How can i fix my Ravenheat CSI 85 AAA (T) 780 series?

can anyone help me please? my ravenheat CSI 85 AAA (T) boiler works absolutely fine in winter mode but when i switch it to summer mode to turn off the heating side of it it looses presure and leaks from the PRV. Goes down to zero pressure then the hot water wont work boiler shuts itself down.I can refill the system works fine again for a little while like an hour water pressure drops again boiler shuts down again.I dont want to keep the heating on all summer just for the boiler to function properly.Many thanks in advance for any help recieved