't get the HVAC to shift off of defrost. It takes vacuum to get the spring loaded diverter door to move off defrost.

 

I had not experience this feeling at all since I bought it back in July. Another guy in the 87-96 forum, Lazy K, thinks it is the antilock valve, RABS, located on the frame rail across from the transmission. He said to check it by taking the large hex screw out and remove the spring and put the bolt back in. I will give that a try also. I think that would make sense, as long as it has that RABS valve on it. Seemed weird for the problem to start the way it did. I will check the drag on the rear brakes also. Any more suggestions I would be glad to here them

 

Be sure about the air in the lines & the rear shoes