i have a built TH400 with Vig 3600 non/lock,TCI Flex plate, and Reverse Manual valve body also a trans brake
And when i put it in reverse it takes a few seconds and then shifts REAL hard and makes a clunking noise, and all of a sudden today i try to shift into third and when i put it in to third it seems like it wants to downshift, or not go into third. what could be wrong with it????

Please Any Suggestions

since the trans brake uses the third gear clutch and so does rev. l think the problem is failed third clutch pack.

Who makes the valve body? Some companies recommend you dont use reverse to back up but put it in nuetral and push the trans brake button to back up. You most likely dont have any engine braking in 1st and 2nd so if you are coasting along at 30mph in 2nd and put it in 3rd it will feel like its downshifting.

If all this started happeneing all of a sudden and it wasnt like that before most likely something let go as John suggested.

how can i find out if its just the third clutch pack and how much will this cost to replace

i also forgot to mention it doesnt feel like downshifting in third till you give it gas in third, if you just put it in third at like 30 it wont do anything till you give it gas

if your third is gone you will be in second and will free wheel or coast and will lock when you give it gas

so if my third is gone how much does it cost to get it fixed or should i just rebuild the whole tranny

the problem is that fixing just third is not a trans guy whats to do because you'll want him to guarrantee his work. that to him means if second breaks he'll have to fix it for free. not many folks like working for the fun of it. l'm afraid you'll have to pay someone the full tilt, sorry

how much does a full rebuild cost

ttt!!!