If you go to remove the restrictor, pressure check (air) from where the restrictor is and make sure there is not alot of hissing that could indicate a broken seal ring at the pump/drum, or a bad front drum piston seal.
You may want to run a flat file over the case where the valve body attaches to make sure there are no high spots causing a bad valve body to case seal.
Since the valve body is out, might as will double check that it's assembled correctly with the shift kit?

I would still try re-adjusting the front band first, just because it's easy.