Install a water tank without losing main pressure
Your check valve is in the wrong place. 

If you're going to do this, you should use three valves -- one to automatically fill the tank, one to maintain the water pressure in your house, and one to keep your water stash private. 

Install a float valve to shut off the tank fill when the tank is full. 

Install a check valve to prevent water from flowing from the house plumbing directly into the tank while allowing the tank to empty into the house plumbing (right side of diagram below). 

Install another check valve to keep your tank from emptying back into the public water supply during power failures (left side of diagram below). 

The combination of these three valves allows the tank to hold water at a much lower pressure than your public water supply, which is necessary to keep the tank from overflowing.