I have my car issue and friend caprice, 2009 and 2012 6l80 trans, I have tow problems
1- at dyno the 4th gear slipping after second run with fully comanded locked
2- when i tested the car in the street, the second and third gear with tcc line pressure zero, after 4th gear the line pressure is working
you can see the 1st vcm at dyno the tcc line pressure is 110 psi, and the second run was 0 line pressure !!!
note: both of cars is same issue