Hydraulic brakes are quite simple in their basic operation, but can sometimes be difficult to troubleshoot.
Basically, a piston presses on a hydraulic fluid, which presses other pistons against the brake shoes.
Total failure is rare, but usually indicates where you should be looking to find the problem.
Typical points of failure are:
- Brake shoes/pads. Shoes are used on drum brakes, pads on disk brakes. While they aren't typically the cause of total failure, they can be a problem.
- Low brake fluid. Sometimes worn brake pads can result in significant piston movement inside the caliper. That can cause low brake fluid inside the reservoir. Check the fluid level.
- Leaky master cylinder or slave cylinder. Usually you'll see fluid leaking around the inside of the wheels or down from the master cylinder.
- Air in the brake line. If there is a poor seal on the master cylinder or if the brake fluid has been allowed to run out, it's possible that you may air in the line. The hydraulic line must be purged of all air for hydraulic brakes to function properly. If you haven't done it before, enlist the help of a mechanically inclined friend who has a little experience with bleeding the brakes.
Of course, it's always possible that a flexible brake line has ruptured or been cut or some other damage has occurred, but that's rare.