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I have a Vw Polo 6n2 from 2000 without ABS. A while back I replaced rear drum brake cylinders and shoes and then I bled them. Concerned with the brake fluid level in the reservoir which went below minimum due to a broken wheel cylinder I went about and tried bleeding the front calipers as well, however the bleed nipples were stuck all rusty. Fast forward to just recently when I got the time I managed to bleed the front calipers. However, the front right caliper is now stuck.

Now, the pedal does travel to the middle point before getting hard, however if I continue to force the pedal a little bit further I hear a what I only can describe as a flute like sound. At this point the pedal moves further down by about a centimeter, IIRC, but only when I push it hard.

I recorded a little video of it which you can listen to here: https://i.imgur.com/37l5mkW.mp4

I should add that the low fluid level shown in the video is not the issue, it did sound exactly the same before it went that low as well as after I filled up the reservoir. At about 0:08 in the video I hold my foot on the pedal on the point where it has pressure and then push harder a few times in short intervals.

It does come from the engine compartment, and using a long screw driver as a hearing aid I think I was able to locate it right at the end of the vacuum hose in connection with the hole on the brake booster. I'm not sure if it's coming from the brake booster itself or the check valve.

I'm not sure it made any sound the first time when I only bled the rear. It was almost a year ago so I don't quite remember. It could have but I'm not sure.

I was about to do a pressure check by pumping up the brakes and then starting the engine, however just then the engine died and wouldn't start again and I strongly suspect a faulty fuel pump. Mainly due to the fact that I barely had any fuel left in the tank for a long time. So I cannot inspect the brakes any further that way. Unless I fix the fuel pump of course, but I'm considering selling or scrapping the car so I'd like to be conservative about time and money.

One thing to note, I cannot build up pressure in the brake booster when the engine is off. The brake goes to the middle just like it does when the engine is on. I did manage to pump the brakes with the engine on for a little while before I ran out of gas. After the engine had died, I did manage to build up pressure to the point where the pedal was really hard and wouldn't move. However, this pressure did decrease after some hard pushing on the pedal.

Is that normal? If I understand this correctly, isn't the sign of a faulty brake booster actually a really hard brake pedal?

Is it possible that this is caused by the stuck front right caliper? My handbrake tends to get stuck in the left rear drum brake as well, but not the right one, if that would mean anything.

As an amateur I wonder, is it

  1. the brake booster,
  2. the check valve,
  3. the master cylinder,
  4. air still in the system,
  5. due to the stuck brake caliper,
  6. nothing to worry about or,
  7. is this normal?

Many many many thanks in advance!

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  • Welcome to Motor Vehicle Maintenance & Repair!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 28, 2022 at 15:07
  • Now that the brake pedal is hard and there is no vacuum left in the reservoir, two questions: (1) Are you still getting the flute-like sound? (2) If you step on the brake hard and hold strong steady pressure on it for a minute or two, does the pedal eventually go all the way to the floor?
    – MTA
    Commented Aug 28, 2022 at 22:15
  • @MTA (1) Yes, but not as pronounced. I'm not sure but it might be slightly less pronounced, like it's waning off a bit, after a few steps on the brake and after I leave it alone for a day or so it's more pronounced again, I think. This time when I pushed the pedal really hard it did not sound at all, only a tiny bit occasionally. I find it strange because the flute sound has been quite audible before. I've been scratching my head on this for a while now. (2) No, not at all, it moves maybe a centimeter at most although I push as hard as I can. I recorded a video: youtu.be/RAIR2axdJdM
    – neortic
    Commented Aug 30, 2022 at 10:45
  • 1
    @neortic OK, so it's not a failing seal in the master cylinder because the pedal stops and does not go further down even after prolonged foot pressure. Just a stab in the dark, but it sounds to me like either the check valve or the actuator on the diaphragm in the brake booster rubbing against its dust seal. In either case, I don't feel that it's dangerous or a warning of sudden loss of braking. More an annoyance than anything. Not confident enough to make this a full answer, but hopefully enough to ease your concerns.
    – MTA
    Commented Aug 30, 2022 at 12:43
  • 1
    @neortic If air is in a brake line or caliper, the pedal will always be soft. The lack of vacuum (supplied by running engine) makes the pedal feel harder than normal. That is expected and OK. After turning off engine, each pedal press will be harder and harder until all vacuum is used up. Side note about frozen caliper: this is sometimes caused by the brake hose for that wheel collapsing internally, making something like a 1-way valve. If you loosen the bleeder for that wheel and fluid shoots out and the caliper un-freezes, it is a bad brake hose. I've had one, easy fix.
    – MTA
    Commented Aug 30, 2022 at 13:52

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