BOLT 0990020 - Caterpillar

Procedure for Releasing Pressure
Release the pressure in the travel circuits before any hydraulic lines or components are disconnected or removed from the travel circuit.
Park the machine on a flat, level surface.
Lower all the implements as flat on the ground as possible.
Stop the engine.
Turn the engine start switch to the ON position without starting the engine.
In order to relieve any pressure that might be present in the pilot system, move the implement control and travel control levers or pedals to FULL STROKE positions.
Turn the engine start switch to the OFF position.
Slowly loosen the fill/vent plug on the hydraulic oil tank and release the pressure.
Tighten the fill/vent plug on the hydraulic tank.
The pressure in the system should now be released. The hydraulic lines and/or hydraulic components can now be removed.