Water side economizer issues

We have been having leaks appear on our econo coils. The units are mcquay swp air handlers with water cooled condensers and water side econo coils. This only happens in the free cooling seasons. The leaks are appearing in the clean out ends of the coils on the rubber o-rings. Coil rating are 400 psi. The piping to the unit also have motorized isolation valves on both the supply and return lines that are energized with the units cooling enable contacts. Our tech noticed that pressures on the lines running we're 175psi but when the isolation valves close the pressure in the lines as well as the coil slowly builds up to between 400-500 psi. My question is could the valves when close creat no room for expansion when the water sitting in the coils that was cold being warmed up by the return air of the air handler cause that much of a pressure increase. Or am I looking at this all wrong.