McAlpine Silentrap Leaking

I recently discovered a leak under my kitchen sink. After much checking it appears to be coming from the relief valve in the Silentrap. I removed the fitting and cleaned out the valve which was really full of gunge. The seal appeared to be in good order so after cleaning I replaced the fitting but it still leaks.
I notice that the sink has been fitted so that the outlet pipe from the trap rises, which means that the relief valve is always full of water. It has worked satisfactorily for over 20 years so why leak now.
I can fit a new trap but is the fact that the valve always is submerged a problem?

 

Leak getting worse so had to act.
Replaced McAlpine Silentap with anti-vac bottle trap.
Quick, simple and nothing to leak. Problem solved.