General Service

In order to properly service and repair available brake systems, a thorough understanding of the various power assist systems is necessary. Two basic types of power assist mechanisms are used: vacuum assist diaphragm assemblies which use engine vacuum and in some cases vacuum pressure developed by an external vacuum pump, and hydraulic pressure assist mechanisms which use pressure developed by an external pump (usually the power steering pump). Both type systems act to multiply the force exerted on the master cylinder piston by the operator in order to increase the hydraulic pressure delivered to the wheel cylinders while decreasing the 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 or bellows utilize this energy to provide brake assistance. The fundamental difference between the different 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, then engine vacuum is 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 top quality, clean brake fluid should be used in brake system, and on hydro boost systems, use of the specified hydraulic fluid in the boost circuit is essential to proper system operation. In addition, on hydro-boost systems, care must be taken not to mix the fluids of the two separate operating circuits, as more seals and valves are used, so use of improper fluids will do much more damage. For the same reason, be sure all dirt is kept out of the system.
The fact that brakes will operate even if the power unit fails gives us a clue to successful power brake service. This means the conventional brake system is left intact and a power unit is simply added to the existing system. Troubleshooting is then exactly the same until we get to the power unit. As with conventional hydraulic brakes, a spongy pedal with power brakes still means air in the system and grease on the linings will still make the brakes grab. Keep in mind, however, that power brakes give a higher line pressure, thus making leaks more critical.
Power units do not require adjustment in normal service. Either they work or they don't. If they don't, the various valves and connections are simply replaced. The only exception is that the power units themselves sometimes have an adjustable connection to the brake pedal or master cylinder. Another thing that is helpful in power brake overhaul is the manner in which the units wear. Repairs are infrequent until the unit has been in service for a long time. Then, when a malfunction does occur, it means the whole unit should be overhauled. Complete overhaul kits are available and all the parts in the kit should be used in the overhaul.