Part 2 of diagnosing F88

So if you have 2 or more trucks that are the same, swap the oil pressure sending unit between the trucks, however in doing so you have to bleed the drive axle brakes and the parking brake as well... Parking brake bleeder is on top of the transmission underneath the transmission cooler lines running over top the trans near the park brake valve body. The issue I had stayed with the part due to swapping it out between trucks, another thing I found was the rail pressure would drop slightly due to brakes being applied hard thus causing the F88 code to pop on... So I adjusted the main pressure up from the 1308 psi book spec to 1450 psi ish... Retested and the main pressure stayed above or at 1300 psi. I also replaced the reducing piston and spring. (Also had custom hydraulic fittings made to install 2 gauges to catch my pressure drops). If you cannot find the correct thread pitch, use a CAT forklift grease fitting and modify the end of it to fit inside either a JIC or JIS hydraulic fitting, have it brazed and it should work. Mainly though the pressure sending unit is the cause of the issue. Bleed the park brake by releasing the park brake switch, have the seat switch engaged, and open the bleeder very slowly or you will get covered in oil. Watch the dash light, if it goes out and comes back on the switch is failing. I spent 5 weeks messing with 2 trucks, 7 weeks between 3 trucks, one had a broken parking brake assembly inside the transmission... When one goes they all go.