THM400 Not Shifting into 3rd Gear??? HELP!

Please Help us, my dad and i just purchased a show quality 1955 chevy and are having a problem with out tranny . . .it WILL shift into 1st and 2nd right away, but WON'T shift into 3rd . . . the vacuum line is connected, it has a saturday night special torque converter, and most likely a shift kit . . .what can we check? Should we put in a tranny that is lying around our garage that works, or could it be a minute problem that we are overlooking?

any help will be greatly appreciated
thanx

We Would like to say thankyou to however it was in this club that anonymously sent us a tip to check the vaccum line. I disconnected the rubber line near the tranny while my dad blew really hard on the carburator end. A chunk of gunk blew out and we now have suction, and best of all we have 3rd gear!!!

Thanx To All
marc and jerry