Pedal Ratio
A simple way to mechanically increase the hydraulic pressure in a brake system is to alter the pedal ratio. Pedal ratios are determined by measuring from the pivot point to the master cylinder pushrod and from the pivot point to the pedal pad. Applying 25 pounds of pressure to a 5:1 brake pedal results in 125 pounds of pressure being applied to the master cylinder; with a longer 6:1 pedal ratio, 150 pounds of pressure is applied.

Boosters Power brakes are something that most of us expect to find on new trucks and the reason is simple enough. Disc brakes don't self-energize so they require more system pressure than drum brakes to push the pads against the discs to stop effectively. In most cases the simplest method of doing this is to add a brake booster of some sort.

Vacuum Boosters
The most common type of brake booster is the vacuum style. A fairly simple device, they are basically a canister divided into two chambers by a diaphragm. On one side of the diaphragm are the brake pedal and a vent; on the other side is a pushrod to the master cylinder and a hose leading to the intake manifold. When the brake pedal is depressed atmospheric pressure is present on the brake pedal side while manifold vacuum draws the air out of the master cylinder side. The higher pressure on the brake pedal side simply helps apply more mechanical pressure to the master cylinder.

Hydraulic Boosters
This type of booster taps into the power steering line and uses hydraulic pressure to supplement the pedal pressure applied to the master cylinder.

Hydraulic boosters are often found on diesel engines as the vacuum type won't work (diesels don't have manifold vacuum) but they work on gas engines just as well. While hydraulic boosters are ideal for diesels, they also work well on gas engines that have low manifold vacuum as the result of an aggressive camshaft.

In operation, fluid flows from the power steering pump, through the brake booster then on to the steering box. The steering box and the brake booster both have return lines, and to accommodate them the pumps used in these applications normally have two return fittings.

When the brakes aren't in use, fluid flows through the booster to the steering box. But when the brake pedal is depressed, pressurized fluid flows through a spool valve into a chamber in the booster. There the fluid applies pressure to a piston, which pushes on the master cylinder providing the power assist.

Also part of the system is a device called an accumulator (some are on the unit, others are remote) that stores enough pressurized fluid for at least one assisted application of the brakes if the engine quits. After that the brakes behave with no assist.

Electric Boosters
Something new on the brake booster scene is the electric type. Used in conjunction with a special master cylinder, hydraulic pressure in the area of 2,500 psi is created by an electric pump. That pressure is then delivered to the master cylinder.

Brake System Valves

Metering valves, also called hold-off valves are used in the brakesystem to better balance the front to rear brakes. The valve does not allow the pressure to rise at the front disc brakes until the pressure at the rear drums has risen sufficiently to overcome the brake shoe springs. At this point the valve opens to allow full pressure to build at the front brakes.

Proportioning valves modulate the pressure to the rear brakes. They minimize rear wheel lockup found in heavy braking and compensate for differences in braking conditions in front disc/rear drum systems. As pressure is applied to the system full pressure is allowed to the rear drums up to a certain point. Beyond that point the pressure to the rear is reduced preventing rear brake lockup.