I installed my shower system, which consists of a mixing valve with two ins(C+H) and two outs (downpout+shower), everything except mounting the actual shower head and downspout. The reason I left those is because I wasn't sure if their sweat joints need to be pressure tested against leakage. The test I did was just for the ins of the mixing valve, and there is no leak.
I understand that the pressure is much less in the downpout/shower nipples because the water is coming out. But the sweat joint (I used brass dropped ear elbows at the end of the copper branches coming out of the valve to join copper with threaded nipples) could still have leaks (although the leak flow-through would be much smaller because of the less pressure as a result of the release). I was thinking to put 1/2" threaded plugs in each elbow female where a nipple would otherwise go and turn the shower on. If there is no leak in the sweat joint of the elbow, then it's all good.
But I was wondering if this is a safe method, i.e. can the mixing valve be damaged if water is released through it without a way out? My question is, is this the best way to test the downspout and shower nipples sweat connection or do something else?