You can do them yourself easy enough. Take the input hose off the block, take the pressure gauge off the block the couple the two. Start the Cat, engage the hiflow and rev the engine to max RPM's. That should then give you the benchmark. Recouple the hose and pressure gauge to there original places. Then place a block of wood under the rotor so it remains stalled, engage the hiflow and rev to max RPM's. The second reading should be 10% lower than the Cat. If the pressure is any lower then the problem is in the hydraulics in the mulcher NOT the Cat. Seeing as the hyd motor has been serviced the control box may not have been and that is the first place I'd look. Adjust the pressure relief valve and see what difference that makes. Your FAE operator manual should explain how to do this. If there's no change it may only a O ring problem a cost maybe $20 to do yourself. I assume the Hydraulic motor service has some sort of warranty so if it's any more that what I have explained then it should go back to the last fixer and you are armed with some facts.