There must be a way for the air to get out. Not much help here but disc brake bleeders are always on the bottom in airplanes because the brakes are pressure bled from the bottom. Just push fluid in the bottom until nothing but fluid comes out the reservoir. Lots easier than pumping pedals, especially when working alone. Is there any chance the people installing bleeders on the bottom in jeeps are bleeding from the bottom up? I don’t know how I would rig a catch bottle but I expect it could be done.