thetford cassette toilet c-200

hi everybody i have a thetford c-200 toilet fitted in my motohome which doesn't flush, i can here the pump going but no water comes through the flush. I've changed what i believe is the pressure valve but still no water coming through. The van is 23 years old so i'm concerned that the wiring may be the problem but i know absolutely nothing about wiring. Am i right to assume this if so can someone please tell me how to rewire the toilet or has antbody else had a problem like this who can enlighten me further.

thanks wayne