What fliud goes in here?

[desertmike345]

I have a Landhope 240F. Looking from the back there is a tube with a yellow plastic cap. The tube goes into the top of the rear end where the lifting arms are. There is no dipstick attached to the cap I have a slight growl coming from the rear end. Need to know what fluid goes here and how do you know if its full. This is not the one for hyd fluid under the seat. Will take a pic tomorrow.

Thanks
Mike