The self-energizing and servo action principles of a typical drum brake may make it unnecessary for the driver to have an especially strong leg, but they can only do so much. If they were designed to wedge the shoes any more tightly against the drums, the brakes would be apt to snatch full on unintentionally, resulting in skids and broken noses. And, of course, disc brakes have no servo action at all. Especially with heavy cars, a system was needed to augment muscle power and make driving easier and safer.

The basic principle behind a typical power brake unit is to use engine vacuum and atmospheric pressure to take the stopping load off the driver’s leg. In most specimens, an air-tight chamber is placed between the firewall and the master cylinder. This contains a large rubber diaphragm and a floating control valve that’s in line with the rod from the pedal. Vacuum from the intake manifold is normally present on both sides of the diaphragm.

When the driver applies the brake pedal, the air valve is opened slightly to admit atmospheric pressure gradually through a filter to the firewall side of the diaphragm, pushing it forward. This, in turn, presses on the master cylinder rod and so transmits hydraulic pressure to the calipers and/or wheel cylinders.

That’s certainly simple enough, but a great deal of subtle engineering has gone into turning this theoretical action into real-world stopping power, and the control valve and the reaction mechanism are the most sophisticated parts. They must provide smooth application, boost sensitivity, and feedback to the driver so he knows how hard he’s applying the brakes.

Quick tests

Whenever you encounter a car that has a stopping problem you suspect could be caused by the power assist system, there are several quick tests you can do. The most basic one is to see if the power unit is indeed applying force to the master cylinder, and it should be performed as follows:

1. With the engine off, pump the brake pedal several times to relieve any residual vacuum.
2. Hold the pedal down with moderate force and start the engine.
3. If the booster is working, the pedal will fall away slightly and then hold solid as vacuum power is added to leg power, and the pedal should be easier to hold down.

If you don’t get the above results, attach your vacuum gauge to the engine to see if it’s generating a normal amount of vacuum. I’ve seen modified engines with high-overlap camshaft grinds, or those in poor condition, that won’t produce enough vacuum to operate the booster effectively.

In cases where there’s sufficient vacuum available, yet the pedal does not react as above, the power unit is probably in need of overhaul or replacement.

A car that passes the basic test, but still seems to have a power system problem should be given a vacuum leak check:

1. Run the engine up to medium speed, release the throttle, and shut off the ignition. This builds vacuum.
2. Wait 90 seconds, then apply the brakes. There should be enough vacuum in the unit for at least two power-assisted applications.
3. If the pedal feels like two or more applications were being aided by vacuum, no leak is present and the booster is probably okay.
4. If you felt no boost after that short wait, the system is not vacuum-tight, and the first thing to examine is the check valve between the hose to the manifold and the booster. Pull the valve out of its grommet and, using lung power, you should find that it passes air in one direction, and not in the other. A check valve that has even the slightest leak should be replaced. Make sure the grommet that it fits into is also intact.
5. If the check valve is okay, the booster is in need of repair or replacement.

Hidden leakage

Sometimes a car will lose brake fluid from the master cylinder reservoir, yet exhibit no evidence of seepage at the lines, connections, calipers, or wheel cylinders. In these cases, engine vacuum may be drawing it through a faulty master cylinder primary piston secondary seal and into the booster, so pull the vacuum hose and see if you can find any brake fluid inside it. It’s always a good idea, by the way, to make sure that both ends of the hose fit tightly on their nipples, and to install clamps if they don’t.

Now for some specific problems and their probable causes. A hard pedal or no-boost condition, as indicated by a test drive or the above checks, may be due to nothing more serious than a disconnected, broken, crimped, collapsed, or plugged vacuum hose, so check that first. Then see if either the check valve or the air inlet filter is blocked. If everything so far is okay, the problem is probably internal. The air valve may be stuck, or maybe the diaphragm or piston seal is leaking.

Lining problems?

But don’t overlook the possibility that poor quality, glazed, contaminated, or worn-out linings are causing the complaint. It would be unfortunate to condemn and replace the booster unit when the condition is actually due to a simple, inexpensive-to-rectify lining problem.

If the brakes apply so violently that you’re afraid to apply them, and there’s nothing wrong with the disc brake pads or drum brake shoes, there’s probably trouble inside the booster. Most probable are a sticking vacuum valve, a broken valve spring, or a leaky reaction diaphragm.

In cases where pedal return is slow, or the brakes don’t release, a broken return spring, excessive seal friction, or a faulty control valve is usually the cause.

It would be imprudent to leave out an important safety note: Whenever you remove a booster, make certain the pushrod/linkage adjustment is exactly as specifications dictate. If it’s too long, the brakes will drag. If it is too short, you’ll be lucky if the car stops at all.

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