Before you pull anything apart, check all the things mentioned and make sure you know where the problem is. I don't know how much of your own work you do but having an expert do the repairs or at least the more delicate parts of repairs and maybe the diagnostics could save you time and money if your not sure what to do. I have had a few customers who haven't rebuilt trannies bring stuff in that they tried to fix only to have to charge them more to repair things because things went together badly. (eg, having to replace a valve body because someone had it apart and mixed everything up, it's not always possible to find manmade problems. It could take hours just to find mixed up springs, whereas wear and tear and common mistakes are usually obvious to a pro).
Just a warning to you about pulling the pump yourself, in case your thinking about it. It would probably be cheaper to take the trans (and convertor) to a shop and have them look at the trans if it comes down to it. If you put the pump in and have something bind up on you it will wreck something internally. If your doing it you need to check clearances, seals, gaskets, clutch teeth are engaged, just to mention a few, get a good book on it.
Any chance you could have the original builder help diagnose it, they would probably be more than willing to help, afterall, it's their reputation on the line if it really was rebuilt right?
1971 Buick Stage 1 iron heads 11.02
(now going to Stage 2 with solid roller)
Thank you, Larry Johnson
Westshore Transmissions Ltd.
1081 Dunford Ave.
Victoria, BC V9B 2S4
(866)459-7070 or (250)478-7070
ATRA Certified Journeyman Transmission Tech
Victoria Transmission Repair
"Where Quality is Automatic and Good Service is Standard”