The Most Common Reasons for a Poor Brake Pedal:

• The bleeder screws on the calipers are not facing up.
• The master cylinder was not bench bled or was not bled completely.
• Defective rebuilt master cylinder with pitted cylinder bore or defective pressure seals.
• Master cylinder bore size too small for the system volume requirements.
• Use of a disc/drum master for a 4 wheel disc system.
• Master cylinder lower than the calipers or wheel cylinders.
• Lines or components near a heat source.
• Lines that loop up higher than the master cylinder and then come back down. Will trap air.
• Low drag metric calipers without the use of a quick take up master cylinder.
• No residual valve to rear drum brakes.
• Drum brake wheel cylinders too large.
• Silicone brake fluid. It can tend to trap air and cause seals to swell. In addition, if Dot 5 is used and it comes in contact with Dot 3/4, it can become a sludged up mess!
• Rear caliper parking brake / pistons not set properly with a rear disc system.
• Rear calipers not being bled properly. Most brake problems with four wheel disc cars comes from the rear.
• Improper pedal adjustment with too much free play.
• Old or inferior quality brake hoses.

How to Properly Diagnose a Brake Problem:

If you have very poor brakes, a spongy pedal or no pedal at all you will have to do some diagnostics to determine where the problem is in your braking system. Is it in the master cylinder, the front brakes or the rear brakes? These tests assume that your system is properly installed and bled. Perform these simple tests to find where the problem is. Once you know where the problem is it will be much easier to fix.

• Disconnect the brake lines from the master cylinder while leaving it on the vehicle.
• Obtain inverted flare plugs for the master cylinder outlets with the correct thread pitch. These are available at any good automotive auto parts store. 
• Plug the master cylinder outlets. Step on the pedal and hold pressure for about 30 seconds. If the pedal remains firm then the master cylinder is good.If the pedal sinks to the floor then the cylinder is bad.
• If the master cylinder is fine, connect the line to the front brakes. If the pedal remains firm then the problem is not coming from the front brakes. If the pedal sinks to the floor or is excessively spongy then the problem is with the front brakes.
• Connect the rear and if the pedal goes bad then the problem is in the rear.

You may also check whether your lack of a pedal comes from the front or the rear quickly this way. If you are sure the master cylinder is good, clamp off the front rubber hoses and try the pedal. If you get a good pedal then you know the problem is from the front calipers. Be very careful not to damage the hoses and try to protect them from damage by placing the hose between something smooth. There is a special tool sold in auto parts stores for this purpose. Try it on the rear if the front is fine.