my rear brakes on the bike suddenly stopped working. i was actually on my way to sell the bike today when i noticed. the brakes grab but with no pressure to stop or even slow down the bike. what could have went wrong because i have no idea what could have happened. the fluid levels are normal and there are no leaks. today was the first time i rode it in about 2 weeks. i parked it to sell it. any help would be greatly appreciated