The water valves are tough to service even if you have the parts to get it done..Pay close attention to the O ring sealing area between the valve and core it needs to be perfectly round and smooth...Some will give up trying to keep it from leaking on the floorpan and do what you have there totally shutting off the water...In my experience most times it's the valve being the problem and the core is usually good hopefully that's your case...I get the whole heater box kit from DMT and refinish the blend doors for new sealing foam and smoother operation while I'm in there..