How to clean the solenoid value of fuel injector testing machine ?

Remove 2 of the things in figure 1 , find the solenoid valve that corresponds to the direct oil in figure 2 . 

power the fuel injector testing machine  , press the oil return button on panel ,

Use the air tube to align the copper tube of the solenoid valve ( see the part finger point to in Figure 2) and blow upward,

most of impurities cleaned , 

if it still doesn't work , try to remove the solenoid valve  and clean it directly  .