Hi. Newbie here. My BIL gave me a .38 S&W revolver, model 10-5, K-frame. It is in nice condition, however when I manually hold the hammer back, it doesn't catch and stay in that position. I took the side plate off and nothing appears to be bent or broken. Some have suggested that police depts had their Model 10's modified to DA only. Does anyone know which parts I would have to change to restore the single action feature? Thanx in advance.