STRIP 3A4408 - Caterpillar

Typical Vickers hydraulic pump with two seals on the shaft.
Typical Tyrone hydraulic pump with two seals on the shaft.... To find the source of leaks on machines with more than one hydraulic pump.
... To keep dirt out of the seal cavity and to prevent oil leakage after assembly until the seals have a good seat.
... The plug is also installed in the drain hole to stop small leaks. The plugs shown in the parts charts are the only plugs that can be used because they can be pushed out of the hole by very low pressure and the oil will then drain from the hole. If the plugs are not installed correctly, or if plugs other than those shown in the parts chart are installed, it is known that the plug can not be pushed out at the rated design pressure.
On machines using water-glycol solution in the hydraulic system, the pumps with two seals do not use a plug in the drain hole. The plug must not be used on these machines because if there is a leak at the seals it is possible the plug will not be pushed out and the water-glycol solution will go into the engine or transmission and cause damage.
Adaptable As: Shown in the chart. INSTALLATION: Any time service work is done on a hydraulic pump check to see that the proper plug is installed in the drain hole. You will find the drain hole near the driven end of the pump, and it can be on the bottom, top or side of the hydraulic pump. Small steel balls are used to plug other drilled holes in this area. The steel balls are necessary and are not to be removed from the drilled holes.
The plugs are to be installed with the closed end toward the pump shaft. If the plugs are installed with the open end toward the pump shaft they may not be pushed out at the rated design pressure.