G1311A pump pressure saw tooth pattern fluctuation

Hello all, after replacing the frit, pump seals and active inlet valve cartridge (the plastic part), rinsing pump heads with IPA and purging the lines, we observed saw tooth pattern fluctuation of the column pressure (the guard column was replaced too). The pressure goes from 240 to 241 bar back and forth, but we didn't see any leaks. What could be the problem? Thank you. 

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