Truck came in the shop needing repairs were not related to the brake issue. Parking brake was operational when it came in shop for an A/C compressor. The cooling fan was running nonstop so the Fan Drive Solenoid was replaced and corrected the issue. Truck was then backed out of the shop for pictures then drove back in because it would not shift out of first gear. Now the parking brake will not release and the low brake pressure light/alarm is on the dash along with a low brake pump pressure code. The external pump on top of the accumulator in between the battery and hydraulic tank will not come on with the key and every test that Cat and I know has been performed with no luck. Any ideas of what im overlooking or not seeing? Any help is greatly appreciated because im absolutely lost at this point.
I vaguely remember a similar fault I had on a 740 years ago, not sure if it’s the same set up for parking on a 725 but I went through all the tests similar to you, it turned out that the hose fitting going into the parking brake valve mounted on trailer unit was actually an orifice, it was blocked. Hope this helps.