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The front Shimano SLX (m7120) hydraulic disc brakes on my trail bikes have lost their mojo. It still brakes but less aggressively than a year ago. Possible causes are worn pads and air bubbles in the oil lines (maybe something else?)

How can I tell what type of service I need for the brakes before going to the LBS?

ysap
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2 Answers2

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Check the pads, there should be at least 1mm of material left, as in the separator spring should be at no risk of contacting the rotor. Replace them if needed.

Check the hose line for kinks which will rob the system of power. You’d need a new hose line if that was the case.

Clean the rotors and pads with brake cleaner or isopropyl alcohol. After cleaning in this way the system will need a short bedding in period.

A small amount of clean sand dumped on top of the callipers/pads just before you ride can help speed up this bedding in. The sand helps to rough up the friction surfaces.

Cleaning the brake system with solvent is not an action you would do regularly due to the need to bed in again.

Once you eliminate contamination of the pads or rotor, or worn out pads you can bleed the brakes.

Shimano MTB brakes can be bled in several ways. You’ll need a Shimano bleed cup.

Mini bleed.

  1. Loosen lever on bar and rotate so the bleed port is the high point.
  2. Remove screw from bleed port. Don’t lose the O-ring.
  3. Install the cup in bleed port. Be gentle with plastic threads. Partially fill cup with a cm of your favourite flavour of mineral oil.
  4. Flick the lever by pulling all the way to bar and releasing. You’ll see an amount of air bubbles exiting the reservoir. Keep flicking several times till the air stops coming. Doesn’t take long.
  5. Plug cup and remove. Reinstall bleed port screw with O-ring. Again be gentle with small aluminium threads.

You might see dark particles in the exiting fluid which would be a signal that a full bleed is needed.

Full bleed. Bottom up.

New fluid is pushed by syringe up from the calliper. The bleed cup captures the old fluid.

Full bleed. Top down aka gravity bleed.

New fluid falls down down from cup to the calliper and collected in a container for disposal.

There’s many online tutorials for all three methods. You can buy kits online for the full bleed. Try the mini bleed first to see if that peps up the performance enough.

If your situation is that you can’t do a full bleed there’s nothing wrong with getting the LBS to deal with the mess and hassle of getting it right. The process does take practise.

Anecdotally, I’ve been mini bleeding my SLX brakes for about 3 years without a full bleed and they are still factory strong.

Warren Burton
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  • Clean the rotors and pads with brake cleaner or isopropyl alcohol. --> quick note that immediately after doing this, brakes were certainly have less power until the pads have re-bedded into the rotors – Paul H Nov 07 '22 at 04:31
  • Wouldn’t sandpaper be preferable to clean sand? I imagine sandpaper is more readily available. – Weiwen Ng Nov 07 '22 at 18:14
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    I live near the coast. Clean sand is abundant. Sanding doesn’t bed pads into the rotors, a small amount of clean hard scratchy stuff between rotor and pad while cycling the brakes can help if it’s available. Squirting water on the callipers can also do the same trick. – Warren Burton Nov 07 '22 at 21:55
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    https://bicycles.stackexchange.com/questions/2193/why-how-should-i-bed-in-disc-brake-pads – Warren Burton Nov 07 '22 at 22:04
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You need to diagnose the issue of why they are poor in braking. Compare them to rear brakes, if your rear brakes are good then you have a valid comparison.

One possibility is that they don't have excessive sponginess, so the amount the lever moves for a given force is the same. In this case, the issue can be:

  • Contaminated brake discs, you may try cleaning them with isopropyl alcohol
  • Contaminated or otherwise bad brake pads, the best solution is to just replace them

You may also want to test different types of pads, if you have organic resin pads (that are cheaper and less noisy), you may get more braking force and longer pad lifetime at the expense of more costly pads, shorter rotor lifetime and more noisy braking, by choosing sintered metallic pads. Note not all rotors work with metallic pads, so you may need to change the rotors in some cases if switching to metallic pads.

Another possibility is brake fading, so the brakes work initially just fine but if going down a steep and long hill, they gradually reduce braking power. In this case, the problem may be that you have resin pads without cooling fins. Switching to metal pads with cooling fins solves the issue.

Third possibility is that the brakes are spongy. To find out if this is the case, take a good bike with a good brake (such as your rear brakes). Compress the lever with as much force as you can, carefully noting how much the lever moves. Repeat the same for the bad brake, and see if the lever moves significantly more, closer to the bars. If this is the case, you need a brake bleed, but you may buy yourself more time before having to do a brake bleed by adjusting the levers so that they are further away from the bars at rest (this only works if you have large enough hands, and only if your lever has this kind of adjustment).

My experience is that hydraulic disc brakes generally don't offer as much braking for heavy persons as good rim brakes would offer. The mechanical advantage of hydraulic disc brakes is good, but with very great braking forces (so great that the rear wheel would rise from the ground for a 100+ kg person), the levers would move too close to the bars. So even with good properly bled systems, there's usually some remaining sponginess which is a real problem of hydraulic disc brakes.

juhist
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