2008 Trans issue??

[Biggsw784]

I have a buddy that just had a new tranny put in his truck. Now when he puts it in reverse it doesnt move until he barely tapps the throttle and it will somewhat catch. I hooked a pressure gauge up to the trans and in reverse he has 75 psi which the suncoast kit that is in his tranny calls for 140-150 psi. In forward gears it works perfect, no issues at all forward. The trans is full of fluid and the filter oring isnt messed up. Is it in the valve body or what?? He got a tranny out of a job 3 truck and had it built and just swapped them out one weekend. His truck is a job 2. Where else can we look to find out this issue?? Also he has a suncoast converter and the converter would lock up on its own sometimes like when coming to a stop or taking off. Would the tcm cause this being its a job 3 tranny and a job 2 truck?? Any help please!!!