To improve the service life and total efficiency of the piston pumps, an alloy steel of HMn58-2 manganese brass and 20CrMo are used as the material for a pin–disc, and the change in the coefficient of friction of the pin–disc is investigated via a friction/wear experiment. A Plint TE-92 tribometer is used to conduct dry and wet wear experiments on the pin–disc. After the experiments, the friction and macroscopic morphology of the sample are analysed. Results show that the wear in the dry friction experiment is not uniform, the coefficient of friction increases quickly, and the sample wears and fails. When using No. 46 anti-wear hydraulic oil as the lubricant in the friction test, the increase rate of the coefficient of friction is significantly lower than that in the dry friction test. At a rotational speed of 2,000 rpm and a load of 1.1 KN, the maximum coefficient of friction is 0.147. When the load is increased to 1,300 or 1,400 N and the speed is 1,600–2,000 rpm, the coefficient of friction is greater than 0.011. A higher rotational speed results in a smaller coefficient of friction, whereas a greater load results in a higher coefficient of friction, using lubricating oil can significantly reduce the coefficient of friction. The effect of rotational speed on the coefficient of friction is greater than the effect of load.