The JFS uses the pressure stored in the brake/JFS-accumulators to start and runs on normal fuel regadless of the position of the fuel master switch. The pressure in the brake/JFS-accumulators should be about 3000 PSI and the pressure indicators are located on the right underside of the aircraft just inbord of the right ventral fin. You can go start 1 or 2 (normally 2) to "blow" (so to speak) one or both bottles to start the JFS. After it engine RPM has reached 12 % it takes between 40 to 60 seconds to recharge the bottles using the B-system hydraulic pressure. It can also be recharged by using a handpump in the left wheel well. You should get a green JFS run light within 30 seconds after starting the JFS and it automatically disconnects when the engine is reaching 50 % RPM (P&W -220). If the JFS is manually turned off, it has to spool down (17 seconds from operating speed) before it can be restarted.
Take a look down on the lower right of this picture. You will see 2 small round guages - those are the JFS pressure guages. The 2 long cylinders down the center of the photo are the JFS bottles where the pressurized fluid/air are stored.
Hope this helps give you a visual reference
City Of Goldsboro Public Utilities
F-16 blk 15/25/42/50. F-15E