A classic car braking system is dependent on the brake fluid’s ability to function to its maximum capability through its hydraulic system.

Over time’, moisture and air will gradually infiltrate the system, taking their toll on its effectiveness.
The driver will experience a  “spongey” sensation when pushing down on the brake pedal or  that the pedal is reaching closer to the floor when braking.
These are indication that air has entered the system through the brake lines, usually as a result of brake fluid levels being allowed to drop to inadequate levels in the master cylinder reservoir.

Bleeding brakes essentially involves removing all of the air from the brake lines, callipers, master cylinder, and wheel cylinders.

When a brake bleeding has been implemented, the brake fluid itself must also be replaced as it likely be contaminated with atmospheric dirt and moisture.  A fresh batch of fluid will inevitably result in improvement of  its braking quality.

As in any mechanical maintenance procedure, when bleeding brakes, it’s important to observe basic safety precautions.  In this case, the parking brake must be firmly set, and manual transmissions must be in neutral.  Automatic transmissions should be  in park.

Make sure the vehicle has cooled down completely and is parked in a level area and that the wheels are charged to prevent any unintentional movement.

When bleeding the brake system, it's important that to use the correct brake fluid. If the  owner's manual  is available  it will advise the type of brake fluid that should be used in the vehicle. If not your local garage or accessory shop should help

 There are four methods for bleeding the brake system: manual, pressure, vacuum, and gravity.

 Regardless of the type of bleeding that is performed, make sure to keep the master cylinder full of brake fluid during the entire process. It is always wise to start the process from the farthest wheel from the master cylinder.

When bleeding a wheel that has disc brakes, attach a clear tube to the bleeder screw on the brake calliper. For drum brakes, attach the clear hose to the bleeder screw on the wheel cylinder, and then place the other end of the hose into a container.

Manual bleeding requires a “helper”, whose role it will be to remain inside the vehicle to apply and release the brakes. Before starting the helper should the brake pedal a few times and then the pressure firm.

 At that point it is possible to open the bleeder screw, as brake fluid flows through the clear tube, pressure will drop, and the pedal will begin to fall toward the floor.
The bleeder valve should be released before the pedal reaches the floor, by the helper releases the pedal.

This procedure can be repeated until a clean, steady stream of brake fluid is observed with no sign of air bubbles. The process can be repeated for the remaining bleeder screws with a close eye being kept on the brake fluid level in the master cylinder.

Another alternative is vacuum bleeding. Vacuum bleeding is accomplished using a vacuum pump that you can purchase at your local AutoZone. Attach the pump to the bleeder screw with a clear hose. Open the bleeder screw and start applying vacuum with the pump.

Once you see a steady stream of clean brake fluid, close the bleeder screw and repeat this process for the remaining bleeder screws, remembering to keep a close eye on the brake fluid level and the master cylinder.

The gravity method is the most basic method of brake bleeding and can be done by one person. as the master cylinder is at the highest point in the car.

All bleeder screws can be opened at the same time, and gravity will pull the fluid slowly through the system. When brake fluid is flowing out of the bleeder screw, it should be free of bubbles. An indication to close the bleeder screw.   Work should begin by working with the bleeder screw furthest away from the master cylinder and continue towards the bleeder screw closest to the master cylinder, closing it last while keeping an eye on the brake fluid level in the master cylinder.

The pressure bleeding method requires special equipment that the average home workshop is not liable to have, as it involves an investment in equipment, If the necessary equipment, known as a “bleeder ball” is available, pressure bleeding is a simple process  that can be carried out quickly and single-handed. The bleeder ball feeds brake fluid to the master cylinder under pressure, keeping it full during the bleeding process.

While brake fluid is pushed through the system at a level between 10 to 15 psi, the bleeder screws can be opened one at a time, forcing air out of the system.  This procedure should be continued till the fluid released from the open bleeder screw is clean and free of bubbles. The bleeder is closed, and the process is repeated for the remaining bleeder screws.

Each of these four methods will be equally effective is carried out properly. Bleeding a brake system is a simple process that can be readily carried out in-house with a minimum of effort and next to no specialist equipment.