I soted it out guys. I contacted Forge Motorsport and talked to a guy called Peter Miles. He said my dump valve is so old that it's almost a classic! Since their dump valves are guaranteed for life and there is no repair kit available that can guarantee to work for my dump valve, he's gonna send me a new twin-piston ram-type dump valve FREE OF CHARGE! Excellent service ain't it guys?!!

Thanks for the help