Just buy either overhaul kit or a new pump assy - replace all the bits and hey presto. Takes about 40 mins to do the kit (depending on your access). Less to replace the whole pump.
Done a few now and its usually been exactly the same fault as yours!!
I've just sent Jabsco an email about the same problem /forums/images/graemlins/shocked.gifThey sent out the same day a new top valve flap and a spring.Whilst fitting these I noticed there was no spring originally fitted,once fitted it works really well with no priming required total cost SFA /forums/images/graemlins/grin.gif