Suzie, you are right about the backflow not being the problem. This has nothing to do with the pressure of your boiler system. The pressure of the system has a lot to do with ho high the highest point of the loop goes. If you have a two story house and the loop goes top floor then the pressure needs to be the static pressure of the water plus 4 psi. The static pressure of the water is the weight of the water that is pushing down on the system from top floor. The higher the loop goes in a structure the higher the static pressure. I would say if the system is heating good with the current pressure I would not worry about this pressure as long as the pipes are not making any noise and the pressure relief vent is not venting. If you have problems with either of the things mentioned then yes drain the tank if you have a steel expansion tank. Turn the boiler off and isolate the tank from the rest of the system. If you have a bladder tank then no.........you need to call an expert for this. I would say you need to call an expert anyhow but just draining the tank is no big deal. The tank will only cause problems if it is completely waterlogged.

Good luck