How To Test a Power Brake Booster

Power Brake Booster Test

Power Brake Booster TestIf the brake pedal feels hard while the engine is running, the power brake booster is not operating correctly.

Power Brake Booster Test 1

  1. With the engine off, pump the brake pedal to remove any residual vacuum in the booster.
  2. Hold pressure on the pedal while you start the engine. When the engine starts, the pedal should drop about a 1/4″, this indicates that the booster is working properly.

Power Brake Booster Test 2

  1. Run the engine a couple of minutes.
  2. Turn the engine off and press the pedal several times slowly. The first pump should be fairly low. The second and third should become slightly firmer. This indicates an airtight booster.

Power Brake Booster Test 3

  1. Start the engine and press the brake pedal, then stop the engine with the pedal still pressed. If the pedal does not drop after holding the pressure on the pedal for 30 seconds, the booster is airtight.



Inspect the Check Valve

  1. Disconnect the vacuum hose where it connects to the intake manifold. Do not disconnect the vacuum line from the booster. Air should not flow when pressure is applied, but should flow when suction is applied. If air flows in both directions or there is no air flow, the valve needs to be replaced.

Verify Vacuum

  1. Check the operating vacuum pressure when the engine is at normal operating temperature. There should be a minimum of 18 in. of vacuum. Vacuum may be increased by properly tuning the engine, checking for vacuum leaks and blockages in vacuum lines.

How a Brake Booster and Master Cylinder Work

Here’s how a brake booster and master cylinder work to stop your vehicle with the press of your brake pedal.

Stopping a heavy, 2000+ lb car is no easy task. It takes a lot of force to press up against the discs and drums on each wheel to stop a car quickly.

The hydraulics of brakes is quite simple. A primary piston (known as the master cylinder) pushes fluid into hydraulic lines that get fed out to the wheels. The piston inside the caliper (or wheel cylinder inside the drum) will expand with the fluid, causing it to glide up against the disc (or drum), slowing the wheel.

The brake booster was developed to sit in between the master cylinder and driver’s pedal, to make it easier for it to press the pedal. While the master cylinder’s diameter is already smaller than that of the caliper pistons, the force required to compress it is still great.

The brake booster works on the principle of vacuum differentials to aid in pushing the master cylinder. On one side, vacuum is sucked from the engine’s intake. At idle, a valve in the diaphragm allows vacuum to be passed through the diaphragm, so that vacuum balances both sides.

When you depress the brakes, that valve moves, sealing off the vacuum side, while allowing filtered atmospheric air to enter the booster from the brake pedal side. This creates a pressure differential between the diaphragm, which helps to force the piston in the master cylinder to compress.

A giant return spring brings the diaphragm back to its rest position when the brake pedal is released.

The master cylinder consists of two small pistons in series. Each piston routes to two diagonally opposite wheels, for redundancy in case one springs a leak or the seal is compromised. Reserve brake fluid is contained in a reservoir above the master cylinder and is sucked into the piston assembly when the brake pedal is pushed.

Periodic brake flushes are required because brake fluid is hydroscopic and will absorb moisture and loose its effectiveness over time. Furthermore brake fluid will wear down with heat, and may become contaminated.

Why Old(er) GM Power Brake Boosters Mount at an Upward Angle

Why Old GM Power Brake Boosters Mount at an Upward AngleThe primary reason is for correct brake pedal geometry. Back in the day, most GM vehicles were offered with both standard manual brakes as well as optionally with power-assisted brakes.

The optimum manual-brake pedal-to-master cylinder push rod ratio (aka “pedal advantage”) is around 6:1, compared to about 4:1 for power-assisted brakes which don’t need as much pedal advantage because they “boost” the force generated by the average human leg.

GM used a common brake pedal with two push rod holes located about 1 to 1½ Inches apart; the upper hole was for the 6:1 manual brakes and the bottom hole yielded the power-brake 4:1 ratio.

When the brake push rod was installed in the lower hole for use with power brakes, achieving the proper push rod arc of travel, center of force, and proper alignment with the brake booster/master cylinder assembly piston center-line required a firewall mounting bracket that positions the assembly at a fairly sharp up-angle.

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