I just changed brake booster out got new one from junkyard. When I started test drive it was like the brakes were halfway applied. Before i got to the end of driveway it stopped by itself. When I cut car off, it lets go. Same model cadillac 93 deville maybe wrong booster? Maybe I should use the same master cylinder that was with booster from junkyard? can u help please?

Have the power brake booster checked, it may have a vacuum leak in the booster diaphragm or a defective brake booster vacuum check valve.

troubles at lights may very well mean a vacuum leak from a faulty brake booster
test by stopping and then applying the park brake and taking your foot of the brake pedal
you can do this test in the garage
start the engine , apply the park brake , place in drive
if it idles alright , put you foot on the brake
if it plays up , replace the brake booster

consider this
when at the stop lights , you will have your foot on the brake as it will still be in drive or in gear and you don't want the car to move
a simple test is to apply the hand brake to stop the car and don't touch the foot brake
if it idles ok but cuts out when you apply the foot brake , replace the brake booster as it has a vacuum leak that is causing the problem
you can do this test in the garage by starting in neutral and allowing the engine to idle , then apply the foot brake
iof it dies it is the booster

May not be the booster as some do make a noise when the brakes are applied. To test press the brakes several times with engine off, Press the pedal down hard and start the engine , the pedal should move down slightly and there should be no whooshing sound . if the above is ok so is the booster.

Hard brake pedal when applied with the engine running. A quick test for the vacuum assist booster is to shut off engine, pump brake pedal 3 or 4 times and hold on the last pump, start engine and see if pedal drops a bit....if you detect a problem with booster you should check for adequate engine vacuum supply to booster when idling . You can sometime hear a hissing or air leaking when applying brake pedal with engine running if booster is defective

_______power brake booster, check vacuum to booster and the check valve for brake booster. primary symtom is high hard pedal that requires greater then normal pedal pressure to stop car. testing booster= pump brakes several times with engine off to deplete stored vacuum. turn on engine with pushing slightly on brake pedal. you should be able to feel the pedal fade away a bit, and then become firm. But not hard. if you feel nothing at the pedal when engine starts. Brake booster is not working. Good-day! make sure vacuum is going to booster with engine running. it may just be a bad vacuum line or check valve.

did you check the brake booster ? how long you use the brake booster? may be have no enough pressure in vacuum brake booster. get more details here for help : http://www.fiverhope.com/
http://www.fiverhope.com/category-67-b0-Brake-booster.html

Not sure if you have done all this.



BRAKE BOOSTER INSPECTION

Functional test:
1. With the engine stopped, Depress the brake pedal several times, then depress the pedal hard and hold that pressure for 15 seconds. If the pedal sinks, the master cylinder, brake line or a brake caliper is faulty
2. Start the engine with the pedal depressed. If the pedal sinks slightly, the vacuum booster is working. If the pedal height does not vary, the booster or the check valve is faulty

Leak Test:
1. Depress the brake pedal with the engine running then stop the engine. If the pedal height does not vary while depressed for 30 seconds, the vacuum booster is OK. If the pedal rises, the booster is faulty
2. With the engine stopped, depress the brake pedal several times using normal pressure. When the pedal is first depressed, it should be low. On consecutive applications, the pedal height should gradually rise. If the pedal position does not vary, check the booster check valve.

Check valve test:
1. Disconnect the brake booster vacuum hose at the booster.
2. Start the engine and let it idle. There should be a vacuum available. If no vacuum is available, the check valve is not working correctly. Replace the check valve and retest.

END TEST

only way i know to test a booster, is to turn engine off. use brakes several times till pedal is hard. then with brake applied,start engine. the brake pedal should then soften.if the pedal stays hard the booster is bad


Read more: http://wiki.answers.com/Q/What_are_symptoms_of_a_bad_power_brake_booster#ixzz2487DHV5I

It it a brake booster problem.I am changing mine this weekend.You can get a new one form autozone for 149.00 or get one at the junkyard for about 70.00.I am installing mine this thanksgiving.I can send you the step by step instructions if you like.I have a 99 jeep grand cherokee limited edition and for once i thought i had a snake in the car..