ok my silverado originally came with the 305 so i got rid of that and put a 350 but from the time i purchase the truck the a/c never blew cold but on the other hand the heat came out nice and toasty. A while back i got some freon and add and that had the truck pretty cold. but after a while its back the warm/hot air. i have heard i mite be a leak some were but were can i start at for it.
You could get some die like the shop guy would do and then look for the die ,that will show you where or just check the seams welds & treades at the nuts, lines.Oh a violet light can help see the leaks the darker the better when using the light.
Try and look at all the hose connections. Leaking refrigerant will show up as slight dirty oil leak at the source. Many times it will have more than one leak. Look at the back of the compressor near the manifold (where the hoses attach), at the condenser (where the hoses attach), at the drier (looks like a aluminum can), at all hoses and hard lines for chaffing, also the low and high pressure switches can possibly leak. The Orings behind these are easily replaced, but the R12 / R134A will have to be evacuated first. Black Orings are R12 and Green Orings are R134A, they are not interchangable between refrigerants If all that looks okay its possible for the leak to be in or at the evaporator, expansion valve. As suggested, the dye method is the best, but the truck will have to be taken to a service center. All this may sound overwhelming but you can do it. Let me know if you have any questions.
