We recently drained the system to repair a leak. After refilling the system, the boilers started leaking from the pop off valves. The pressure at the boilers is 45 psi and sometimes goes up to 65 psi when they are heating causing the pop off to drip. The part that I don't understand is that I have 12 psi on several parts of the loop. I read 45 psi at both boilers, at the cooling tower, and outlet of the pump. I read 12 psi at all of the WSHP units, and on the return side of the pump. I believe all the air is out of the system and the expansion tank is okay. I figured the problem was in the 3-way mixing valve but after disassembling it and checking everything, it seems to work properly. Can anyone tell me what I'm missing? I don't have a lot of experience on this type of system. Thanks!