Contaminated brake fluid
The reduced braking power and softness of the brake lever are causes of air trapped in the braking system. Even though this job is very unpleasant, it is very necessary.
The traditional method
This process involves connecting a tube on the bleeder valve and placing it in a container.
You can reduce the time needed to bleed the brakes by installing aftermarket one-way speed valves in place of the factory installed ones. The special part with these valves is that they only allow fluid to get out, but not to come in, this way no air will get in also. All you need to do is repeatedly press the brake lever until the fluid is clear.
You can also reduce the working time by sucking the brake fluid. You can buy such a pump that does this from a service. You attach the pump to the bleeder valve and it does the work for you. Make sure you keep the master cylinder full while the bleeding takes place.