Ideal Classic Pressure drop...please help!?

I have been trying to fix this problem for nearly a year with no success. The heating works fine till it goes off and then within a minute the pressure drops to 0. We had someone out to service the boiler and said the expansion vessel was ok and it must be a leak...no signs of leak anywhere so we used an internal leak sealant... didn't work ..the radiators have been bled..don't know what else to do ..don't want to call another bloke out only to be told it's a leak nothing he can do again.
Is there anything else it can be? Can anyone help me get to the bottom of this?