Yes the 73 models did have a water valve with bypass , but only cars that had factory A/C , if you have just the heater you are right , no valve or bypass and water ran through it all the time controlled by the manual damper door , wound up with the NOS valve I have when I worked at the ford dealer at the time I had a 73 with air , got two to have a spare , been carrying this one around for over 40 years , one day someone will need it along with a never installed pinto AM radio ,