Whilst servicing my son in laws car, Ifound that the power steering fluid level was low and needed topping up.

I checked the system for leaks and couldn't find any problems or fluid escapes.

I have searched the various forums and it seams to be quite common for the fluid to need replenishing back to the max level.

I may have found the answer to this in the "manufacturers tips and maintenance book" for the car.

It states:

Power assisted steering

You should never leave the steering wheel at full lock for longer than 15 seconds when the engine is running. The hydralic oil will be heated to a high temperature by the PAS pump as a result of the extended period of the steering lock.

The power assisted steering system could be damaged as a result

Furthermore, the system will make noises every time a full lock is engaged whilst the vehicle is stationary as the PAS pump is placed under high pressure as a result. The idling speed of the engine will also dip briefly as a result.

Is this common knowledge and just me who was not aware?

Hope this is of use to others, and perhaps this is the cause of the disappearing fluid (Evaporating?).

Doug

i've heard the noise and noticed the car idle drop but never really thought about it.

might do in future

Yup knew that, had to explain it quite a few times to customers so your not alone.