1. Check for a low brake fluid level in the master cylinder reservoir. The fluid level is normally visible through the plastic of the reservoir, and it should be up to the full mark on the reservoir side. On older vehicles with metal reservoirs you have to take the top off and look inside. If the level is low, add new brake fluid to bring it up to the full mark. A low level likely indicates a fluid leak, so carefully inspect the system for signs of leakage. Check all system components for signs of fluid on the outside. Look very carefully at the brake line connections at the master cylinder, the flexible brake lines at each wheel and the rubber seals on the caliper and brake pistons--any connection point is a likely place for a leak. If you discover a leak at a connection point try tightening the connection to stop the leak. Some connections are secured with hose clamps while other have threaded connectors. If you can't stop the leak, the line must be replaced.
2. Check for looseness in the pushrod linkage at the brake pedal and at the power booster and master cylinder. Tighten up any loose link points by loosening the lock nuts at either end of the pushrod that is attached near the top of the brake pedal lever, and rotate the pushrod to increase or decrease pedal height as desired. Tighten the locknuts securely when you are finished. Take care not to adjust the pedal to either end of its travel; doing so will pull the master cylinder away from its normal rest position.
3. Bleed the brake lines. Air trapped in the brake lines or piston cylinders will cause a mushy feeling at the brake pedal. Use a baster to remove the old brake fluid from the master cylinder reservoir, then refill with new fluid. Do the back wheels first, followed by the front wheels. Working from wheel to wheel, remove the wheel and locate the bleeder connection on the brake or caliper piston. Slip a piece of thin tubing over the nipple on the connection and run the tubing to a container on the ground. Open the bleeder connection by loosening the nut at the base. Let the fluid run out until new fluid is visible (sometimes the brakes will have to be pumped a few times to keep the fluid moving). Be very careful not to let the level in the master cylinder reservoir go to the bottom. Close the bleeder connection securely, refill the master cylinder reservoir with new brake fluid and move on to the next wheel. Dispose of the old brake fluid in accordance with local regulations.
4. Check drum brake self-adjusting mechanisms for proper operation. Back the car up quickly and brake hard to try to coax a seized self-adjusting mechanism into working. Repeat several times, then open up each drum and inspect the star wheel and lock tab. Clean the mechanism thoroughly, apply anti-seize compound to the star wheel teeth, then lubricate the star wheel and lock tab pivot points with brake grease. Replace any defective parts.
5. Check the operation of the master cylinder. Remove the master cylinder reservoir cap and empty the fluid with the help of a syringe or baster. Disconnect the brake lines from the the master cylinder and plug the line connections on the master cylinder with suitable caps or plugs, taking care not to damage the soft connections. Refill the reservoir with new brake fluid and replace the reservoir cap. Start the car and press and hold the brake pedal. A firm and high brake pedal that does not sink over time indicates that the master cylinder is operating properly. If the pedal slowly sinks over time, or if it feels soft, low or mushy, the master cylinder is defective and must be replaced.