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I've performed these 2 tests with success:
Test if the Vacuum Hose is the Problem
If the brake booster is not working, it might be the vacuum hose. Here is a simple test to check the health and integrity of the vacuum hose.
1. Park the car.
2. Turn off the engine.
3. Pump the brakes five to six times using no more pressure than you would usually apply when braking. This action allows more air into the brake booster and bleeds any residual vacuum.
4. Now, start the engine, and keep your foot pushing down on the pedal. Note what happens to the pedal.
With the engine running, the engine sucks the air out of the brake booster, restoring the vacuum. Ideally, the pedal should fall towards the floor with you applying light and steady pressure. If the pedal pushes back against your foot, then there is a problem. This could be a blocked vacuum hose, a leak in the vacuum hose, or a leak in the brake booster check valve.
Test if the Brake Booster Can Maintain a Vacuum
The next test checks the integrity of the brake booster. If the booster cannot maintain a vacuum after you turn off the engine, the problem is the brake booster itself, or the failure of the brake booster check valve. To perform this test, do the following:
1. Park the car.
2. Take your foot off the brake pedal and start the engine.
3. Let the engine idle for two minutes.
4. Just before you turn off the engine, push down on the brake pedal and hold it.
5. Turn the engine off and continue holding your foot on the pedal for another 30 seconds.
If the brake pedal stays where it is, then the brake booster can maintain a vacuum. If the pedal starts to rise, it could be time to replace the power brake booster. To be sure, perform the test below to test the brake booster check valve.
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