What's wrong with my karcher 620?

Hi
I have got a karcher 620 had from new so a few years old but it's not had a lot of use it stopped working a few years ago so left it in the garage to gather dust. Yesterday I decided to take it apart to see what was wrong with it i didn't realise there would be oil in it so a bit of mess, once apart I cleaned it all and now it's like new but still not pressurising the water, the non return valves a seem to be seating correctly the motor is turning when switch on so I filled it up with oil again and it back together and still the same fault.
It's just not pressurising the water can anybody give some advice.
Thank you