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.

Check the park brake solenoid wiring harness adapter... I'm guessing you have a newer era truck. Solenoid has 4 wires vs 2 wires for the old era. New the solenoid head points towards the mast and is black in color, old era is screwed into the valve body of the parking brake valve body is silver and points to the radiator. I just had the same issue happen with a F88 code. I used an ohm meter and checked for a short to ground within the wiring harness, and determined the issue was the wiring not the parking brake. I also checked the pump pressure and changed the hydraulic oil and filters which made a bit of difference. Also check the suction line to the brake pump.. if it gets air around the clamps you will get weird readings.

Any chance someone has a little more info on this code? And a code F90? No electric park brake.

This code is low hydraulic pressure to the park brake system. The truck should be taken out of service as soon as possible. The park brake is an internal clutch pack system. You will probably need a manual for the pressure test procedure. The F88 code can also be set if the truck runs out of fuel and the park brake pressure drops below 700psi.