Dunkirk Furnace pressure too high

I am a homeowner with a 22 year old Dunkirk forced hot water furnace with 5 zones. We recently had leakage from the overflow pipe because the pressure was too high (over 30psi). The furnace works fine and we have never lost heat. Regardless, the repair guy came to look at the pressure & the water overflow. He replaced the overflow tank and the release valve (?). He flushed the system and checked to make sure the water flow was sufficient (it was). He says the furnace works fine but he can not figure out why the pressure is still at 30psi. He called Dunkirk (the manufacturer) and they had him troubleshoot but even they can not figure out why the pressure is still at 30. The furnace is working fine, the heat comes on and off with no problem. The only thing that may or may not be relevant is that the water that overflows seems to be high in minerals because once dry, there are powdery deposits on the floor. They told me to "wait a few days" & watch for leakage again. Any ideas?