Ref Maxol 402 boiler switch off?

Just moved in and the Maxol 402 boiler keeps cutting out after removing rads for painting. Have bled the system but boiler is maKing a squeaking sound. There are two relief valves above the boiler that allow air to escape which I've also tried to bled. One shows a build up of calcium/lime scale as if it's been leaking. I'm sure it's air but can't get it out of the system. Any ideas please?