Thats interesting.... Thanks for the update... If you didn't have the smaller pump handy, I wonder if putting a valve on the output of the pump might work as well? Im guessing not, because air would flow through the valve much faster than liquid... Might not be able to maintain any consistency either...
Or another thought.... Rheostat speed control on the pump motor... DC motors are easy to speed control, by simply lowering the voltage... but I don't know if these pumps would take to it?