I rebuilt my dual path, it was my first time doing it. I found lots of internal things wrong, fixed them all and it is running today. I used the workshop manual.
The workshop manual can be difficult to understand, might need to read it more than once. Basic settings are the first thing I'd check, linkage, fluid level check, otherwise....
I recall during teardown there is three small intermediate cylinders between the valve body and forward drum center support. You have to remove these, but the o-rings harden up making it a challenge. Some of the shops force teardown, not saying your shop did this. If you see July9 post on this forum you can see its common to things not to go as planned.
Instead of causing damage I made custom tools to remove these cylinders. Also, I made three new ones out of 6061 aluminum and have spare set. There are replacement pieces available in delrin(aftermarket), aluminum(OEM), steel(OEM). orings you can get no problem.
Also, a common area to look at is the babbitted bearing that the main shaft rides on, also wondering if it was cracked on yours. Another place is the shifter that looks like a rooster's crest inside, they crack from age. You have to remove the pan to inspect this. It would've been caught at teardown.