Why wont my 1978 Chevy c10 shift into passing gear?

[justmetoday86]

Ok I just bought a 1978 C10 chevy shortwide, very nice truck. I'm having a problem with it going into passing gear though? I just want to know if anyone knows how to fix it or has simular problem. It has a turbo 400 transmission with a stock 350 in it. I plan on making it a beefy daily driver. I'm really good with motors but not much knowledge of transmission. Any help?

 

[CarmanNguyen]

Vacuum modulator,cheap and easy to replace,first to be sure there is vacuum and hose is not off.