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I have a fairly heavy e-bike and I'm a fairly heavy rider. My bike has mechanical disc brakes, and around 5 months ago I was having an issue where the brakes became useless, as in when I applied them they did nothing.

I think this was due to the extension of the brake cables so I tried to tighten them. After I did this when I pulled the brakes they seem to catch for a second then they would disengage with a click sound.

I ended up taking it to the bike shop who said they repaired by adjusting the brake cables. It was fine for a few months but again the problem seems to have occurred (though in this case it's just the front brake). I did try tightening the brake cable and it seems to have made it better (but I have yet to properly test this).

Why does this happen? I got the impression from the repair it was a pretty straight forward, anyone have any suggestion on how to fix this?

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As you describe it, it looks to me like the normal pad wear, that happens with all mechanical brakes (rim or disc): as you use the brakes, you wear pads (=material is removed), so you need to periodically adjust tension to compensate for that wear (and when you change the pads, you also need to "cancel" the adjustments you've done as the pads wore).

The "catch during a second and then disengage" seems to like insufficient torque on the adjustment bolt used to pinch the cable.

This video explains the basics of mechanical brake maintenance:

(It's for a drop bar bike, the principle remains the same on a flat bar bike, except that the barrel adjuster is rather found on the brake lever rather than on the caliper)

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    I think thats it sorted thanks! I think the issue was the brake pads wore down meaning the caliper was going passed its maximum range. In any case I did essentially what Chris H suggested and seemed to have sorted it thanks.
    – John Tracey
    Commented Mar 2 at 14:25
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I agree with the other answer that the pinch bolt might not be tight enough, but that could be a symptom of something else: If you haven't adjusted the fixed pad for some time, you have to pull harder than normal on the lever.

Most mechanical disc brakes move one pad with the cable, and that presses the rotor against the other pad. As this fixed pad wears, it needs to be screwed in further (on the ones I have; others may use a different adjustment), working through the wheel. If you don't do this, you're pulling hard just to flex the rotor enough to touch the 2nd pad. Even worse, it can touch the brake body first, giving very little friction. I reckon you need to adjust the fixed pad every other time you adjust the cable, if not every time. The fixed pad wears unevenly before this point, so will need to settle down a bit after adjustment.

Some mechanical disc brakes have an adjustment for the moving pad that's not the cable tension. I've never worked on those, but it's worth checking.

So if your brakes are a bit weak, and you pull extra hard, that can make the cable slip.

One other situation I've faced is that when the brake is in need of adjustment and so has extra travel, if the cable moves stiffly the brake lever can spring back and catch the wrong bit of the cable head (where it meets the cable, rather than under the head of the mushroom shape). That effectively shortens the cable, until it slips back to the normal position. You would experience that as a biting point that over many rides needs progressively longer pulls on the lever, then suddenly shorter pulls, but suddenly longer again.

Finally, if this catch and then slip behaviour can't be fixed with adjusting the cable tension and fixed pad, check for uneven pad wear, and especially damaged pads.

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    A third way that the engage-click-disengage can happen is when the barrel adjuster is screwed out all the way. In that case, it may first hold a bit with its first thread, but then fall one thread deeper in the hole as the force increases. After all, brake levers are usually made from aluminum, and a thread or two of aluminum simply cannot withstand the tension of the brake cable while braking.
    – cmaster - reinstate monica
    Commented Mar 3 at 11:28

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