That valve is there for a reason. If you redirect the bypassed water back to your water tank, it will heat up the water in the tank, especially if you have a small tank and even more so as the tank level drops towards empty where the hot water has a greater effect on the remaining cold water.
The purpose of that valve is to protect the machine from getting excessively hot and when it senses the temp is too high allows hot water to be dumped into the recovery tank. So if by redirecting it to the water tank the machine cannot cool itself because instead of getting cold water to cool itself, it's getting warm or hot water, you end up with a situation where the machine gets ever hotter and end up with a very expensive problem on your hands. The valve may only need to be taken to bits as sometimes they clog up with junk and get stuck open. But it could also just be doing its job and be symptomatic of another problem occurring in the system.
Simon