Turn the boiler off at the mains.
Find the PV fill valve which looks like a car tyre valve with just a threaded part sticking out which is often red. They can be accessed from the top of the boiler (sometimes under a panel) or through the gubbins behind the front drop-panel. On some boilers (Ravenheat) it can be ridiculously difficult to get at, sometimes impossible.
Press the central pin. If water comes out you need a new Pressure Vessel
Drain enough water from the heating system via a drain cock, to allow the pressure gauge to drop to Zero. Then let an extra 10 litres of water out, or leave the drain cock open if none will come. This may draw air into the system.
Use any (car or bike) air pump with a built-in gauge to pump up the PV to eg (See Boiler Instructions) 10 psi (0.7 bar). This pressure isn’t too critical, and it will as you have found drop over time.
Remove and refit the pump to see how much the pressure drops when you remove it and inevitably lose some air.
If the pump doesn’t have a built in gauge, pump it up a bit high, then keep taking readings with a push-on gauge. Every time you do, the pressure will drop a bit.
During this phase it is IMPERATIVE that water or air is allowed to escape from the heating system as it is pumped out of the PV. Otherwise the system pressure will resist your pumping and give a meaningless reading. If you aren’t sure, let some more water out when you have the air pressure at the right level. If that makes the air pressure drop, pump a bit more and release water and so on, until it doesn’t.
The PV may be say 10 litres. If it was full of water you have to displace all of that water from the system side by your pumping up the air in the vessel. That’s a whole bucket of water!
Use Filling Loop to bring pressure back to 1 bar and switch boiler back on.
Some air may have got into the system when you let the water out but most should go out through the Automatic Air Vent and a rad or two may collect some air too. Recheck the pressure with the boiler off.
IF IN DOUBT CALL AN ENGINEER.
a) DO NOT let water into a hot boiler. Let it cool first.
b) DO NOT let water in too quickly. 20 seconds for 0.5 to 1 bar should be OK.
c) DO NOT take the pressure above 1.5 bar- even if the system is warm as the switch may have stuck.
d) DO NOT allow the pressure to go too high when refilling or you will need to drain water out from a drain cock. Turn it off early to see if the gauge keeps rising a bit.
e) NEVER use the Pressure Relief Valve to let water out of a system – it will leak afterwards unless of course it was already leaking.