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Old 10-28-08, 09:57 PM   #1
banerjek
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Sinking brake pedal

Our car has had an interesting issue for awhile. If you hold the brake firmly while stopped for around 30 seconds, the pedal suddenly sinks in about an inch. The sinking is accompanied by a sound that is sort of like exhaling. The brakes don't appear to let go, and the pedal feels firm until it sinks in (and then it is firm again). The pedal has never sunken except when stopped.

I haven't bled the brakes yet because this is not the normal spongy feel you get when there is air in the line and I've been too lazy to investigate this. Braking performance is good under all conditions.

Is what I have most likely benign, or is there some potential for there to be a real problem? Part of the reason I haven't worried about it is that I've noticed the exact same behavior on some of the fleet cars I've driven.
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Old 10-28-08, 10:02 PM   #2
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I don't really know my way around the mechanics of cars, but I suspect it has to do with the power brakes doing something while you're just sitting there. After all, you can try the following, with the car off (and thus power brakes off). The first stomp of the brakes is relatively easy. Try again, and it's harder. It doesn't get much harder after that, though, since any built-up hydraulic assist is already used up. Press on that same brake pedal (having already seriously firmed up the pedal by stomping repeatedly) while turning the car on, and after a moment, the brake pedal sinks as the power brakes come back into play. I've never noticed what you described while sitting around, but then I usually don't hold the brakes all that firmly, so I have no clue whether what you describe is normal.
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Old 10-28-08, 10:38 PM   #3
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Try bleeding the brakes, but it's more likely a vacuum line issue to the brake booster. Replace the vacuum line and see if that works.
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Old 10-28-08, 10:44 PM   #4
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How is the fluid level?
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Old 10-28-08, 10:55 PM   #5
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It could be low fluid...a vaccum leak or a bad booster.
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Old 10-28-08, 11:37 PM   #6
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I've been out of the car biz for a while now, but I remember a few models doing that regularly. I believe it was a Toyota trait, and was no worry. Has the car always done it, or did it just pop up?
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Old 10-29-08, 01:02 AM   #7
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Sounds like a Toyota trait. I remember an old pickup I had doing that, as well as a Camry and a Corolla.

The vehicle will brake normally. 30 seconds after stopping, the vehicle will try to lurch forward slightly.

Only guess I can make is that its something with the power brakes.
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Old 10-29-08, 05:37 AM   #8
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it's a brake booster issue of some sort. the "exhaling" sound is a tip-off.

the brake booster works by having a diaphragm with a vacuum on both sides. when you apply force to the brake pedal, it opens a valve which allows atmospheric pressure to enter one side. this assists your foot in pushing in the master cylinder's piston.

the sound you are hearing is probably air rushing through the valve.

whether this is a problem or not is beyond me. i'd just be aware that there is a possibility of a booster failure. which will simply mean it will take (a lot) more pressure to actuate the brakes.
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Old 10-29-08, 05:44 AM   #9
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Definitely sounds like your muffler bearings are shot.
New ones here: http://kalecoauto.com/index.php?main...products_id=10
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Old 10-29-08, 05:52 AM   #10
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Quote:
Originally Posted by jhota View Post
it's a brake booster issue of some sort. the "exhaling" sound is a tip-off.

the brake booster works by having a diaphragm with a vacuum on both sides. when you apply force to the brake pedal, it opens a valve which allows atmospheric pressure to enter one side. this assists your foot in pushing in the master cylinder's piston.

the sound you are hearing is probably air rushing through the valve.

whether this is a problem or not is beyond me. i'd just be aware that there is a possibility of a booster failure. which will simply mean it will take (a lot) more pressure to actuate the brakes.
+1
Probably a ruptured diaphragm in the brake booster. Replace the booster or go without a booster.
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Old 10-29-08, 08:39 AM   #11
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Dangit I was wrong. *stomps foot into ground*
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Old 10-29-08, 08:45 AM   #12
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I too suspect a vacuum line.
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Old 10-29-08, 10:00 AM   #13
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Quote:
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the brake booster works by having a diaphragm with a vacuum on both sides. when you apply force to the brake pedal, it opens a valve which allows atmospheric pressure to enter one side. this assists your foot in pushing in the master cylinder's piston.

the sound you are hearing is probably air rushing through the valve.
Fluid is good. I think the car has always done it.

