[71C10Guy]

I have a '71 C10 with a 350 and 700r4 swapped in. A few years ago I was pulling out of my driveway and suddenly lost drive and all other gears. I ended up letting it sit because I had just had the transmission rebuilt and I was pissed, then life got in the way and the whole ordeal was put on the back burner until recently.
Well I figured out the fluid pump isn't pumping by pulling the cooler line. So I'm wondering if anyone can give me advice on figuring out what in the pump went wrong and if I can fix the pump itself or if I need to just get a new one?

I don't know what the shop did when they replaced the transmission, it was a small shop in a small town but they claimed they had experience with 700s. I'm thinking there's no way they cracked the pump when installing the transmission as it lasted about a year and a half before it went out, and as far as I know it would have never worked if they did that. On the receipt it said they replaced the transmission with a reman instead of rebuilding my original one.