Recently I purchased a new Merida Crossway, and while the bike itself has been fantastic, I've been having almost non-stop issues with the mechanical disk brakes.
They began a loud squeal not too long ago, and while I did take it back to the shop where I was told they were sanded down and cleaned, the squealing soon began again. Taking matters into my own hands, I have removed them, cleaned both the pads and the disk, and made sure that they were aligned and set properly. While this works temporarily, they then begin to squeal again after only a few minutes of use. Straight after taking it out for a test, I removed the pads once more and found they were covered in this horrid black soot, similar to what I removed before the test.

What's causing this soot? Should I do a more thorough job of cleaning the pads? Will they need to be replaced?
Hopefully someone here can help, I've had nothing but problems with these brakes.

I think the cheapest solution is to replace the brake pads. Also make sure you clean the rotors with alcohol (denatured, not jack daniels)

GCN has a couple of good videos on fixes for noisy pads. Main points are - Straight / True rotors, Pads square with rotors, no oils or any product on either pad or rotor. And it helps to break them in properly.

Pads are porous, so sanding then only works with light contamination. Replace the pads, clean the rotors.

The "horrid black soot" is brake dust caused by the wear of the pads and is quite normal.

Merida is the bike factory,
You need to learn what company they bought the brakes from, and the model ,
to narrow down the 'which pads' quest..

They bought them rather than make them in house.

Thanks for the tips and the video folks, I know the type of brake the bike uses so I'll hunt around for a new set of pads. Knew the soot was produced by the pads but the buildup seems to correlate with the squeal - not sure what else could be causing it as both rotors and pads appear clean. I'll experiment with the technique tried in the video, sounds like it's worth a try. Thanks!

Are the pads aligned right? Does the action of the pad bow the rotor ie deflection? The pads should pinch clamp the rotor & not move the rotor laterally.

With a couple of exceptions most all the cable operated disk brakes only move one pad. For there to be no rubbing when not using the brake the other pad, often called the fixed pad, needs some gap between it and the rotor. So when the moving pad clamps against the rotor the rotor is pushed a tad and against the fixed pad.

Troul's comments are, in a perfect world, correct for a caliper where both pads are moved in during the braking action. But even with a dual moving pad caliper there is often a slight off center pad movement and a slight rotor pushing. Andy

Yep this is correct for the Crossway, only one pad moves inwards. I've made sure that the pads are aligned properly so I highly doubt that the issue lies there. My only guess is that there is some form of contamination within the pads which is causing the squeal, I don't know what else it could be at this point as all other variables have been ruled out.

i visually observe the rotor deflection from standing height distance to determine if the braking is adjusted to my satisfactory needs. If I can see rotor move inward when apply the brake, I'll adjust out the fixed pad until it is not doing so. Most of the time, the squealing brakes cancels out when improving the braking feel during the adjustment.

what works for me may not work for others.

Troul- Your goals are correct IMO. And I do much what you aim for too. But I tend to split hairs and don't like rotor rub when not braking so I leave a tad of gap between the fixed pad and the rotor. My customers don't musually have the skills or motivation to fine tune things if they remove and reinstall a wheel, so my tuning condition has a tad more gap for this then what I might do for myself.

BTW when one repositions a pad (usually the fixed side) it's relationship WRT the rotor can often slightly change. The pad shifts it's angle, it rotates within the caliper and/or the pad's backing changes the overlap with the caliper's body. All these slightly change the vibration that we hear as squeal. Andy