Fluid in reservoir too low? Vacuum at breather when extending cylinder.

[lhfarm]

Back with my 1949 farm jeep hydraulic system. One way cylinder (gravity down). The original system had no pressure relief valve and was closed - not vented. I have added a pressure relief valve and a breather cap. When I put the cylinder under a little load (~300 lbs) it works fine. I can hear a sucking sound when fully extended. If I bump the weight to ~500 lbs, it lifts fine, but when I lower, it "burps" fluid out the vent tube. I placed a finger of a rubber glove over the filler/vent tube and could see the vacuum on extension and the finger blow up as I lowered the small load.

The reservoir is under the jeep bed and I don't have a way of measuring the amount of fluid in the tank. I've been afraid of overfilling, but thinking there might not be enough and that I'm sucking air and then expelling it. The filler tube comes out of the side at the top of reservoir with a 90 degree elbow. Not sure I can easily remove it. Any suggestions for measuring the amount of fluid or should I slowly add fluid until the sucking sound stops?

Thanks,

 

[CalG]

If the tank is over filled, what is the worst that could happen? 'Puke out fluid from the filler? Get ready, and try it.

You might try pushing a dipstick down the fill port just to see where the fluid level is now. a long spring works.

 

[lhfarm]

If the tank is over filled, what is the worst that could happen? 'Puke out fluid from the filler? Get ready, and try it.

You might try pushing a dipstick down the fill port just to see where the fluid level is now. a long spring works.

The filler port is on the side of the reservoir, with a pipe coming out of the rear of the jeep. My "dipstick" would need to make 2 90 degree turns. So I would have to remove the pipe to get anything into the tank to measure the current level.

The fill it till it pukes may be my only choice, but I know from experience that flying hydraulic fluid is messy. I should have asked a more direct question, like how much expansion space should you allow in a reservoir?