Broken pressure washer Karcher K2 (Oakridge - RG21)

Pressure washer - broken. Karcher K2 Compact.
Its in parts at the moment as shown in the second picture.
Motor runs but not pressure is built up. The motor runs non stop, not just when the trigger is pressed.
I've taken it apart to check, nothing obvious. I suspect the cylinder part head needs replacing. It will also need some new oil to put back together.

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