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. Perform Brake Pedal Reserve Check. Retest.
TEST 4 - VACUUM HOLD TEST
NOTE: If the adapters and gauge is not available, perform alternate Test 4 at end of this procedure.
^ Remove the check valve from the power brake booster.
^ Install a fitting in the rubber grommet 14.61-15.88 mm (0.575-0.625 inch), lightly lubricated with alcohol or silicone oil, taking care not to cut grommet.
^ Install the check valve between the shutoff valve and engine so that flow through the check valve occurs in the normal direction.
^ Complete the setup as shown using Rotunda Vacuum Tester or equivalent.
^ With the shutoff valve open, run the engine to medium speed, release accelerator pedal and turn the engine "OFF". Read the vacuum gauge.
^ After three minutes, read the vacuum gauge a second time.
Is the second reading within 6.75 kPa (2 in-Hg) of the first reading?
YES - The Booster and check valve are holding vacuum. Restore vehicle and go to Test 6.
NO - There is a vacuum leak. Go to Test 5.
TEST 5 - VACUUM HOLD TEST
^ Start the engine and run to medium speed, release the accelerator pedal, close the shutoff valve and read the vacuum gauge.
^ Shut the engine "OFF".
^ After three minutes, read the vacuum gauge a second time.
^ The second reading should be within 6.75 kPa (2 in-Hg) of the first reading.
Is the second reading within 6.75 kPa (2 in-Hg) of the first reading?
YES - Replace the brake booster check valve then repeat Test 1. Service and Repair
NO - Replace the brake booster then repeat Test 1. Service and Repair
ALTERNATE TEST 4 - VACUUM HOLD TEST
NOTE: Perform this step only if the adapters and gauge is Test 4 are not available.
^ 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 6.
TEST 6 - SPONGY PEDAL CHECK
^ Place the transmission in "PARK"
^ "START" the engine.
^ Apply the brake pedal.
Does pedal feel spongy?
YES - Go to Brakes - System Diagnosis Pinpoint Test F.
NO - Booster OK.
TEST 7 - 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. Refer to Adjustments.
NO - Booster OK.