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A milking machine liner (DeLaval WC01) was evaluated for optimum performance characteristics for vacuum level and pulsator ratio. Parameters assessed were milk yield, peak flow, average milk flow, milking duration, kickoffs, and manual adjustments. Nine vacuum and pulsator ratio combinations were used to milk 147 cows twice daily for a period of 18 days. Each vacuum and ratio combination was used twice in reverse order. The vacuum settings were 42, 46, and 49 kPa. Pulsator chamber ratios were 60:40, 65:35, and 70:30. Peak flow and average flow differed significantly among ratios and vacuum. Milk yield was greater at a ratio of 65:35 than yield at 60:40 and 70:30 ratios (P< .05). Kickoffs and unit adjustments were not significantly different for ratio; however, the higher vacuum resulted in fewer manual adjustments. While no significant differences in kickoffs were evident, the operators felt the cows were more comfortable at the median vacuum. Considering milk yield and cow comfort, it was suggested the optimum operating vacuum for this liner is 46 kPa at a 65:35 ratio.