pressure on Main Combi Eco

Was doing a few jobs today for an old lady who's just moved into a flat and noticed her boiler pressure was just in the red so offered to top it up for her. First problem was no filling loop - the cold feed and return connections were blanked off and couldn't find a loop plumbed in anywhere else. So I went off and bought a generic loop and turned off the boiler to start draining it down and noticed the pressure jumped back up to a normal level. Tried it a few times and found the same - as soon as the boiler fires the pressure drops. I've only been gas qualified for six months or so, so not too experienced with boilers but have always found the pressure goes up once the pump kicks in. Any ideas people?

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