Most common problem for this condition is the 3-4 clutch pack just being worn out, it's definitely a weak point in these transmissions. Sometimes the machining is bad where the pump slides over the input shaft teflon seals too which will cause lower pressure to the clutch plates and they'll wear prematurely.
What parts did you give the person to "rebuild" it? To get the pump off you need to pull the pan off after you get the trans out to get the TCC solenoid out before hand. To get the TCC solenoid out you have to take the TCC PWM valve out first. It's held into the valve body with a clip that can be taken out with a small pick. They tend to go flying if you aren't careful so be sure to watch where it flies if it comes out quickly.
Then take the two 10mm bolts out that hold the TCC solenoid in. Pull it out. Then you'll have to get the bell housing off. They are a T50 Torx+, not a regular torx. After that's off the pump comes out after the 7 13mm bolts are taken out. Be sure to take the small seal off the input shaft before you lift the pump up. There was a time where our special tool was MIA after a renovation in the shop and I had to get them out without it. Sometimes it's easier said than done. Some will slide up fairly easy and others can be a bit of a pain. Grab the pump shaft and wiggle while pulling up and it should come out. Sometimes I'd take a screwdriver with a rag over the tip to gently pry up on the edge of the pump to help if it was stuck.
Once that's out, pop the 2-4 band off of it's anchor pin in the case, pull up on the input shaft and the input drum and reverse input drum will all come out as one assembly. Take the reverse input drum off of the input drum. Be sure to keep track of where all of the thrust bearings and washers go. The 3-4 clutch back is the top clutch pack in the input drum. Take the snap ring out and the top plate. Remove the boost springs from around the outer edge of the drum and remove the clutch plates. Chances are they're smoked and you'll need new fibre and steel plates.
Getting everything back together can be a bit tricky when trying to get all the clutch plates to line up. Put the reverse input drum onto the input housing, make sure the thrust washer and bearing are in place. Rotate the reverse input drum back and forth and you'll feel and hear the clutches dropping down. After it's fully in place then place the whole assembly into the case and do the same thing, this one is usually a bit trickier. Once that's all in, put the band back in place and now you're ready to put the pump back in. Use vaseline to hold the plastic thrust ring onto the bottom of the pump when putting it in. Make sure you use a new gasket for the top and bottom of the pump. The one on the inside is a paper type and the top one is a large ring that the bell housing holds in place. The torque specs are 22lb ft for the oil pump bolts and 52lb ft for the bell housing. Reinstall the TCC solenoid and TCC PWM valve. Put a new filter in, pan back on and your done!
Wow, I kinda got carried away.
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2000 Silverado ECSB- 5.3 CAI, Pacesetter LTs, ORY- Pipe, Magnaflow XL, E-fan, Ported T/B , Nelson Tuned, Built 4l60e, Circle D Stall, 3.42s G80, 2-4 Drop, Hertz Highs, Rockford 501s 
Originally Posted by LamboDoNoMo
