Deere spec on a vacuum fan is 3-5 GPM each making a total of 10 GPM for the dual fan setup.

Thinking about simple things
-- Is the reservoir full of oil?
-- What is the reservoir oil temperature after x minutes of running?
-- Is the oil in the reservoir showing signs of aeration?
-- Are the pressure lines from the pump plumbed to the correct side of the fan motor inlet?
-- Do the vacuum fans have the check valve that was added to later model years of 1770s?