2000 sportster.
i put a new tire on for the season, went to take a ride and i almost went through the fence wondering where my back brake went.

all i did was take the wheel off, change the tire and put it back on. i didn't pull the break pads; though i should have they are pretty worn.
so the disk went right in not having to press the caliper pistons.
i checked the fluid level. it seemed low so i added some dot 5. bled the line while the bike was sitting level. pumped up pressure and the foot lever stays pretty good and firm until i get down the road and get to using the brakes a hand full of times. if i pump the brake it comes back slightly but not much.

if i sit and pump the lever at a stop the lever goes back to good and able to fully stop the tire while at a decent speed. but, it loses pressure after the rear brakes have been used a hand full of times.
i did adjust the lever in front of the master cylinder reservoir. this gave me a few more brakes on the test down the road.
i also noticed when i apply the rear brake the brake light doesn't come on. i shorted out the wires at the pressure switch and the light comes on. i will replace this switch, i don't believe it will matter much what kind i get as long as the replacement switches at the same pressure and all that...can this bad switch cause my problem? i wouldn't think so.
... this doesn't make sense to me. all i did was change the tire.
i plan on figuring this out tomorrow after work does anyone have advice where to start? it would be much appreciated. id love to get out on the road.