If your hydraulic pump is intermittently running then you have a leak. Period. The amount of air in the system only affects the leak rate because air leaks faster than oil. If oil is not leaking out of the system then you have a port to port leak. This type of a leak can occur anywhere there is an actuator or valve.  Before you start rebuilding system components it may be a good idea to isolate the leak by capping the lines and monitoring the pressure decay. A ball valve with a couple of pressure gages "T"ed to each end installed in the line will allow you to pressurize the system and then close the valve to see which side of the valve has the leak.

There are four valves (two on each line) inside the pump where a leak may develop. There are two Over Pressure Relief (OPR) valves and the two ends of the cross piloted check valve.  The internals of the cross piloted check valve are pictured below.. This valve uses the pressure side to pilot the return valve open.  If the pump is not running the system is isolated. Contamination  is the most common source of damage to the valve seats.

Randomly changing hydraulic fluid is NOT recommended. MIL-H-5606 is qualified for use with Buna-N (rubber), the compound most commonly used for elastomer O-rings.  Many ATFs have additives that are incompatible with some elastomers, Buna-N among them, causing seals to become brittle or soft. This is why there are "types" of ATF.  As the legendary Mooney  story goes, any fluid, even urine, can be used for hydraulic fluid, for a while.  If you are interested in long term operation,  use only qualified fluids.

Regarding airspeed and ground speed, it makes NO sense to do performance comparisons using a random frame of reference. Sitting at my desk I have a ground speed of better than 1,000 Kts. Of course, for my ground reference I have selected a famous stationary landmark. The Eagle's descent stage.......at Tranquility Base......on the moon.

Regards
Brent Regan