If you have a bad check valve from one of the bays ( which sounds like the case), when you put the check valve inline from the tank to the distribution panel for the bays, you should definitely find out which bay is causing your problem. More than likely, when one of the bays are using a high pressure function, the check valve is not sealing and letting some of that pressure flow back through the spot free line and into your holding tank. When you put a check valve inline from the holding tank, that pressure has to go somewhere. Depending on what type of hose you are running to the bay for spot free, it will more than likely blow that line.