You are correct. Entrapped air is the problem.
You will have to cycle the wheels a lot more than ten times. Do not hit the bump stops if you can help it. The closer to the stop you get without actually hitting it the greater the volume of fluid you will move (next to nothing at center) but you want to stay off the stop if you can; as it will damage the pump with foam as a lubricant instead of fluid to protect it.
The spindle valve inside the gear box acts like a ramp with greater fluid flow the closer it gets to the stop. It is actuated by a rack and pinion that behaves like a poppet valve with flow increasing as it lifts off the seat. As it travels through center it is closed off isolating each circuit and doesn't flow until the valve starts to open in the other circuit to move the wheel back again.
You also have to verify there are no leaks as the lines will suck air on the return side, but few see a problem because there isn't fluid squirting out under high pressure.