Hydraulic reservoir venting

[franklin2]

On my Case 480c I noticed each time I pulled the plug to put some hydraulic oil in the reservoir, it would puff air out under a slight pressure. I saw in the manual that is has a breather that needs to be cleaned out once in awhile, it has a access plug on top of the hood area beside the fuel tank filler.

I finally got around to prying up the plug and taking the breather vent assembly out. Of course it puffed again. I took it apart, it has a sintered brass filter on the bottom, but on the top part it has a spring and a washer. It seems to work like a check valve, letting air in if it needs it, but not letting air out. Everything was clear, the check assembly is why it's puffing air.

Does anyone know the theory behind this? As you lose fluid you need to let air in to prevent a vacuum in the tanks, but why not let air out also?

 

[old-iron-habit]

Not sure about the Case but many tanks run under a lesser pressure to help avoid air induced cavitation from retirning oil.

 

[John C.]

When the hydraulic tank is at or below the level of the hydraulic pump, putting a little air pressure on top of the level of the hydraulic oil forces oil into the pump all the time. This became critical when piston pumps started to become the technology of choice for hydraulic systems. It is general practice that when you start up in the morning you should cycle the implements so the level in the tank goes down and up a couple of times. That way air is sucked into the tank and not released.

There is the one issue with this system in that if you want to install attachments that have quick couplers you will have to release the pressure somehow in order to install them. Same goes if you are going to loosen any fittings in the system.

 

[franklin2]

Thanks for the replies.