1200 Agilent Prep HPLC Binary Pump flow-rate problem

I use 1200 series prep hplc which includes binary pumps. The pump B gives lower flow rate with respect to the command value. I measured the accuracy which falls to app. 50% at 5 and 10 mL/min with IPA. However, Pump A works fine. I cleaned the pump head - seals, plungers, pistons, inlet and outlet ball valves. Also cleaned the filter frit, and filter cup. Pump B specifications - P/N: G1361A, Firmware: A.06.53, S/N: DE60556274. Pump A specifications - P/N: G1361A, Firmware: A.07.01, S/N: DE60555760. I appreciate if anyone can help me to find out the cause.

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