Chaffoteaux + Maury help/advice?

Hi, I have a Chaffoteaux + Maury but no idea what model, It heats up the rads and water but when it gets to the temp of the thermostat is set at it tries to come back on but crashes so i have to reset it. The pressure is very low, no leaks anywhere that I can see either. We have just moved in an worried that it's going to cost a fortune to fix.

I have a few pictures if that would help.
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