honeywell am-1 mixing valve issues

I have used these valves in the past with no real troubles to speak of.But recently we had several leak at the top where the adjustment knob is.You would try to adjust and the hex nut would move a little and water would start to drip and you would think if you tightened it would stop. Nope,it would shoot out and no extra o ring.Several others the o ring would blow out where the unions connect and no heat was put to valve.At least 6 of these in a row on new installs costing us time,aggravation,$$...thx