worcester 9.24 rsf pressure problem?

If I start at pressure 1.1, when I switch on it rises to 1.6, then switches off at the right temp. Every time it switches on and off again, the pressure rises slightly. I left it on for 4 hours, then switched off and let it cool completely. The max. pressure reached was 1.8 but when cool was now 1.3. Does anyone have any ideas?
Thanks