I have a Nilfisk Alto domestic pressure washer that does the same. (Motor cycles with lance off so losing pressure somewhere) It is under warranty so I took it to a repair agent. After replacing seals it still didnt work and it is thought the motor has been damaged by frost as I didnt drain / insulate it before winter.
The unit still works but doesnt reach full pressure.
This answer may not help but if your washer worked before winter but doesnt now it could be the cause.
Also worth checking the lance nossle is clear and inlet filter.
Keith, May 2009
