STI units you say
any particular model ? because some are DIFFERENT
Is the grease in there slightly hardened ? Have they been sitting unused for a while ? Or is it that they've been heavily rained and pressure washed over the years and are devoid of any internal lube ?
If its just been sitting then it could be the grease is stiff and needs freeing up. If this is the case undo the lower cover screws about 1/2 way and spray in a tiny bit of a spray lube. These lubes are in a carrier solvent so it will wash away the grease if you use too much, this isnt what you want even though it leaves behind an oily residue that will work but will cause premature wear and you'll have to top it up quite a lot also.
Spraying in a tiny bit then working the lever back and forth till it functions properly ( having the cable fitted is handy to force it to return when you pull either trigger,just keep the pressure up on the cable)
If you suspect its completely dry inside ,take the lower cover off completely. If you have an pace grease gun( rc-7s) youre cooking but otherwise you'll have to get another grease injector from the lbs/ebay and use that. Handy bit of kit the old grease gun
Basically squirt in a bit into the WORKING areas and actuate the lever to help spread it internally.
Any other questions ?
1976 Raleigh Chipper,1991 Muddy Fox Adventurer,1991 Claud Butler Ventoux 400 ,1992 Raleigh Diablo ,1992 Rudy Project, 1995 Cannondale M200 ,1997 Santa Cruz Tazmon ,1997 Specialized Stumpjumper ,1997 Specialized Rockhopper ,1997 Marin Quake 9.0,1998 Marin Pallisade trail ,1998 Raleigh Torus,1999 Orange Clockwork , 2005 Kona Hahanna ,2007 On-One Inbred ,2010 Orange Pure7 And of course The Ti
The Marvellous Mechanical Hope Collection ,and other Hope niceties