Brake pedal drops 2 inches at red lights -

[Shadowfax]

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.

A year ago I had an issue with one caliper siezed so 3 would grip and then the 4th would break loose, but they assured me that isn't the cause this time.

Any ideas?

[jdandy]

Brian.......Perhaps some air in the brake lines? Maybe a full brake bleed with fresh fluid will solve the issue. The braking system is the most important part of the entire vehicle. Anything out of the ordinary is cause for concern and should be corrected.

[soundqcar]

If you hadn't just replaced the Master Cylinder, I'd say it's likely to be leaking on you. That said, I concur with Dan. Might still have some air in the system. Tough to get it all out if not done properly. The AC in the car cycling on and off definitely changes the engine vacuum and could result in a softer brake pedal....although 2" is a bit much.

[Shadowfax]

I may have gotten to the cause with some suggestions from another site.

There is a One-Way Vacuum switch between the brake booster which could be bad.
As per Honda, it is normal behavior when AC is on and engine idling speeds change, so does the vacuum.

But I never remember having this issue while owning this vehicle for 10 years and there was also a Master Cylinder issue at the same time that was dealt with.

I am taking it back Friday for them to check the valve now that I know under what exact conditions it happens.

Thanks for the suggestions tho

[Shadowfax]

So they replaced the new Master Cylinder....again. Have not driven enough but appears to be fixed.