If there is a spur on the hammer, and I cock the hammer and it doesn't hold, I am going to assume the interior parts are either worn out or broken. End of story for me.
As an aside, before purchasing an old gun I always cock the hammer and with my finger off the trigger put some forward pressure on the hammer with my thumb. Of course, before working the action on any firearm I first ask the owner/dealer if he minds and I have of course determined for myself that the gun is unloaded. If I can't work the action, I walk away. If thumb pressure on the hammer causes the hammer to fall without my finger on the trigger, then most likely the hammer or sear is worn. I consider any firearm that will not hold full cock to be unsafe. Just this week I was about to buy an old S&W Regulation Police until it failed my thumb push test. I thanked the dealer, whom I know fairly well, and gave the gun back to him.