Brake assist fade

gsobol · 06-13-09, 07:51 PM

I was standing in traffic and I've noticed that my car started moving forward even though that I haven't taken my foot off the brake. I've applied more pressure and the car stopped, but few seconds later started moving again. I've done a test, when you pump the brake in very small increments, NOT releasing the pedal all the way up (!) (approx 20-30 pumps), the pedal will go all the way to the floor and releasing it a millimeter will allow the car to roll forward. Releasing the pedal all the way up returns things to normal. I've owned the car over 3 years, and that's the first time I've noticed that. None of my previous cars exhibited such a behavior, although I've had brake failure on the Lincoln that was caused by a failed relay (electrical brake assist). Any suggestions, or should I take it straight to the dealer?

snhrph · 06-13-09, 08:04 PM

I believe, based on your description, that the problem is most likely with your master cylinder. Did any warning lights come on when you noticed the malfunction or during the test?

gsobol · 06-14-09, 05:31 AM

Quote:

Originally Posted by snhrph

I believe, based on your description, that the problem is most likely with your master cylinder. Did any warning lights come on when you noticed the malfunction or during the test?

Nope, no warning lights whatsoever. My concern was exactly the same, that there is a leakage of brake fluid within master cylinder (around main piston). Just wanted to know if others have noticed the same thing.

mann777 · 06-14-09, 11:02 PM

what you are facing is loss of brake residual pressure when pressed. This has happened to me, I believe this happen due to change is external temprature, which may cause a locked up pressure to decrease in external temprature. This reduces holding level of the brakes.
One thing if you experience next time, get the vehicle parked by the side , ensure it a flat surface, put the parking brakes on, Ensure the Gear on "P".
Now pump the brake pedal to around 15-20 times, And release it. Wait for few seconds - Pump again for 5-10 times, Release it, One last time pump once ,ensure the foot brake pedal is depressed,completely, if the pedal has gone fully depressed and bottomed out , that means there is no pressure build up, Need to service you Master Brake Cylinder all four wheels and completely bleed the brake fluid out, Replace them with new one.
IF you foot brake pedal is depressed half way, and it has stalled, that means your master brake cylinder pressure build up is just fine. You are good to go. There is no trouble shooting further. let us know your outcome

gsobol · 06-15-09, 08:37 AM

Makes no difference how many times I pump up the brakes, I can still push the pedal all the way to the floor. I encounter resistance, but I can push it through. The brakes appear to keep the car stopped, and I haven't observed it creeping forward any more, but the worst thing is to not to trust your brakes. You know the feeling - I stopped now, will I stop the next time?

mann777 · 06-15-09, 11:41 AM

Gs, sorry I missed the part, you need to completely stop the engine to a off state, Remove the key and start the pedal kick and depress the brake pedal several times around 40 times until you find no vacuum is left in the booster. And Stop for a minute, then again start to push the pedal this time you will a certain resistance , Now visually see what distance is the resistance felt, is it halfway mark or at the 3/4 mark, if it is at 3/4 mark then you need to the brake overhaul ie Complete remove the brake fluid and have an inspection of the pads also, including the parking brake pads. Now replace with new fluid after the complete bleed process is over. I will give you an hint: When a pressure in power supply system is released the reaction force becomes light and the stroke becomes longer.
Now at the second stage, Push the ignition to on and now depress the pedal to 20 times. Give it a minute and now check the brake reservoir in the engine bay, you will see it getting up to to the brim and it will be a slow process that the level will start to decrease. Go back to your car . Now start your car, depress the pedal to a few times. And then give a final release, Take a test drive in a less traffic or congested area or empty traffic road, and do a sudden burst of acceleration and then brake isntantly again do a burst, Till you feel the spongy feeling on brake is getting dissappeared and you will get a firm braking environment result. let us know.

gsobol · 06-17-09, 12:53 PM

OK, I tried what you suggested. Seemed to work. Pedal goes about 1/2 way with engine off, I can't depress it all the way to the floor without risking braking the chair and my foot in the process. Why is it possible to depress the pedal all the way down when the engine is on, but not when it's off ????

gsobol · 06-25-09, 02:40 PM

Confirmed, dead master cylinder. Took it to the dealer after another incident. Weird only 30K on the truck!!!

mann777 · 06-28-09, 12:21 AM

My first post said that : Master Cylinder have to be serviced, My gut feeling would be the dealer would not replace completely but try to replace the parts, Just to make sure if you are giving teh job to the dealer, just make a marking on the cylinders, Once you get back you would know for sure what they have done.

gsobol · 06-28-09, 01:24 PM

Too late. The trucks's at the dealer now. I guess I'll find out what they have replaced once I pick it up. I have no option but to trust that they will do a good job. .... I hope ......