As I understand it, the tank should seal and even though it is vented, should hold some pressure to help reduce water contamination? Curious if that is accurate? I am pretty confident my main lid deal is totally gone. The whole side of the tank looks "washed down" from oil so I have a feeling someone opened it and never sealed it right. I am wondering if that would create any other issues than just an air leak there? From what I read, you should get a little pressure relief when opening the filler cap and I do not. Due to the level of fluid contamination, I am just trying to pinpoint problem areas.
That tank should hold a pretty good bit of pressure, can't remember how much. The main reason it does is to ensure a pressure feed to the pumps.
From memory also when opening - or after you have opened the tank - you are to put all your rams at full extension ( ie rams full of oil ) then screw the tank cap back on -- this will ensure you have pressurized your system as when you suck back the rams the air pressure increases( as oil volume returns to tank) - if this does not then you do have an air leak somewhere
There is a valve that allows the tank to pressurize but wont allow it to hold a vacuum. The reason you want the tank pressurized is to prevent cavitation. Raise the boom and stick out will pressurize the tank. Some excavators like the 345 use boost to pressurize the tank.
The breaker relief valve on the tank is a 3J-3738 according to the book. It's been cancelled and replaced by a 6E-3528. However I can't find anywhere the specification of what pressure it's supposed to relieve at. As Lee says, if you pull it off the hydraulic tank and suck through it, air should pass straight through the check valve. It should appear to be blocked if you try to blow through it. Usually these type of valves relieve at around 10-15psi and are designed to pressurize the tank to stop the implement pump cavitating.
The usual spec for reservoir pressure is 7 to 14 PSI. Basically just cycling your boom up and down a couple of times will pressurize the tank. There could be a small breather valve on top of the tank or a lot of manufacturers use a radiator cap on the fill port. Usually when I find a machine with a lot of oil down the sides of the tank I suspect someone filled it when the boom, stick and bucket were in the wrong position and when they collapsed all the cylinders the tank ran over. The other reason is that someone spilled the oil because they didn't have a funnel.