Assume you had it in bits - there is (well, on the Yam unit, but I have a wee theroy they all buy them from the same supplier!) a non return / releif valve which is very easy to loose or unseat the tiny ball bearing that operates it if the unit is in pieces.
Also from memory there should be an over pressure releif valve somewhere i nthe system to stop it blowing up when the piston (s) hit top. I rebuilt mine with that valve wrong way round first time!
Does it sound like it's churning fluid? (i.e it's pumping round in circles rather than not seeing fluid?)