if theres a leak this is what causing the pressure loss. u will need to get this fixed before the pressure will rise. the pressure vessel is like a tank with a inflated balloon in it. when the pressure increases the balloon decrease in size to accommodate the expanded water. if the balloon is deflated or punctured then it wont handle the increase in pressure causing a surge in pressure causing the water to leak outside through the pressure relief valve. my guess by what your saying is that the pressure vessel has a leak on the outside tank and will need replacing.