John Deere 450H PCP trouble code F3C6

2002 dozer, 4700 hours, SN: 90722xx, the name plate was damaged so last two digits are unknown. Trouble code history: Replaced left motor speed senor (F373) by following instructions from the Technical Manual TM1743 (June 2004 version). Instructed to then calibrate and found F3C6 toward end of calibration. Went through flow chart to validate adjustments of the PCP and PDCV. Found the PCP diff' pressure at 5-psig and adjusted to null, then swapped ports and validated reading. We used a two port pressure diff' gauge for measurements. Manual switch at PCP works well so closed circuit hydraulics appear good. NEXT, the PDCV null adjustment. The initial sample of the right port ramped up with ease with only an 1/8 turn of the null adjusting set screw, but could not get the expected 50-psig on the left port, only rose to 18-psig. When hunting for pressure rise from left port we heard the hydraulic strain within the pump and quickly turned back other direction. We did at one point bottom out the null adjusting set screw when trying to reach pressure from the left port. Seeing that we could not get the pressure up on the left port we returned the set screw to original position. Tested PCP manual switch and tracks moved as expected. Charge pressure within specification at 300 psig. NEXT, TCU Diag readings at 1,500 rpm. Found difference in mV between Rstr and Lstr (175.2 and 311.6 respectively). Found similar large difference from Rctr and Lctr (157.5 and 337.0 respectively). All other mV readings appear normal. My questions are this, given the large difference in mV readings for right and left track I think we need to replace the PCP (PN: AT220775), and given that we could not reach the expected 50-psig should we consider replacing either the feed-back linkage or the whole PDCV? Decal on PDCV is KVEBZ0715. Dozer is up on blocks so we do not have to travel during calibration mode. Appreciate some insight on this topic