auralisRS ... I don't know if it's any help to you but I just overhauled the PAS system on my 3 door (see link below )

The pump feed hose (reservoir to pump) was cracked in several places (even though it was not leaking) and the small diameter return hose had become porous (even though it looked alright) and I was gradually losing fluid from there (along the length of the hose, not just one point). I don't see how this would let air in though as the system operates under pressure

Whilst running it (to leak check it) I noticed that the fluid splashes around a lot (no bubbles though) and the internal of the bottle has two sections so the returning fluid doest mess the supply fluid ... I know this cause it was a bastard to clean

Do any of your flexiable hoses appear wet with no obvious damage ? Are all your clamps tight? Is your pump noisy (or vibrating)? Pumps hate air in the fluid and this may indicate a fluid supply problem, or maybe your pump is at fault ...

Maybe the cap is leaking and letting the pump draw in air through the reservoir ...