the reserve fills when you fill your tank. not 100% sure on this, but on most fuel pet cocks i have seen, there is two tubes that are up in the tank, which the gas flows thru into the gasline. one is taller then the other, the taller being normal "on" position, the other being a shorter "reserve" position. when the gas is low in the tank and goes beneath the normal higher tube "on" postion pick up tube, you switch to "reserve" and gas flows in to the line thru the lower "reserve" pick up tube. kinda a way for you to know your low on gas.
1985 KXT 250 Tecate - kx cylinder, kx ignition, bassani headpipe, answer silencer, v-force reeds, 38mm airstyker carb, +2 franks swinger, raptor rear shock.
1996 YSF240 Blaster - maxxed out!!!
Life's journey is not to arrive at the grave safely in a well preserved body, but rather to skid in sideways, totally worn out, shouting "HOLY SH-T, what a ride!"