Frequent Brake Vacuum Booster Noise

A Prius cannot use vacuum to boost the brakes, because the engine is not always running to pull a vacuum.

So the Prius does not use vacuum in the braking system for anything. Instead, it uses an electric pump that pumps brake fluid to high pressure in a nitrogen-filled accumulator.

The pump sounds a bit like a rattlesnake, or a joy buzzer. From your description, the pump is probably what you are hearing.

When the brake system is in good condition, you will only hear the pump every few times you use the brakes. If the brakes are not touched, you should be able to sit in the car and read short books between runs of the pump.

If you are hearing the pump run more often than that, and especially if it is as often as 15 seconds, the system is in poor condition.

There are two assemblies under the hood:

(Because of some PriusChat breakage from the recent server migration, that image doesn't show up. You can view it here.)

The lower one in the drawing contains the pump, but the upper one contains most of the valves in the brake system. When the pump is running often, even though the sound you hear is from the lower unit with the pump, the problem is most often in the upper unit with the valves. One of the valves may be allowing some brake fluid to pass when it should be completely closed, and the pump needs to run more often to make up for it.

People for whom cost is no object might just replace both assemblies at once and avoid having to think further about which one has the problem. Others will usually start with the unit on top.

 

That year and that mileage should be covered under Dealer Warranty. There's a TSB for it.

 

RCMN-13V235-0909
RCMN-13V235-9056

^ These are the numbers on the pdf format TSB's I cadged. You could search for those. I'd post them, but that's no longer possible, since the server replacement.

 

Here is one version of the Toyota extended support on the brake booster. However Toyota requires one of the codes. The typical repair is $2500.

 

You are getting the best. Chap likes the master cylinder only; when mine went out years ago I got both and its good to know both parts were new. I now have more miles on the warranty parts than I had when the original parts failed. Some guys want minimum, Toyota and I prefer reliability.

 

I'm not sure it's a matter of what I 'like' ... I've mentioned before that Toyota changes both parts, and that's what you'd do if money's no object. Sometimes people post who are outside of the free replacement coverage and who say that money is an object for them, and to those people I'll say that if you're going to start with one part or the other, the odds are on the upper one (brake booster assembly, also known as master cylinder) because that's where most of the little valves are to wear out.

As to the question in #7, that upper part ("brake booster assembly" in the drawing) is also known as the master cylinder. And it's also known as the actuator, and as the skid control ECU. In gen 3, all those things are in that part.

The part number prefix of the booster/master cylinder by itself is not 04000-, it's something else (47050-, I think?). 04000- prefixes are for "kits" that include a thing itself and some other stuff, like a gasket maybe, that you're going to need when you change it.

It's useful to know the part numbers for those kits, because sometimes the kit price is lower than the price for the thing by itself.