I bought a new 2006 Honda CRV in Jan of 2006. Several months later, the brakes failed completely on the freeway (no accident, thankfully) and the dealer replaced the master cylinder based on the history of the problem. I just took it in to the dealer (different Honda dealer) for a 30,000K mile service and told them that when the air conditioning is on and I am at a stoplight, the brake pedal gets soft and goes down about an inch or two. The dealer repairma tried it and said the brake went all the way to the floor with the a/c on. They then told me that it was okay, that the “pressure” in the a/c system and brake system are shared and it is okay to drive. I’ve got 27k miles on it and need to drive across country next week. Is my car safe to drive or do I have another brake or master cylinder problem? Thanks in advance for any advice. I’m totally amechanical.
You do have a problem. A brake problem. The pressure is not shared with the air conditioner and the brake pedal should never go to the floor.
I suspect you may have a slave cylinder problem that they missed. I hope you have documented what has happened. I would want something in writing from the dealer that the brakes are OK and give it to my next of kin. No car with a brake problem is safe.The brake pedal should not sink when the AC system cycles on and off, and it should NEVER go to the floor under any circumstances, and if, indeed, the pedal went to the floor the vehicle is not safe.
I’m not sure what “pressure” the mechanic thinks is shared by the braking system and the AC, but there is little, if anything, shared by both.
The AC system may use vacuum to operate air flow control flaps, and the brake booster is vacuum operated, so maybe that’s the connection.
I think someone else should look at this vehicle. Problems with the brakes are not something to ignore.
I just thought I’d chime in with my 2 cents. My family has nothing but honda’s. We have a 2000 Accord that, when the air is on will do a similar thing with the brake pedal. This is what I’ve rationalized.
You have your foot on the brake, stopped at a red light. The AC compressor is on, and then it goes off. When it goes off, the pedal goes down a little bit more. What I think is happening is that when the AC compressor turns off, there is a slight increase in RPM. But since your foot is off the accelerator, the throttle valve is still closed and the increase in RPM causes an increase in the vacuum. Since the vacuum gets more powerful, the brake booster has a little more umph and for the same application of force from your foot the booster will apply more force to the brakes, causing the pedal to go further. In plain english, the vacuum increases and the pedal gets sucked down further when the AC compressor turns off.
I’ve experienced this most pronounced on the 2000 Accord, and less pronounced on the 2005 Civic, and hardly at all on my 97 CRV.
I have a Honda Accord, a Subaru Legacy, and a Ford Ranger. The cycling of the AC compressor has no effect on the brake pedal in any of them.
A slight movement of the pedal, a fraction of an inch, perhaps, would not worry me much, but the original post said the brake pedal went to the floor, and that should never happen under any circumstances.
There is no direct connection between the AC and the brake system. However, when you press the brake pedal you are assisted by a brake booster that used the engine’s vacuum to help. It does this via a diaphragmmatic assist known as a “brake booster”. The engine’s vacuum comes from the pistons attempting to pull air in while restricted by the closed throttle plate.
This is a longshot wild guess…but if when the AC is engaged the compressor loads down the crank too much and the ECU responds by increasing the idle speed too much the vacuum could drop enough to be felt via the drop in brake boost. However, that would not make the pedal drop. It would only make the pedal feel harder and you might perceive this as a drop. Understand that I’m really really stretching here.
The real problem is the pedal dropping to the floor. You have a direct hydraulic connection to the brake pads via the pistons in the Master Cylinder compressing the hydraulic fluid and the fluid pushing the slave pistons in the cylinders to force the pads against the disks. The pads pushing against the disks stop the fluid and the fluid stops the piston in the MC, which stops the pedal. The only way the pedal can go to the floor is if the fluid when compressed leaks past the seals in the master or slave piston chambers…or leaks out to the outside world. If it were leaking to the outside world you’d lose your brakes. I’m inclined to think it’s not in a slave cylinder because that would not prevent the system as a whole from pressurizing, it would only prevent that specific cylinder from operating.
In short, I think you may still have a master cylinder problem. That or a leak are really the only two things that will allow the pedal to go to the floor.
could also be improperly bled fluid, or fluid that has, for whatever reason, boiled.
Do you have ABS? If so, I think there is a connection with the cycling of the AC. It may somehow cause the ABS to relive the pressure temporarily. This is NOT a normal condition, its a defect in the ABS pump. I don’t know the details but I heard about this once before in the foggy past of my memory.
I agree with the vacuum theory. When the A/C compressor cycles off or on it causes a change in engine speed which causes a small fluctuation in vacuum. The additional vacuum will increase power assist, which will cause the pedal to sink down some.
You might have the dealer check out the check valve on the booster, but I think that all is in order.
Exactly how hard did you push the pedal to get it to the floor? Any brake pedal is floorable if you push hard enough.
-Matt
Just a note here. Yes, brake fluid is described as an “incompressible” fluid. But this isn’t 100% accurate. Any fluid has some compressability to it. Otherwise the brake pedal wouldn’t move at all, except to take up the freeplay between the peg and the pushrod. A brake pedal connected to a perfect brake system will still move if you push it hard enough.
Brake fluid simply offers less energy lost to compression as opposed to say, air, or nitrogen, or some other gas. And brake fluid is also very difficult to boil.
-Matt
Thanks so much, everyone, for your helpful comments. The car does have ABS and the dealer did mention that the a/c and brakes share a “vacuum”, not “pressure”. Based on this discussion, I’m taking it to another mechanic to look at the situation. I don’t like the way my will reads right now so definitely need to have my brakes in order! Thanks again. This is my first posting and it has been very, very helpful. I’ll add more info if anything interesting comes up. DMKDMK