AWC 6 Bar pressure relief valve passing water?

Water seems to pass only on morning initial start up of Worcester Bosch Greenstar 12i/24i system boiler. I have stripped the valve and it didn't seem too dirty, but I cleaned it and reassembled. This seemed to do the trick, but a couple of days later the same thing is occuring. Will I have to renew the valve or could it be something else that is causing the problem. Any advice gratefully received.