Vacuum Brake Booster: Description and Operation

Two basic types of power assist mechanisms are used: vacuum assist diaphragm assemblies, which use engine vacuum or, in some cases vacuum pressure developed by an external vacuum pump. The second type is a hydraulic pressure assist mechanism, which use pressure developed by an external pump (usually the power steering pump). Both systems act to increase the force exerted on the master cylinder piston by the operator. This in turn increases the hydraulic pressure delivered to the wheel cylinders, while decreasing driver effort necessary to obtain acceptable stopping performance.
Vacuum assist units are similar in operation and get their energy by opposing engine vacuum to atmospheric pressure. A piston and cylinder, flexible diaphragm (bellows) utilize this energy to provide brake assistance. The fundamental difference between these types of vacuum assist systems lies simply in how the diaphragm within the power unit is suspended when the brakes are not applied.
In order to properly diagnose vacuum assist system malfunctions it is important to know whether the diaphragm within a power unit is air suspended or vacuum suspended. Air-suspended units are under atmospheric pressure until the brakes are applied. Engine vacuum is then admitted, causing the piston or diaphragm to move (or the bellows to collapse). Vacuum-suspended types are balanced with engine vacuum until the brake pedal is depressed, allowing atmospheric pressure to unbalance the unit and apply force to the brake system.
Regardless of whether the brakes are vacuum or hydraulically assisted, certain general service procedures apply. Only specified, clean brake fluid should be used in brake system. On hydro boost systems, use of the specified hydraulic fluid in the boost circuit is essential to proper system operation. Care must be taken not to mix the fluids of the two separate operating circuits. Use of improper fluids, or contaminated fluid will cause damage to the seals and valves.


DELCO-MORAINE TANDEM DIAPHRAGM TYPE

Fig. 10 Delco-Moraine Tandem Diaphragm Booster Exploded View:

Fig. 11 Power Head Assembly Exploded View. Delco-Moraine Tandem Diaphragm Booster:

This unit utilizes a vacuum power chamber, consisting of a front and rear shell, housing divider, front and rear diaphragm, plate assemblies, hydraulic pushrod and a diaphragm return spring, Figs. 10 and 11.
The unit operates in much the same manner as the single diaphragm unit described previously. The diaphragm and plate assemblies utilize the pressure differential between the engine intake manifold vacuum and atmospheric pressure to assist in braking.