Our 2000 ML430 has intermittent loss of power steering when manouvering the car at very low speed, such as when parking, ur doing a U turn from a standstill. When driving the car normally the steering is okay, but I think it's a bit heavier than it should be.

There are no fluid leaks that I am aware of. I have replaced the power steering pump and the O-ring seal for the fluid reservoir. I have also bled the system several times.

Any ideas about the cause of this problem would be much appreciated