I have an Elddis Firestorm 505 built around 2004. It has a CRYSTAL Maxi Pump and I was experiencing exactly this problem. Under the seat I found a blue in-line non return valve that was easily removed but worked fine.
However I noticed in the housing where the pump plugs into the caravan, that there is another non-return valve (NRV).
A very helpful shop in Alcester sold me a Water Inlet and Non Ret/Val MI101036PK (£7.50)which I had to dis-assemble for the inner part. With a pen and an assistant outside / inside the caravan I was able to remove the black O ring (visible inside the caravan) and then replace the spring unit and O ring. This solved the problem.
I hope it's of help to you too.
I've had the same problem, bought a new non return valve, still the same. bought a new drain tap, still no luck.
PROBLEM FIXED bought a truma sring non return valve 40060- 88000 Perfect.
Brian .