how do i know the master cylinder is bad ?
It means your master cylinder needs to be replaced or rebuilt. The seals inside the cylinder can go bad and cause this to happen.
possible have a bad master cylinder
Brake fluid contaminated with moisture and dirt along with wear would do it.
The gasket to your clutch master cylinder is probably broken or cracked. or your master cylinder is bad.
Look for leaking fluid at the slave or master cylinder.
A symptom you almost always feel is a brake pedal that will not hold. You stop, hold the brake, and the pedal will slowly move toward the floor. This is a sure sign the master cylinder is bad. A leaking master cylinder or one that sticks, is also a sign is is defective. When you replace the master cylinder replace all the brake fluid in the brake system and bleed the brakes. Moisture in the brake fluid will destroy a master cylinder and this may be why it failed.Brake fluid leaking from the master cylinder. When you stop and hold your foot on the break the brake pedal will slowly go to the floor. Either of these symptoms means your master cylinder must be replaced.
a leak or a bad master cylinder
If the master cylinder reservoir is empty, you have a leak somewhere. If the master cylinder reservoir is full, the master cylinder is bad.
Usually the rubber seals go bad. On the other hand, if you live in a very humid area you might have a little water in the hyudraulic fluid and it could have rusted the master cylinder. They aren't usually terribly expensive to replace if you do it yourself.
Normally this is caused by a Master Cylinder that has failed. Replace the Master Cylinder. Normally this is caused by the loss of fluid in the brake system. The fluid reservoir needs to be checked first. It possibly may be caused by a bad master cylinder but the most common cause is loss of brake fluid.
May be a bad master cylinder, a bad sending unit from the master cylinder or the parking brake or the parking brake is not completely disengaging.