sound like you have a internal leak ,i have made a adapter to my leakdown tester so i can test the ports in the trans (valvebody remowed) ,but that still not test the valve body
in the past i have a weak shift to third (turbo action vb)
and burned frictions ,change to a griner vb and that solved the problem ,have one teori about burnd frictions in 3rd
when you have tranbrake or highstall do not the converter dumps hot oil into the area of the directdrum ?
my trans was getting super hot in late rounds going bracketracing agains slower cars hade to whait at the tree at full stall (footbraking)
one time i cracked a clutchplate