well then i would double check the pressure relief valves and make sure the system is not set too high causing it to lock up. It may be exerting too much stress on the coupling causing it to break.

One other thing important to remember is to lube the coupling if it is all metal when installing, if there are any rubber parts then do not use regular grease, i would use lithium. If you are installing it dry that may be causing premature wear weakening it.