Hydraulic fluid level in tank

[Doug Allman]

2004 Marquis with C-12. Have a hydraulic tank in upper right hand corner (passenger side) of engine compartment. On the tank is a fill cap and a site gauge. There are no other fittings on tank that I can see or feel and the fill cap does not have a dipstick attached.

Replaced fluid last year, Dextron III type, and filled to site gauge.

Had fluid running out on several occasions after we had gone a ways.

Just completed yearly filter change, messy, and now want to find correct level for fluid. Anyone know how to do this?

[Jerry Emert]

My manual says 23 quarts with filter.

[Edward Buker]

Doug,

Depending on the capacity and the heat of the fluid where they place the sight glass is a bit of a guess. Fill to maybe 3/4 of an inch below the sight glass cold and then when you have run it enough to have the fluid be at temperature, check it again. You want the fluid at temp to have a little air space at the top of the tank because the fill pipe has no real volume to it. If the sight glass is not useful based on overflow and the expansion issue, find the level you like and make a dipstick out of a material that will not shed particles. The level of this tank is not critical, it is just a matter of a slight change in capacity if you run it down a bit to accommodate expansion.

Later Ed

[steve zannella]

Does anyone know if the hydraulic reservoir has to have a vented cap?
My reservoir has a vented cap which looks like a gas tank cap and oil is always leaking out .

[Doug Allman]

Good question Steve as I have same on my reservoir but is a plastic cap that is much like a new style gas cap. Always oily on reservoir.

[LaMonte Monnell]

That's a good question. I will have to look at my cap tomorrow.

I don't think mine is vented as I have no issues with residue or any leakage!

[Edward Buker]

If there is no additional vent port on the tank then the cap would have to be vented to allow for fluid volume change with heating and cooling. I also have never examined mine closely. If there is a separate vent then I suspect the cap would not be vented.

Later Ed

[Dave Atherton]

Gentleman, hydraulic tank vent, your hydraulic tank needs to have a big enough vent either
in the cap or else where. The hydraulic flow that is required from hydraulic pump has to return
back to the hydraulic tank. Reason for vent cap or vent, thing what called back pressure will be
Restricted and slow hydraulic flow to valve. Rule of thumb with a what's called a closed center
hydraulics system return flow, cannot exceed 25 % system pressure. With vent cap leaking or
staying wet hydraulic fluid could be over full or wrong spec hydraulic fluid, or restricted vent opening.
With open center hydraulics there is no vent on hydraulic tank to prevent cativation with hydraulic
pump and open center hydraulic valves. Dave Atherton Retired Cat Mechanic

[Gerald Farris]

Steve,
Yes your cap has to be vented, and if it is leaking more than just an oily film, which is normal, the reservoir is probably slightly overfilled.

Gerald 

[Richard LaLande]

following

[Michael Rump]

All,

I have been having the same problem with leakage and I would like to run the tank a little closer to the sight glass level.  The attached pic's are of my tank label, and the vent cap (looks like a gas cap). 

The leakage I observe seems to be coming out of the tank fill tube versus the vent hole in the cap.  I don't think the cap 'o' ring seals properly to the fill tube seating surface. 

Not sure what the fix is, but perhaps collectively we can come up with a vented cap that won't leak!

Thanks and safe travels,

[Dick Simonis]

I just had the fluid and filter changed at BCS and was advised that the level should be slightly below the sight glass when full or it would leak when hot.