Pistons are not working or master cylinder is not working .
If you unbolt the line from the master cylinder to the rear break and put your finger over the hole and push on the pedal you should feel pressure on your finger.
If you have pressure at the master cylinder then its the caliper.
Unbolt the caliper from the carrier and with pads in place stick a large screw driver length wise in between the pads and have some one push on the pedal. You should see the pistons come out. If not the square O rings inside the calipers are bad. Replace them don't cheat.
Auto parts store sells a brake fluid test kit for $3.95 break fluid should last for 4 to 5 rears. Your waisting your money if you do it every year.
Please add your state and model of bike you have. Click user CP @ top of page and then far left click edit signature. Thanks