Okay, I know that it is not a Jeep and not a common tow rig but I need some help.

The truck is a 1995 GMC Topkick with a CAT 3116 engine and an automatic transmission. It is a 25950 GVW with hydroelctric(?) brakes. There is a brake control box mounted on the kick plate in the driver side area.

The premise of this system is that when the vehicle is placed in park the parking brake engages and when taken out of park the parking brake disengages. There is also a brake control on the dash that ties into the system.

The problem: when the truck runs for a long time unloaded or moderate hauls loaded the parking brake system stops working correctly. Either the parking brake will not engage or will not disengage. To add to that on the control box it shows a digital readout 00 when in park 99 when in gear. As the truck gets hotter the read out will sometimes drop to 60 while driving which makes me parking brake light on the dash flash. I do not want to take a chance with the brakes locking at highway speeds. Also when the parking brakes do act up the control on the dash is no longer effective. If the truck is turned off and allowed to cool down the problem goes away.

I know that there was a recall in later year Topkick for a pressure switch and that has already been replaced.

Any suggestions?