LINK AS-TRACK 5594918 - Caterpillar

Never remove the fill/vent plug from the hydraulic tank if the oil is hot.Air can enter the system and cause pump damage.
Illustration 1 g00661492
Park the machine on level ground. Lower the bucket to the ground with the stick in the vertical position.
Illustration 2 g00666125
(A) High temperature range (B) Low temperature range
For a cold machine, maintain the hydraulic oil level in the low temperature range. For a machine that is at normal operating temperature, maintain the hydraulic oil level in the high temperature range.
Perform Steps 3.a through 3.f if the oil level is low.Note: Refer to Operation and Maintenance Manual, "General Hazard Information" for information on Containing Fluid Spillage.
Illustration 3 g00666077
Remove bolts (1), washers (2) and access cover (3) from the top of the hydraulic tank.
Illustration 4 g00665049
Relieve the internal pressure from the hydraulic tank by loosening the pressure cap. After the pressure is relieved, remove the pressure cap.
Add hydraulic oil to the tank through the opening for the pressure cap.
Illustration 5 g00101462
Inspect the pressure cap. Replace the cap if damage is evident.
Clean the pressure cap and install the cap.
Install access cover (3), washers (2) and bolts (1) .