72 Transmission information

[saddletramp]

Hi Guys i need alittle help my 72 C,20 has a Turbo Hydra -Matic this truck had a manual transmission in it the PO swapped to the Automatic i dont no much alse about it i hoping to get some answers while driving it it shifts almost to 20mp then between 30 mp after that i dont feel or hear it shifting again when should it shift again ? does it shift when it you get,s up to highway speeds with more vacuum and i was told its a very sought after transmission can some one explain this transmission for me much appreciated. Saddletramp