Random brake failure

As I drive places I see my brakes work, I can step lightly and it stops. If I need to step very hard for something, my brakes fail to work, the same way for a moment, because the next time I need them, its not lightly that makes it stop, I can think , I can, yet I am still moving, to stop I must give it all I have. That light brake feelimg returns in a moment later. The fear of not being able to stop my car.

If your brakes do not work, you should be afraid. Do not drive this car anywhere or you could cause an accident. It sounds like your car has a bad brake master cylinder.

I strongly urge you to not drive this vehicle until it’s fixed. Have it towed to your trusted mechanic.

What’s likely failing is the brake master cylinder. When you push the pedal, it pushes two pistons in the master cylinder. The pistons push fluid under pressure to the calipers, which clamp the rotors to which the wheels are secured, stopping the car. When you push it lightly you’re apparanetly not creating enough pressure in the fluid to cause it to bypass the worn out seals on the MC pistons, but when you push it hard the seals cannot hold the pressure and are failing, thus not pushing fluid to the wheel calipers.

This is an extremely dangerous condition, both because in an emergency you’ll be unable to stop and because the seal will ultimately fail totally, very likely in the very near future.

Please, I urge you, do not drive this vehicle. It’s a disaster waiting to happen.

You may have a defective brake booster. It sounds like it is not replenishing vacuum properly. Often, a leaky master cylinder can leak fluid into the booster and damage the valving.

In any case, get it fixed soon.

While I am having a hard time wading through the OP’s fractured syntax, one thing comes through loud and clear, namely a problem with his/her brakes.

Any time there is a problem with being able to stop your car, the only appropriate action is to immediately drive the car (or tow the car) to a reputable mechanic. Continuing to observe the condition is not a safe approach, and asking for online help is not going to help the OP to stop the car.

Please–for the sake of pedestrians, bicyclists, and drivers with whom you share the streets and roads–get this car to a mechanic tomorrow, before you wind up in a serious accident.

Thanks the reply its helping. The car is near 10 years old, it did this worse 10 years ago, yet when it was under warranty the Chevy dealer said there was nothing wrong with the brakes doing this, they only did what the service adviser wrote, replace something or add something, rather than its this problem, find it, fix it. My main problem at the time was to get them to fix the air conditioning, it alway leaked freon, they thought that adding freon only would solve the problem, never did. I went there near 10 times to have them fix it, then the brake problem problem happened, he tested my brake pedal while sitting in park with the door open, then wrote replace master cylinder I thought, not sure he really did, it might of been the brake booster that was the problem, the problem was still there, i asked again, no problem they said, will drive it over night to their home and back, still could not see the problem. Then when I needed my boots replaced on the CV Joints I found a place that does the boots and brakes, when they cleaned the CV Joint then replaced with new grease, they cleaned every thing around the brakes too, then replaced everything with out doing anything to the brakes, just put them back on, the problem stopped yet I would get this squeak on a turn or backing out the driveway. Then I had this noise when I braked at a stop, I found someone near me to work on my brakes, the squeak in a turn went away, backing out noise went away, when I stop the noise went away. They told me it was leaking in the pistons in the back, they replaced the part that holds the piston and seals, in the drum brakes, cleaned the area there and replaced the front brake pads, they said the brake shoes were good, not replaced. Maybe the Chevy dealer never replaced the master cylinder and not noticeable until I brake harder than usual, like you wrote it bypasses on a harder stop, rather than build up pressure. Something creates the pressure, to press lightly on the brake pedal that is adequate, yet its not adequate after being pressed hard for a moment, then accelerate then brake, next its extreme force to stop, yet its adequate after not using it for a moment. How you explain the pressure in the master cylinder, how its reacting is good, I press slightly, yet not enough to bypass, yet press hard causes it to bypass creating not enough pressure, yet pressure builds later in a moment. Could the brake booster be doing it, it feels like squeezing a a bag full of water, I sometimes wonder if I am rolling backwards, as if I took my foot off it slightly, then wonder if I imagine it.

The brake booster uses the engine’s vacuum to pull a diaphragm and assist you inapplying the brakes. The diaphragm pulls the rod that your pedal pushes, that rod that goes directly into the master cylinder and pushes the pistons.

A malfunctioning booster will make the pedal feel hard, and it can malfunction intermittantly. The reason I suggested the MC was because I understood from your post that you were intermittantly losing your brakes, and while a bad booster will make the pedal hard it will not cause you to lose your brakes entirely.

Having said that, if the MC is in fact defective and is leaking past the rod and into the booster it can damage the booster, as Hillta27 suggested. A shop will look at this when they do the diagnosis.

Whatever the final diagnosius turns out to be, the MC, the booster, or both, the vehicle should not be driven until it’s repaired. It’s very dangerous in this condition.

I noticed what I wrote recently could be wrote differently, perhaps. I noticed that using the brakes in my car, can be strange. If I press lightly on the brakes I get a sudden response, it feels like a fluid filled bag I felt with my hands, a little bit creates a controlled stop. To test a theory, I found a safe place, I found that pressing hard does get a response, it does stop suddenly, maybe not as soon as I like perhaps, yet not skid, when I accelerate again from a stop, then press looking for a response, I find it feeling hard and solid, not responsive after a light touch, to stop I must exert all I have to get some response from it, the response is not adequate, the thought I have is I am pressing yet still moving, must put all I can to get it to stop, yet that fluid feeling and resposive touch does return in a moment, just by doing nothing put wait a moment.

If you have figured out how to recreate the problem, take the car to a mechanic who is very familiar with brakes and have them take the steps you have to recreate the problem, with you in the car. If a mechanic can experience the problem, they can diagnose it and repair it.

Yes good idea. Because the chevy dealer did not think anything was wrong, I tried to find a reason for it happening, I think I found a reason where someone can see it happen now, thanks for the help, perhaps I will get it fixed now. Now someone to fix the problem, if I try the right person. Bye, thanks.

This new description makes it sound more like a booster malfunction. But either way, it needs to be hands-on diagnosed and repaired. Making assumptions about brake problems, or driving with intermittantly malfundtion brakes, is dangerous.