MASCOT May - June 2005

How Does it Work? - No 12 - The Brakes Part 2 - Hydraulically Operated Systems

The Tool Chest

Hydraulic brake operating systems are very efficient and have been used on the majority of vehicles since the early 1930s. Notable exceptions are the 1939 Singer Bantam and Roadster models from 1939, which, in order to keep the costs down, were equipped with Girling rod brakes right up to 1950, when the 4AB, because of its independent front suspension, was fitted with hydraulic brakes at the front, although it still had Girling mechanicals at the rear.

Fig 1 shows the layout of a basic hydraulic system, and Fig 2 the detail of a typical hydraulic brake. Movement of the foot pedal pushes on the piston in the master cylinder, forcing hydraulic fluid along the pipes to the wheel cylinders. Further movement of the pedal then moves the pistons in the wheel cylinders outwards, bringing the brake shoes into contact with the drums. Unlike mechanical brakes, the system is fully compensated - no one brake can apply until all the shoes are in contact with their drums, at which point the pedal feels 'solid'. Extra force applied at the pedal will then cause the fluid to become pressurised and the force will be applied to the brake shoes. When the brake pedal is released, the shoe return springs force the fluid back to the master cylinder.

You can see, therefore, that because of the 'automatic' compensation, any unevenness in braking effort between the wheels, which would cause the brakes to 'pull' to one side, is not, as often thought, due to incorrect adjustment, but to a fault in the brake itself, such as a seized piston in the wheel cylinder, or oil, grease or brake fluid leaked onto the linings.

The rubber seals in the wheel cylinders depend on the fluid pressure to hold them to the cylinder walls to prevent leakage. To ensure a slight residual pressure in the system, a check valve is fitted in the master cylinder. Fig 3 shows a typical Master Cylinder and the insets the operation of the check valve. When the pedal is depressed, fluid is forced out past the one-way cup valve. When the pedal is released, pressure created by the shoe return springs forces the fluid back to the master cylinder, lifting the valve cup off its seating against the piston return spring, which closes the valve when a pre-determined residual pressure has been reached. As the piston returns to the end of its travel, a small 'By-Pass' port is uncovered, which allows any excess fluid in the pipes due to expansion of the fluid through heat, to return to the reservoir. Without this, pressure would build up in the brake lines and the brakes would not release fully. The same problem can occur if the push rod between pedal and piston is adjusted so that it is too long, which would result in the piston not returning fully to uncover the By-Pass port. This is one of the more common causes of brakes binding, and why it is important that there is a small amount of free clearance at the pedal before the push-rod contacts the piston.

It is also necessary to allow for replenishment of fluid 'lost' due to contraction in cold conditions and to compensate for the adjustment of worn brake linings, which, depending on the type of adjuster, means that the pistons do not retract as far. This is achieved by the Inlet Port, which is drilled just behind the main piston seal and allows fluid to enter around the waist of the piston. An outer seal prevents fluid from leaking out, and holes in the piston flange allow fluid to pass the main seal to replenish any loss.

For all practical purposes, brake fluid can be considered incompressible, so the pedal should feel solid when depressed. Any sponginess is usually due to air in the system, although I have seen a flexible brake pipe 'ballooning' under pressure, which also resulted in the pedal feeling spongy. To enable any air to be removed bleed valves are incorporated in the wheel cylinders see Fig 4.

A reliable procedure for bleeding hydraulic brakes is as follows:

1. Ensure fluid reservoir is full. Jack up vehicle and place on axle stands. Release the handbrake, making sure it goes fully off. If necessary, slacken off the cable adjustment or disconnect the linkage. Then, if your brakes have snail cam adjusters as per Fig 2, slacken off the adjusters fully so that the wheel cylinder pistons are fully retracted. Ask a colleague to sit in the car and operate the brake pedal to your instructions.

2. Starting at the wheel cylinder farthest from the master cylinder, attach a bleeder tube to the bleeder valve and immerse the other end of the tube in a jar containing hydraulic fluid.

3. Open the valve and call: "Press pedal slowly to floor and hold," then close the valve and call: "Release the pedal." Watch for bubbles appearing from the bleeder tube in the jar as the pedal is pressed.

4. Repeat this operation until air bubbles cease to appear in the jar of fluid. This can take 4 or more pumps of the pedal. Check the fluid level in the reservoir and top up as necessary.

5. Repeat the operation at the other wheel cylinders. Adjust all brake shoes and pump pedal until it becomes solid. If any sponginess remains, repeat the bleeding operation. Readjust the handbrake cable ensuring there is a small amount of slack.