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danny
danny, Mechanic
Category: Car
Satisfied Customers: 5063
Experience:  Mechanic at school corp bus garage! Experience in fleet diesel and regular automotive.
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When I hit my brake pedal while driving, it has no resistance

Resolved Question:

When I hit my brake pedal while driving, it has no resistance and hits the floor. the car still slows and stops, but takes longer to do so. I can not immediately stop the car if I needed to. Before this started happening sometimes the car shutters lightly when I drive with out hitting the brakes, and when I put my foot UNDER the brake pedal and lifted up on the pedal it'd stop shuttering. Sometimes once in awhile it would even sound of grinding when I hit the brakes and would stop for a while if I slam the brakes. That problem comes and goes. I recently had ALL 4 rotors and pads (ceramic, used the brake grease) replaced with new parts. As far as I can tell I am not losing ANY brake fluid. I believe I have a sticky caliper, but that wouldn't cause the pedal problem where it has no resistance and hits the floor. I have not opened up the hydraulic system to the brakes, so I don't think it could have gotten any air in the system. There is no proportion/ metering valve. I am also a bit car savvy myself.

I will respond ASAP if I can when you answer so expect a quick reply to all answers. I have it set up to send me a text once someone helps me.

Submitted: 4 years ago.
Category: Car
Expert:  danny replied 4 years ago.

I am stil going for air and you need to rebleed system. or you have a malfuntion in your vacum brake booster check valve. Here is a little test for air in there.

 

 

  • Run the engine at idle for approximately 1 minute, and stop it after applying vacuum to the booster. Depress the brake pedal normally to change the vacuum to the atmospheric pressure. Check that distance between the brake pedal and floor panel gradually increases.
  • Depress brake pedal while engine is running, and stop engine with pedal depressed. The pedal stroke should not change after holding pedal down for 30 seconds.

CAUTION: Depressing pedal interval is approximately 5 seconds.

Customer: replied 4 years ago.

How would air get inside the hydraulic system with out exposure? I have not worked on the brakes at all other then to replace the rotors and pads which does not allow any opportunity for air to get into the lines. If there was any leaks I should be losing fluid and Im not.

 

You'll have to give me a chance to perform your diagnostic tests.

 

I am hoping more for a bleed the system problem (especially since I was about to replace the brake fluid soon anyways) rather then a booster problem. Booster problems can be expensive.

 

If it is a booster problem is it fixable or would I need new parts, like a new booster?

 

Customer: replied 4 years ago.

How do you apply vacuum to the brake booster exactly (I don't have much booster experience)? Doesn't the vacuum always occur as long as the hose is attached?

*****UPDATE 1***** I tried to follow your instructions as best as I could but I am having a hard time understanding what I should be doing... Can you help clear them up better so I can undersand them better. Anyhow when I disconnect the brake booster hose I hear a quick short suddenly release of air (PSSHHH) and if the engine is running while I pull the tube off or if I start the engine with the tube off, the tube has a vacuum. Now the problem seems to happen only when you slowly compress the pedal down while driving. If you slam on the brakes now 9 time ouf of 10 it'll suddenly stop the car. However if you try to perform a gradual stop, the brake pedal sinks to the floor before it begins to stop the car at all, and if you gradually press the pedal and let it sink, and then try to stop the car suddenly you get this real wierd hard to describe nois that seems to come from the front of the car, seems to be accompanied by a grind now and its almost like back of the car hops (it is rear wheel drive). I never experienced that before....

 

If you PM me I will send you my cell phone # XXXXX we can talk in person, probably be a bit more effective.



Edited by BMSeaton on 2/10/2010 at 7:12 AM EST
Expert:  danny replied 4 years ago.
I understand how you want to chat on the phone about this but we are not allowed to work out side this question box. we are not allowed to send or recieve any personal info either. I will explain the vacum for you.
Expert:  danny replied 4 years ago.

we are just talking about pumping up the brake pedal like so.

 

With the engine stopped, change the vacuum to the atmospheric pressure by depressing the brake pedal several times. Then with brake pedal fully depressed, start the engine and when the vacuum pressure reaches the standard, check that the clearance between the brake pedal and floor panel decreases.

 

So do this before doing the the first procedure I sent. Put tube back on and your normal vacum will come back. then go through these steps. if it fails then replace the check valve.

Customer: replied 4 years ago.
Sorry I haven't gotten back, I plan to soon. I have not had a chance to test this yet, busy schedule.
Expert:  danny replied 4 years ago.
Me too. Let me know how it goes.
Customer: replied 4 years ago.

Turns out it was the brake master cylinder. I was receiving bypass inside the cylinder which was not an external leak hense why I wasn't losing any fluid. However the cylinder inside the brake master cylinder was not displacing the brake fluid properly... And when the brakes are applied slowly, the fluid is allowed to leak by the cylinder, while the pedal hits the floor, the cylinder still is able to activate the secondary brakes (rear brakes which only power 20% of your brake power) and thus the car still stops, but slower to do so. When you hit the brakes hard, the cylinder pushes on the brake fluid fast and hard enough that the fluid is displaced and acts normal because it is not given the time and ability to leak past the cylinder.

 

I swapped out the brake master cylinder and the problem went away.

Expert:  danny replied 4 years ago.
Good job. I am glad your brake issue is resolved.
danny, Mechanic
Category: Car
Satisfied Customers: 5063
Experience: Mechanic at school corp bus garage! Experience in fleet diesel and regular automotive.
danny and other Car Specialists are ready to help you
Customer: replied 4 years ago.
Thanks for your help. I am going to accept this because you were a help.

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