worcester 240 combi please help

[diamond007]

Hi my combi kept loosing pressure and ending up at 0. so i took the front off etc and found the leak comeing from the air relieve valve in top left corner.
so i fitted a new one
This has stopped the leak but it now looks like i have another prob the pressure keeps going beyond 3.5 bar and when it cools back down it is at half a bar so i top it up to 1.5 then it happens again.
Any ideas of what it will be. and am i damaging the boiler by keep topping it up

 

[bobw1956]

Check the expansion vestle, may need represurising,or the diaphram may be passing water,in which case a new ex;vestle needed

 

[powell30]

Also might need to replace the PRV as constant lifting as would happen in your situation weakens it.....change it (10-15 quid if that)