THM440 slips in 3rd gear

I have a 89 gran prix with a THM 440 trans.I just overhauled the tranny.It works fine untill it warms up all the way and then it slips like crazy in the drive position.1st 2nd,and overdrive work fine.This is the same problem I was having before except before I rebuilt it it sliped all the time.Even when it was cold.I changed the modulator,readjusted it,Checked the vacum going to the modulator.It has 40 PSI at idle.The cluthches that I took out were burnt.They still had some fiber left on them but they were pitted and black from slipping.I,m I gona have to pull this thing back apart or what should I check next?I appreciate any help I can get on this.

Did you air check the drum before installing? You will need to give the VB a once over make sure no valves are stuck then look at the sealing rings and check the drum for cracks

Did you air check the drum before installing? You will need to give the VB a once over make sure no valves are stuck then look at the sealing rings and check the drum for cracksThanks for the info.I think you are correct.I,ve been told to check the third gear roller clutch too.Is there a way to check this thing in the car?Like a oilpressure test?Thanks again.

go drive the car cold and hot and see what the line pressure is then and while in 3rd