High pitched noise when brake pedal is pressed firmly (when stopped, it’s not pad wear indicators)

I’m getting a high pitched noise/whine/squeel… (like air being let out of a balloon sort of sound) when I’m pressing the brake pedal while stopped. I heard this as I was stopped in the garage and pressing on pedal. I had the wife press the pedal while I looked around underneath, at fittings on the calipers and lines that I could see from the wheel wells. Next I popped the hood wondering about the master cylinder or the booster and the sound was louder, seemingly coming from that area. I have not (yet) discovered any leaks.

What could this be? Any experience with it? Next steps?

I’m trying to load a video but the site is not letting me…maybe something with IPhone?

Ooof. I’m just getting issues fixed from a ‘get off’ (get off the road) this winter, almost caught up...

 

Diaphragm leaking in brake booster?

 

To be 100% honest, brake boosters are a bit of an unknown to me beyond my vague understanding of them based on what their name implies. I don't really know the ins and outs of them.

Is that a curtains type thing for a brake booster?

Trucks 21 years old and has 238k on the clock.

 

At that mileage installing a new master cylinder/brake booster set is money well spent. Simple job, I just did mine recently.

 

That is good to hear, thanks. I have always heard mystic talk about brake boosters going and it sounded like an expensive part, but I didn't really know.

I guess I better start looking into parts and prices.

 

I purchased mine from NAPA. Make sure to use a proper line wrench on the brake lines. Do a search for a booster replace thread. You’ll be fine.

 

Awesome, thank you Sir. Yes, I've got a good set of line wrenches for bleeds, and I'm just now doing that exact search

 

Pretty sure Timmy the tool man has a video about this job.