Simspon Riviera continually filling with water?

My approx 20 yr old Simpson Riviera washing machine is constantly filling with water - the level in the machine stays the same as the water drains out throughout the agitation cycle. I'm not very mechanically-minded, but ideas of causes, cost of repairs, can I or a friend do it, are most appreciated.

Extra info - I've just started using the machine again after approx 3 years in storage.

Thanks very much.