You have a rusted steel line or the wheel cylinder blew out. Neither of those are especially expensive or difficult to repair. The exception would be if the rear brake was worn out and grinding metal-on-metal and you ignored it. Be aware though that as a result of pressing the brake pedal further than normal, that can damage it on any vehicle more than about a year or two old, so don't be surprised if after the repairs are completed at the rear, your mechanic has to tell you a new master cylinder is also needed. There's no way for him to know that right away but many shops will warn you ahead of time to prevent surprises later. Even if your master cylinder is okay, some damage may not show up for a few days. When and if it does, it results in a slowly sinking brake pedal when you hold steady pressure on it, and the red warning light will turn on again.

Answered by caradiodoc (expert)
17,289 answers provided