air in brake system
Air in the brake system is one of the most common causes of a soft brake that goes all the way down without braking, or braking a bit in the end.
To fix this, bleed out the air.
Click HERE for the how to video
leaking brake lines
If a brake like is leaking it will let air inside and eventually empty the master cylinder, thus letting only air inside with little to no fluid.
Inspect brake lines, and replace if necessary.
Braided brake lines are a good upgrade.
low brake fluid
If there isn't enough brake fluid in the master cylinder air comes in and makes the brakes soft.
Inspect the brake fluid level and top up if necessary.
corroded brake calipers
Rusty and corroded brake lines can get small holes where fluid runs out and air gets in. Check connection points between brake lines and calipers.
If this is the case, buy a new caliper.
Leaking master cylinder
A leaking master cylinder or master cylinder cap can let air inside and make brakes soft.
Check for leaks an replace if necessary.