TH400 no 3rd

[jccustoms]

Hey guys, lets start off by saying this TCI Th400 has been a pain in my a**. Here is what i have:
68 Chevelle
496- 750-800hp
3.73 12 bolt rear
Th400 full manual reverse shift vb
3500 stall
I purchased the transmission before my build was done so i dont have a warranty on it. from day one i dont have 3rd gear. i will try to make this short.... i swapped out the valve body with a new hughes vb thinking i had issues with my 2-3 valve, i swapped it because summit gave me my cash back for my old one since i purchased the trans through them. still no 3rd, ive checked and checked and checked my linkage and it all good. i got frustrated and took it to the local transmission shop and they said they found a slightly torn center support seal (cost me $600) guess what??? still no 3rd. oil level is correct. Hooked up pressure gauge and there is no change from 2nd to 3rd, like it just stays in second. any thoughts or suggestions would greatly help.

[Bigblockscott]

What fluid are you using in it?

[jccustoms]

The first go around was TCI's max shift ATF, it has Valvoline synthetic ATF in it now

[Bigblockscott]

Many years ago in my TH400 I ran Royal Purple ATF. Trans lasted about a year before I started losing gears. Pulled the trans and brough it in to my builder, he told me to not run synthetic fluids and to get just regular old parts store brand Dex III. Been close to 8 years now living behind a 600HP 496 BBC in a 3700LB station wagon. It gets "John Force'd" every time I drive it, so its lived a pretty rough life, but still going strong.

Did you verify that the trans dip stick is correct with the pan off?? Lots of times "full" on a dip stick is not correct.

[jccustoms]

Thanks for your response, ive double/triple checked the dipstick as well. full line is at the bottom of the trans case, if not a hair above