To obtain a first approach. you may do the following test:
1.- Unplug the compressor
2.- Remove the pulley protector
3.- remove the air filters
4.- Cover with a hand one air inlet
5.- Re confirm that the compressor is disconnected
6.- Turn by hand the pump pulley
7.- Check if the piston produces and maintains vaccum
8.- Repeat with the other cylinder.
With respect to the second issue, it seems that only one cylinder is working. In this ad:
I have seen that the FAD ( free air delivery) is 428 Liters per minute. As a cubic foot is equivalent to 28.3168 liters (30.48cm x 30.48cm x 30.48cm= 28,316.8 cm3= 28.3 Lt), 428 liters are aprox 15.1 cubic feet. Half of this volume is 7.5 CF, near to your calculations.
This is only an approch, please wait for an expert and more complete opinion in this forum.