keep breaking gate valve stems

The crews are newly formed and do not have alot of experience. Just recently we have found 4 broken 24-inch gate valves. I suspect the crews are using a hydraulic operator and are setting the force of the operator too high.On a 24-inch gate valve, probably a Chapman installed in the early 1900s, what is the force that the hydraulic operator should be putting on the stem? The water utility I work for has purchased a couple of Wachs valve operating machines. A 24-inch transmission main is being cleaned and cement mortor lined. There were two 24-inch gate valves with broken 4-inch bypass valves. I had an operations crew go out and check all the valves on near the two broken valves. Typically all valves are supposed to be open. The crews went out and tried to close & open all the valves with the valve machine. In 2001 someone else had operated the valves and closed the valves with about 2500 pounds of force set on the machine. This year three more valves were broken with about 1300 pounds of force. We are petty sure the stems are breaking. I can not (do not know where to) find any information on how much force should be applid to the valves when operating them. I have not confirmed this, but I believe the stems are 2-inch diameter with acme threads made out forged bronze alloy.
From the originalgate valve