High Pressure in the Coolant Expansion Tank

Sorry earlier I was not paying attention... I was writing in english and trying to listen to a someone speaking in French which is horribly confusing to my old brain.

it almost sounds like some kind of airlock or vapor lock in the system. The system heats up and coolant expands overflowing then later you have a low level. The high pressure would indicate somewhere things are pretty hot as Marmot suggested.

A long time ago I ran into something similar with similar and we could not figure it out for some time as there were some long pipe runs to the heat exchanger and we could not get the thing corrected. The expansion tank was the high point but the system had some pockets acting like a sink drain j trap trapping air. We did solve the problem by using a vacuum pump to evacuate the system to about 1/4 or less of an atmosphere of pressure absolute, and sucking the coolant in from the bottom of a container... as we could never fix the air pocket or locate either.

An air block is certainly possible. If there is a low point drain you might attach a fitting there and fill from the bottom.

Can you please advise how you removed the airlock as I seem to have a similar problem.