grant 70/90 outdoor, sealed system boiler?

I have been losing pressure and some water from system for some time now could'nt find a leak any where indoors I suspected air expansion vessel was faulty so replaced it, made no differance then found that the pressure relief valve was releasing water when the boiler was running so replaced it. The new one is doing the same so something else is faulty and causing eccessive pressure or heat in the system i suspect the pump not working properly or fauty thermostat I would be grateful for any ideas