B7800 hydraulic check?

[BeezFun]

Be sure to pull the stick out and dip it again for the actual reading: this machine (and the B2910) will develop an air pocket where the end of the stick goes and will read incorrectly when you first pull it (may show nothing on the stick)

Dip it a second time and you will get a correct reading. Place the stick against your finger to determine the wet area if you can't easily see it.

Same thing with my 2710. I've looked at the parts diagram, it's just a tube that comes off the side of the transmission. Maybe the air heats up and expands and pushes the fluid down out of the tube.