Vacuum Brake Booster: Testing and Inspection

PRE-CHECKS
^ Inspect all hoses and connections. All unused vacuum connectors should be capped. Hoses and their connections should be properly secured in good condition with no holes or no collapsed areas.
^ Inspect the power brake booster check valve on the power brake booster for damage.
^ Check the hydraulic brake system for leaks or insufficient fluid.

TEST 1 - OPERATION CHECK
^ Place the transmission in "PARK", and the engine "OFF".
^ Apply the brake pedal several times to exhaust all vacuum in the system.
^ Depress the brake pedal and hold it in the applied position. "START" the engine. If the vacuum system is operating, the brake pedal will move downward under constant foot pressure.
Is movement felt?
YES - Vacuum and booster functioning. Go to Test 4.
NO - The vacuum power brake booster system is not functioning. Proceed to Test 2.

TEST 2 - VACUUM SUPPLY TEST
^ With the transmission still in "PARK", and the engine "RUNNING".
^ Remove the power brake booster check valve from the power brake booster. Manifold vacuum should be available at the check valve.
Is vacuum present ?
YES - Replace the power brake booster. Retest.
NO - Make sure all unused vacuum outlets are properly capped, hose connectors are properly secured and vacuum hoses are in good condition. Repair as needed. If no leaks are found, test engine for low vacuum. Go to Test 3.

TEST 3 - RETEST
When manifold vacuum is available to the power brake booster, reconnect the power brake booster check valve and repeat Test 1. If no downward movement of the brake pedal is felt, replace the power brake booster. Retest.

TEST 4 - VACUUM HOLD TEST
^ Operate the engine a minimum of 10 seconds at fast idle.
^ Stop the engine and let the vehicle stand for 10 minutes.
^ Apply the brake pedal with approximately 89 N (20 lbs) of force. The pedal feel (brake application) should be the same as that noted with the engine operating.
Does the brake pedal feel hard (no power assist)?
YES - Replace the check valve, then repeat the test. If the brake pedal still feels hard, replace the power brake booster.
NO - Go to Test 5

TEST 5 - SPONGY PEDAL CHECK
^ Place the transmission in "PARK"
^ "START" the engine.
^ Apply the brake pedal.
Does pedal feel spongy?
YES - Go to Brakes and Traction Control - System Diagnosis Pinpoint Test F.

NO - Booster OK.

TEST 6 - PUSH ROD LENGTH
A power brake booster that is suspected of having an improper push rod length will indicate either of the following:
^ A push rod which is too long will prevent the brake master cylinder piston from completely releasing hydraulic pressure causing brakes to drag.
^ A push rod which is too short will increase brake pedal travel causing a clunking or groaning noise from the power brake booster.
Are either complaints present ?
YES - Adjust push-rod. Adjustments
NO - Booster OK.