So I just replaced all four pads on my Trek hybrid's cantilever brakes (...for the first time in eleven years, he admits sheepishly).

Recognizing the potential for my own mechanical ineptitude, I then brought the bike into the LBS to have them adjust the brakes: center them, set the spacing, tweak the cable tension, whatever the heck professional wrenches do when an idiot like me brings a bike in and says "Brakes Need Adjusting."

And they did a fine job, inasmuch as the bike stops firmly, assuredly, and decisively. Except the front brake also makes A GODAWFUL CATERWAULING SQUEAL when I use it. I'm talking serious diesel bus/garbage truck kind of brake squealing, a louder, more horrific shreiking than even war-mongering banshees would emit. It's deafening, it's embarassing, and it's relentless. Mostly happens when I'm going down hill and/or coming to a quick stop. Once it starts, no matter how gently I try modulating the brakes, it just keeps going, like a siren. People four blocks away are ducking & covering.

And the back brakes, which are the exact same pad, are perfectly silent.

Any ideas? I beg of you...my ride partners beseech you...my neighbors implore you.

Thanks.

Well, I'm not a mechanic, but I have run into the same problem. I've had some success with 2 different approaches. The first you've already done, replace the pads. The second is to take rubbing alcohol and wipe the edge of the rim to remove any oil or rubber build-up on the rim. Be careful, it's not a good idea to let the alcohol go directly on to the pads. This process worked very well for me on my son's old Schwinn roadie. It has the side benefit of improving your stopping power slightly.
Have you checked with Sheldon Brown's website?

In my experience this is caused by oil on the rims.
To get rid of it, I clean the rims thoroughly with alcohol (I've had the best success with denatured, but rubbing works OK) and then lightly clean/ resurface the pads with a file or sandpaper.

I could be wrong but it sounds like the pads might need to be toed. The front part of the pad needs to touch the rim before the rear part of the pad. Loosen the pad a little and put a something thin like a credit card between the rear part of the pad and the rim. Center the pads and have someone apply the brakes. Tighten the pad. That's what I did to get rid of squeak and vibration.

Take the bike back to whoever installed the new brakes. I agree with SBW - it is likely that your pads need to be toed. It is easy to do if you have a 10mm wrench and a 5mm allen key, but they should take care of it for you. Squeaky breaks can often lead to bigger problems.

Welcome to the world of cantilever brakes. This problem has plagued mankind for millenia. Finding the right answer is like a long spiritual journey (through hell), exploring all the possibilities of pad angles, rim surfacing, brake boosters, and different pad compounds. The answer will be different for every bike.

Uh, dude, if ya don't know the answer, just say so. ( )

In addition to taking it back to whoever you paid to do the job right, and checking the toe, take the tire off the wheel, aggressively sand the section of the rim that the brake shoes contact with 320 grit, clean with brakecleaner. Then sand the pads with 60 grit or a file.

Toeing it sounds like to me. Allen key and it can be adjusted by hand lickity split when you're riding so you KNOW it's gone. Had the same thing happen when I changed pads out recently.

My commuter bike has cantis (Avid 6s), and like yours, the fronts squeal loudly. I have adjusted every which way - toeing, moving up/down etc. Sometimes the squeal will stop for a few miles, then it comes back.
I have learned to consider this a feature - when approaching a runner or another cyclist who I am passing, rather than calling out 'on your left' or some such thing, I just apply front brake gently, and they get out of my way. My wife knows when I am coming home since she hears me from a block away. If they ever do stop squealing I will have to learn to adapt.

The stock pads on my hybrid ended up very loud after a while. I'm talking bugle blast loud. People literally trying to jump out of their skin if I braked near them. Put on some new pads, problem solved to this day.

its impossible...toe in, toe out, sand down, sand up..nothing works...at least for me

This should be the first thing you try. For me it's been the No.1 solution to silencing the squealing

Two of my old Raleighs were the same way. New pads solved both. They were calipers though.

Hi Bob! Your hilarious description above EXACTLY describes my ride this Saturday. Were you able to solve this "squalling like a scalded cat" noise? What solution worked?

I had the same problem on my new Cross Check. The front (never the back) Shimano BR-R550 cantilever brakes squealed dreadfully on any firm stop, wet or dry. I cleaned the rims to surgical specifications, cleaned and sanded the pads, changed to Kool Stop Salmon pads, toed them in, toed them out, set them flat, tried different wheels, nothing worked. In despiration I even exchanged the cantilevers brakes for Avid SD7s and matching V-brake levers and had exactly the same problems.

What FINALLY cured the problem was to change the MTB-style front brake pads and holders for road bike equavalents. I installed Kool Stop Dura-type pad holders and Salmon pads and the squeal is finally gone. All I can figure is that the shorter holders and pads are more rigid and, thefore more vibration resistant. Braking did not suffer one bit from this change.

I was dealing with this issue last week after installing a new pair of brakes and pads, turns out the factory pads that came with my Avid Shorty 4's are crap. I'm including pictures so you can compare, as you can see the first picture (the factory for the 4's) have 3 sections, the middle being quite long and which I believe was causing the issues by not providing enough flex, and trust me, I did everything humanly possible before switching them out. The second picture is of the Avid ultimate pads which have 4 sections providing I believe more flexibility which I believe is what fixed this issue because once I installed the new pads replacing the factory pads I was good to go. Not sure if this is your issue but worth considering.

Is the OP supposed to do all this before or after he returns the bike to have the brakes done right? It does however beg the question, is making sure the brakes don't squeal part of a brake job? Supposing the LBS did everything right, are they still to be blamed because the brakes squeal?

Anyway, assuming the OP would rather try and fix the problem himself, I first say check for toe in. If that looks right, I just today had to deal with brake squeal from some V Brakes; suddenly started about 100mi on some new Kool Stop pads. Cleaning the rim brake surface didn't work, but the it did look uneven and glazed. Took some 220 grit sandpaper to it (sorry no 320, but it seems it got the job done in 1/3 less time) until the surface looked clean and even. I did a test ride and the squealing is gone. BTW, I have numerous bikes with cantilevers, V brakes, calipers (single and double pivot) and disc brakes (hydro and mechanical). Occasionally one will start to squeal for one reason or another. If I ever had a bike where I couldn't quiet the squeal, I would get rid of it and that is something I rarely ever do..

The pads are too hard.....or the pivot is worn/loose...or both.

If the pads don't breakdown while braking it will glaze the rim and pad,causing chatter.

If the pivot is worn/loose,no amount of breakdown will silence them because the arm can't control the toe-in while braking,causing the lead-in edge of the pad to dig into the rim,making it chatter. You can try giving it more toe-in than normal,than when the pad seats fully,it will preload the arm more when braking.

The front makes noise,the rear doesn't......which one do you normally use most...the front.I'm guessing the pivot is worn.