safety check for followthrough

so after about 18-20 years sense trigger job I am getting follow through to half cock but not every time going to take apart an see if I can tell which surface needs attention , so what surface is most often bad? (colt series 70 about 1977 I think when I bought it still has original sear hammer disconnect and grip safety)

More than likely the soft sear wore and is not catching the hammer like when it was a young pup.
Jon

Could just be your trigger stop screw. Try loosening it a bit.

I can't see any thing that looks obvious so I touched hammer and sear with stone and put back together same thing, only get followthrough when releasing slide with finger off of trigger and it only goes to half cock.
so what is the best brand for drop in setup. thinking about KC's CMP roll trigger but not sure I want roll, what would be next option?

Your sear is probably too short.  To fix like situations, I've used a few Nowlin matched hammers and sears.  Dropped right in and trigger pull was excellent, less that $100.
Phil

thanks for all the help
Decided to dip in to Roll trigger world, ordered the Battle Axe from KC

Somewhere, I think on this list, it was suggested that what you experience could appear if there was too little take-up in the trigger.  Can someone confirm or deny this as a possibility?  I have a wad gun that does this if I drop the slide on an empty chamber.  As long as it has ammo to slow it down slightly, it holds fine.

I've experienced too little takeup as well as too much overtravel allowing hammer to fall to halfcock.
But other factors also can contribute.
Jon