Low and Reverse Servo, for 2002 to 2011 Ford 5R55N, 5R55W and 5R55S Automatic Transmission 

I had a company power flush the transmission and immediately after that my reverse slips. All other gears work great including manual 1st gear. I have had the reverse servo piston replaced and there was nothing wrong with it, but the shop never checked that reverse valve or the o-ring when they were in there. Is it possible that there is debris in the passages of the reverse valve preventing it from applying full pressure to the band? Thanks