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Hydraulic fluid needs to be kept clean of foreign debris to avoid damaging/destroying hydraulic fluid lines. It is also known to be hydrophillic; Absorbing water leads to a lower boiling point, which can introduce air into the system and render it incapable of functioning. To this end, caps for hydraulic fluid reservoirs incorporate some sort of seal to inhibit dirt and water from entering the system.

There are a number of different styles of cap that seem to be prevalent across different manufacturers, see images:

[Ford]: http://i.stack.imgur.com/lFHT8.jpg

[Dumptruck]: http://i.stack.imgur.com/vEluX.jpg

[Mercedes/GM but I can't get the GM photo to upload]: http://i.stack.imgur.com/RdLVJ.jpg

The first 2 have small inlets that allow for the reservoir to equalize pressure (Or at least I assume that is what the tiny holes are for). The last one does not seem to have any such inlets, just the rubber seal.

Why are these holes in the caps? Why do some applications have inlets and some do not?

  • I don't see how the answer to this could be opinion based. IMO this question should not be closed. – DucatiKiller Oct 11 '16 at 0:41
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A hydraulic system uses a pump to create fluid pressure to move cylinders (typically). As the pump builds pressure, it pulls fluid from the oil tank. In turn the pressurized fluid extends the cylinder. When the cylinder is retracted, the fluid is returned to the tank.

The tank is not typically under pressure. To keep it that way, vents are used to allow the fluid to flow easily and avoid pressure building in the tank. If pressure is allowed to build up in the tank, it can cause leaks and hose failure on the low pressure side of the system.

Each system is designed different, so some use the cap to vent the tank, while others have a vent elsewhere, therefore, the cap may not always have a hole.

Sources:

Hydraulic Systems Need Venting to Prevent Damage

Does a Hydraulic Reservoir Need Venting?

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Early system caps were vented to atmosphere where as later systems use a convoluted membrane to follow the fluid level thus keeping the resevoir sealed removing contact with atmospheric moisture and the cap is vented to atmosphere above the membrane.

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