580ck fuel tank pressure. Where’s vent?

a new behavior/issue with my 1971 Case 580ck. I went to start her the other day and she just cranked and wouldn’t fire. It was 40+ deg and should have. I opened the bleeder on top of the fuel filter and nothing came out. Seemed strange as I was sure I had half a tank. Opened the fuel cap to look inside and as soon as I did it hissed (air flow) and the bleeder on the filter started to flow fuel. I closed the bleeder and the gas cap and she fired right up. Used her for about an hour with no issue. When I was done I thought about the hiss and opened the fuel cap and again she hissed and even gurgled some fuel out of the cap at me.

So the fuel system is air bound. I am not sure if it was positive or neg pressure but am sure I’ve never had the tank spit diesel at me before.

So the question is how does the fuel tank vent? My best guess is that the vent is clogged. But I have no idea how the tank is vented. The cap is obviously sealed as the air exchange when I opened it. This is completely new behavior and not sure what’s up.

Any ideas?

Some have a hose connection at the top of the tank and the vent hose runs down the side of it. I have seen some with a breather filter in the line also (its like an inline fuel filter). You may also have a vented cap that is plugged up, seen those too.

Air hissing noise and the fuel flow situation that you described is typical of a plugged vent in the cap.
Should be a vented cap on it. Some of them need to be taken apart to clean them. The type of cap I referring to doe's not have a hole in the top of the shell.
They allow air to enter between the outer chrome shell and an inner plate.
It is much easier to replace the cap with a new one.

Mine is in the cap on the old ck.

Yup.. I always thought it was in the cap too.

You guys were right again (as usual). The fuel cap on the old CK is supposed to be vented. Mine was full of crud. A few minutes with carb clean and an air hose and she seems as good as new. The pic shows the vent holes (green arrows) and under the tabs that lock it onto the tank (red) are also holes that connect. So green arrow (outside air) is connected to red dots (tank side) or at least are when not full of crud.