In a few weeks time with cooler weather you will probably be reversing this and reconnecting the heater but still with the same problem. Surely far better to replace the valve, speak to Chris @Redline Components, he has them in stock and nearly always next day delivery. If you raise the car as high as possible at the rear and syphon all the water out of the expansion tank you will have little water in the heater matrix to clear up after disconnection. After any changes lower and refill expansion tank.