vailant 824 combi?

i keep loosing pressure ,the filling loop is not passing as i have just checked. the higher the the heating temputure i set it on the boiler the quicker it moves up to 3bar and blows off , within half an hour normally, if i set it at approx. 60 it may take 24 hours to lose its pressure, can anyone help.. i am a ex installer but not to sure of internal problems....cheers