i work on them for a living.

if you take it to bits, there should be an electric pump (possibly an engine?)
that should drive a pump head.. if you got the bottle to take it to bits
there should be some little pistons inside this pump head (dont take the electric/diesel engine to bits ).


the problem will be either the little O ring seals around these pistons.
Or possibly the 1 way valves that let the water Into the pump head..

very easy to fix mate