sounds like most problems could be associated with a pressure vessel fault. You say when heating goes off (ch system cools) pressure is down.
If you noe top up to 1 bar and run central heating, what reading would read after say half hour an hour???
Will it be reading 3 bar +? Is it letting water out the relief pipe outside during this time?
If yes then pressure vessel fooked or flat and needing recharge.
t
February 2010