STI shifter problem

[dude]

I have one of these old STI shifters from the early 90's

https://drive.google.com/file/d/0By-S838OebDHUnRnSDd2b1dRajQ/view?usp=sharing

I can shift between the center and small chainwheels, but not from the center up to the big one.
The button goes all the way without resistance.

Any way to fix this or is is screwed for good?

Thanks in advance!

[Painkiller]

its about a 50 50 shot at saving it. remove it from the lever, remove plastic bottom cover. do not remove nut holding the guts together. pic out any old grease you can. then spray the crap out of it with wd-40 to try and free up the pawls. then spray with engine cleaner scrub the best you can with an old tooth brush, rinse with hot water, dry well with hair dryer and re lube with oil, not wd-40 as it is not a lube. If this doe not work it is a lost cause. I can dig around and see if I have an extra you can have if I have one.

[dude]

I'll be away for a few days, but I'll definitely give this a try when I'm back.

Thanks a million!