The cap assembly should be checked in accordance with customer specifications. If there are
no customer specifications, the typical practice in GKN is to check there is no leak after
pressing.
This check consists of putting fluid (oil or water) on one side of the cap and then applying
pressurized air from the other side of the cap. The presence of bubbles in the fluid indicates
an incorrect cap assembly. Figure 10 below describes the principle of the leak checking.