Question about Brake Pedal dropping towards the floor while sitting at red lights

H

Hyfi

Addicted Member
I seem to have an odd issue with my 07 Rav4.

Every once in a while after coming to a stop and applying the same pressure to the brake pedal, it slowly drops a few inches towards the floor.

I have had all new brakes, rotors, caliper pins and bushings, and lastly a new Master Cylinder. The issue is still present.

My wife usually drives it but I drove it all weekend to see what I could figure out and the only thing I can correlate to when it happens, is the Air Conditioning cycling on and off which changes the RPMs of the engine.

I was told that the only other thing in the brake system would be the Vacuum Assist, but if that was failing it would be harder to brake all together.

I spoke with the Service manager at Toyota and they were stumped. I am taking the car back to my mechanic Friday for another round of looking for an answer but thought maybe someone here would have an idea what may be causing the issue.

There is no issues braking, just the pedal dropping every once in a while for no reason.
 
Sounds like air in the system, or one of the fittings is not fully tight, so the pressure leaks off when you push hard.
If everything is done correctly, a firm pedal should stay firm.
 
They also bled the system 2 so far. There are no visible leaks. It could be a bad 3rd party new master but that should have repeatable issues, not just random.

The engine goes from a high idle to a lower idle with the air on when it happens, that is why I was thinking something just slightly off with the vacuum.
 
If this only happens while the AC is engaged, it`s most likely due to the changes in engine load from the compressor cycling on & off....
At idle, that Rav 4 engine is only putting out maybe 35-40 HP, and if the compressor is eating up about 30 to 40% of that power, the result will be quite noticeable....
 
pedal dropping slowly usually is brake fluid seeping by the piston in the master cylinder, look for a wet spot behind the carpet up above the brake pedal or on the firewall behind the master, also check to see if fluid level is dropping
 
I suppose that's better than having the pedal hit the floor while coming UP to a red light ...

Is there a proportioning valve that splits the air assist between the wheels? That goes, and I suppose it could confuse the computer.

And no - I wouldn't call it normal. It may be typical of that particular machine, but I'd consider it a defect. I assUme your dealer has checked for recalls or at least reported the problem and checked all the service bulletins.
 
switch AC off whilst at lights ..or use the handbrake .. or maybe you call it the emergency brake like we used to call it in the old days .
i can see a few ways around this . one is have the ac wired through a relay to brake lights so it turns off when brake is pressed . another is speed up the idle but only when ac is on . this might not be an easy task .
 
Hyfi said:
I seem to have an odd issue with my 07 Rav4.

Every once in a while after coming to a stop and applying the same pressure to the brake pedal, it slowly drops a few inches towards the floor.

I have had all new brakes, rotors, caliper pins and bushings, and lastly a new Master Cylinder. The issue is still present.

My wife usually drives it but I drove it all weekend to see what I could figure out and the only thing I can correlate to when it happens, is the Air Conditioning cycling on and off which changes the RPMs of the engine.

I was told that the only other thing in the brake system would be the Vacuum Assist, but if that was failing it would be harder to brake all together.

I spoke with the Service manager at Toyota and they were stumped. I am taking the car back to my mechanic Friday for another round of looking for an answer but thought maybe someone here would have an idea what may be causing the issue.

There is no issues braking, just the pedal dropping every once in a while for no reason.
Click to expand...

I do not know much about a RAV 4, but being a 2007, it has ABS correct? I have experienced and repaired this issue for other peoples vehicles and the solution was varied, just like all the replies here, it can be many many things. If it were a leak or a dangerous drop in pressure somewhere in the system, you should see a warning lite. If your fluid level does not drop, it is not leaking. Did you originally take it to be repaired because of this symptom or did it occur after a repair for another reason? The symptom seems to be related to engine speed, engine vacuum is highest at idle, so if you have a fluctuation at idle only, it must be vacuum related. When it drops...will it return if you pump the pedal? I assume you have an automatic transmission, I would experiment by putting it in neutral in your driveway, that removes a little load on the engine...see if it behaves the same way doing that. If not, pump the pedal a few times and see what happens. If the engine RPM stumbles when you pump the brakes, definite booster issue. Another check for that is to start the engine with no accessories running (the AC and radio cover noises), push the pedal to the floor, if you hear air rushing or hissing, booster is bad for sure and at that point the booster check valve should come with the new booster anyway. All of the answers I see in this thread are plausible, but the RPM drop when it occurs in this symptom leads me to vacuum related issues.

That is about all I know, hope it helps.
 
I have had mine in the old CR-V drop slightly towards the floor at a light when the AC would kick on--not a couple of inches, but I could feel it. I chalked it up to a change in engine vacuum many years ago, and it's never been a problem.
 
Thanks for all the help. I spoke with the mechanics yesterday and will take it in for them again on Friday.

This issue must have been happening at the same time we experienced the Master Cylinder issue and was not found because it was cooler out and we were not using the AC.

Now that the full brakes, calipers, and MC have been replaced, this symptom became more apparent as well as having the AC on all the time now.

Since I don't remember this happening, the distance it drops, over the 10 years I owned this vehicle, it can't be "Normal" as per Honda. A half inch, maybe but not 1-2 inches.

And yes, if pumped, it comes back up, but I had this happen 2x at the same light Sunday after stopping, then creeping up a little. Pedal came back up and when the AC cycled, it dropped again.
 
Something similar on my 79 Chrysler Cordoba with Power Brakes, turned out to be the old vacuum booster leaking air in.
Swapped in a new unit (major PITA) and the problem went away.
 
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