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Prius Squeak at Altitude

my 2007 Prius also has a chirp/squeak in the mountains and I didn’t buy the “condensation” answer given. A quick search of the Prius Online Discussion Forum turned up this from poster JeffD:

At high altitudes, any air bubbles in the brake fluid grow due to the lower pressure. It is air bubbles in the fluid that cause the “Chirp”. “Bends” are due to nitrogen bubbles in the blood that for due to excess nitrogen dissolved in the blood at very high pressure due to water depth.

link to the discussion:

My '04 Prius does the same thing at high altitude. The Toyota dealer in Lake Tahoe said that there is a part in the brake system (and I forget the name, but it’s something weird like “phantom brake actuator”) that can be replaced, but I’ve never had the time to leave my car there while up skiing. The sea level Toyota dealer won’t replace the part because they can’t reproduce the noise. I just ignore it and it’s had no obvious effect on braking performance.

Just out of curiosity, When does this “chirping” occur… and what ties it to the brakes.
ie it only occurs when you press the brakes… or it occurs when you let off the brakes… or it simply occurs when you go to Tahoe,

and/or do cars that sell in Tahoe experience this problen all/some/none of the time.

And assuming it is “the bends” how often does the car have the bends… once woudl seem normal. Then it is “unbent”.

the chirp occurs for me while the brakes are being depressed and above ~5000 ft or so. it happens pretty reliably with every brake press the entire time i am at altitude…which is probably at most 3-5 days. the sound goes away with descent immediately.

i dont see any reason to “fix” this.

this has happened to us on several mountains in the Sierras and Southern California…it’s not just Tahoe!

A Prius is very different than normal cars in many of it’s systems. Braking, or better yet slowing down is done by regeneration from it’s electric motor. The amount of power the motor will regenerate is a controled by the rate of slowdown dictated by the brake pedel preasure. This method of slowing transfers from regeneration to the brake calipers appling pressure to the calipers at 4 - 6 MPH. This is switch a very noticeable, most Prius owners lighten up on the brake pedal to minimize the wiggle the car does in the transition. I believe the caliper being applied is causing the chirp. So why in the mountains, maybe morning rotor rust, More agressive preasure required because of the steep hill, etc. That part Ray had right.

XMAN23 / AKA Prius Man

sorry to keep this alive, but it is not related to “more agressive pressure required because of the steep hill.” the chirp happens on perfectly level roads that just happen to be in a mountain range.

also, if i understand “morning rust” correctly, then the squeak should stop after using the brakes a few times, right? this is not what happens…the brakes don’t just make the squeak at the beginning of the drive. in fact, i recall recently having driven for hours (starting at ~sea level) and not hearing the squeak until braking to turn into the trailhead parking lot at ~8000 ft.

I agree with kellec (I was the first replier in this discussion). The dealer in Tahoe said it was related to the brakes. The chirp definitely sounds like it is related to the passage of air through an orifice. It happens the whole time I’m at altitude then goes away at below 4-5000 feet or so.

I’ve driven a lot of Prii at this point and found that the brakes, especially in cold weather, squeak when wet. What specifically is happening is that water gets in and affects the pump that repressurizes the friction brakes. This repressurization happens periodically, but mostly while braking - so you will occasionally get this weird sound when not braking, or just sitting there stopped.

I’m not sure if this is the sound heard in the original post though; I’ve driven a couple Priuses below 0F, and one above 5000ft, but never any higher than that - I’ve never gotten something I would necessarily call a “chirp” - the sound is more like a duck quack, and sounds a little bit like the horn honking. It can get you very annoyed looks from drivers around you.

The way you solve this problem is you let it dry a few days. It will go away on its own. I don’t know if it has any negative impact on brake performance, short-term or long-term, though I haven’t noticed any and I’ve experienced it 6 or 7 times.

Sorry to keep this topic going but I meant to comment on it when I first heard the car talk explanation on the air.

I also don’t buy the “condensation” answer. We have two Prii (2004 and 2005) and have driven them both on several long trips in all seasons to high elevations. We’ve been in deserts in the summer with very low humidity. Both cars squeaked when the brakes were applied and the squeaking always went away when drove down to lower elevation. The braking performance doesn’t seem to be affected with or without the squeak. The symptom is definitely altitude (air pressure) related and not associated with condensation. In both cars the sound seems to originate from the front right area of the car.

Usually all that you need to do is to bleed the brakes (sorry, but you need the special Toyota computer tool to do this - the dealer has one) to eliminate this “chirp”. There is a TSB describing the full problem and the repair procedure. If it happens at altitude as described here you probably will need to do the procedure at altitude (try in Denver).

My 2004 had this noise and bleeding the right front brake cured it.