The explanations here make a lot of sense. I'll see if I can nail the problem exactly. This is the first car I've had with power brakes, so pressing harder on the brake is not a big deal. However, anything that doesn't appear to be working properly drives my wife crazy so I need figure this out.
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Old 10-29-08, 10:06 AM   #14
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brakes are for wussy
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Phobias are for irrational fears. Fear of junk ripping badgers is perfectly rational. Those things are nasty.
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Old 10-29-08, 11:03 AM   #15
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Go with a fixed gear.
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Old 10-29-08, 11:14 AM   #16
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Brake Booster......replace it,its not gonna get better,and when it quits yer brakes are gonna lose stopping ability....longer stopping distances.
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Old 10-29-08, 11:52 AM   #17
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i don't have a (functioning) booster on my daily driver, and it stops fine. it just needs a little more pressure.

as long as you're not elderly or a cripple, you should be able to stop a car safely with or without power brakes. they're just there to save effort.
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Old 10-29-08, 12:30 PM   #18
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On my truck when it would loose the brake booster if I did not fix it the master cylinder would go soon after.
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Old 10-29-08, 02:42 PM   #19
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Problems with brake-booster will ONLY change the amount of force required for any kind of braking effort. Such as needing 20-lbs of pedal-force to push to sink it to the floor versus 100-lbs. However, it should NOT be sinking to the floor regardless of whether the brake-booster is working or not.

The issue you have is bad seals in the master-cylinder. Usually due to running it over rusty bores due to water-accumulation. Brake-fluid loves to suck in water from the atmosphere. In cold weather, the solubility of water in brake-fluid goes down and water precipitates out and causes rust. Make sure you change your brake-fluid once a year.
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Old 10-29-08, 03:17 PM   #20
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This car is only one year old, so seals and fluids should be in decent condition. Looks like I need to verify that the problem is 100% replicable and take it in since I'm still covered by warranty.
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Old 10-29-08, 08:14 PM   #21
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Problems with brake-booster will ONLY change the amount of force required for any kind of braking effort. Such as needing 20-lbs of pedal-force to push to sink it to the floor versus 100-lbs. However, it should NOT be sinking to the floor regardless of whether the brake-booster is working or not.

The issue you have is bad seals in the master-cylinder. Usually due to running it over rusty bores due to water-accumulation. Brake-fluid loves to suck in water from the atmosphere. In cold weather, the solubility of water in brake-fluid goes down and water precipitates out and causes rust. Make sure you change your brake-fluid once a year.
Agreed. Very common with hydraulic brakes. I've rebuilt MC's because of that reason. Some inner cyclinders can become corroded to the point where new seals won't help. Most shops will just replace the entire unit, depending on the car and part availablity. Honing and polishing a damaged cylinder bore is a specialized task, and many shops won't waste their time trying.
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Old 10-29-08, 08:17 PM   #22
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This car is only one year old, so seals and fluids should be in decent condition. Looks like I need to verify that the problem is 100% replicable and take it in since I'm still covered by warranty.
Curious... report the prognosis when you find out.
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Old 10-29-08, 08:19 PM   #23
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BJ - did the problem begin with any changing weather? Cooler temps of late? Does the problem go away once engine is warmed up?
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Old 10-30-08, 06:30 AM   #24
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Problem seems to occur only when the engine is warmed up, and I have not yet made any associations between its occurrence and weather conditions. Last night, I unsuccessfully tried to duplicate it with the engine off. Later, when I had to drive 1.5 miles to pick up my wife from work, I was unable to reproduce the problem while I was waiting for her.

Since the problem does not appear to be consistently replicable, what causes can I eliminate?
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Old 10-30-08, 07:10 AM   #25
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Try bleeding the brakes, but it's more likely a vacuum line issue to the brake booster. Replace the vacuum line and see if that works.
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