Dumb question. Everyone I know is under the impression that if you vent the pressure out of the tank then it'll relieve pressure on whatever fitting or hose your loosening. Isn't the tank pressure behind the pump?
There is a lot of sources of pressure.... opening the tank is only one, low pressure source.
Most (note the word MOST) circuits will be open back to tank when the engine isn't running. However it is always good practice to wiggle all the controls to relieve any pressure in a hydraulic system in addition to cracking the tank cap before loosening hoses or connections. On a hydraulic system with a main and a pilot circuit then even doing the aforementioned is not necessarily going to relieve pressure. Always approach said systems with caution and assume that any hydraulic component or fitting that you plan on disconnecting could potentially be under some kind of pressure.
The air in the tank acts as a spring that pushed oil into the suction side of the pump. Since all the return oil goes back to the tank, in most cases freely, the pressure felt on the suction side of the pump is also being felt at the discharge side of the pump when the engine is off. Nige is dead on though that the oil trapped between the valve and any of the circuits will still be held there. If there is weight on that circuit, such as a boom in the air, there will still be enough pressure to at least make a mess and at worst cause some physical damage if released suddenly. I like to open the cap, turn on the key, open the hydraulic control safety lever and cycle the controls until all the pressure is released before opening a fitting.
Most spool valve banks in modern circuits have check valves that open from the tank return passages to every work port to backfill any circuits that create a vacuum, with the possible exception of travel and swing motor spools. Helps eliminate pulling dirt and air into the system, possible pump cavitation, odd behavior etc. As long as the tank holds air pressure, it's fairly likely that that pressure will be present on every hose, tube, or fitting connected to a valve spool, not to mention the case drain system. YMMV
Thanks for the replies.