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Dave Nova
Dave Nova, Cadillac Technician
Category: Cadillac
Satisfied Customers: 1522
Experience:  ASE Master Certification. GM World Class Certification
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Have a 09 sts prem lux package bought new started last fall

Customer Question

have a 09 sts prem lux package bought new started last fall if car is parked for several days , when started it has no power assist on brakes. If i prestart and let it run for several minutes brakes ok..........
JA: Will the headlights turn on? Is the battery fully charged?
Customer: yes for both
JA: Are you fixing your STS yourself? What have you tried so far?
Customer: Car has been to dealer twice and no trouble found. On start up this morning had to idel about 2 min for power assist to come on......
JA: Anything else you want the mechanic to know before I connect you?
Customer: that is the only trouble I have had sence new.
Submitted: 9 months ago.
Category: Cadillac
Customer: replied 9 months ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  david craig replied 9 months ago.

Hi Im David, thanks for visiting the site. theres not much else that can be wrong with the vehicle other than. 1) brake booster is bad.

2) the check valve at the brake booster vacuum line is bad. 3)theres a vacuum leak at the intake manifold where the vacuum hose for the brake booster goes.

Expert:  david craig replied 9 months ago.

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.

Expert:  david craig replied 9 months ago.

Testing the Brake Booster Check Valve

Before replacing the entire booster, make sure to test the check valve. The check valve is located where the vacuum hose connects to the brake booster. Check valves are often made of plastic and prone to failing or developing leaks around the gasket where they enter the brake booster. To perform the test, do the following:

1. Park the car
2. Pop the hood.
3. Take a can of brake cleaner and spray around the check valve gasket. Check to see if a suction is pulling the cleaner into the booster.

If there is a suction at the check valve it could be faulty and should be replaced. If you are replacing the booster, it usually come with new check valves.

Expert:  david craig replied 9 months ago.

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.

Expert:  david craig replied 9 months ago.

do you still need assistance?

Expert:  david craig replied 9 months ago.


Expert:  david craig replied 9 months ago.

checking in to see how its going