Ariston SX20M combi boiler help please?

[acottrell]

Can anyone help me? The boiler loses pressure when using central heating. I cannot find any leaks however water is being released from the pressue valve outside. I have replaced the valve but still it comes out. If i use the hot water only the preasure drops after about a couple of weeks. If i use the central heating its goes down within a couple of hours.I cannot find any other leaks from pipes or taps etc.
Thanks Ade

 

[twgas]

pressure/expansion vessel needs recharging - see FAQs

 

[PEDANTICVINDICTIVEMAN]

Take the dust cap off, press the plunger in the middle of the schreider valve, if water comes out the Pressure vessel needs replacing, if air or nothing comes out it only needs recharging.