Stop and check first
Weak brakes are a safety problem. If your bike does not slow properly, stop riding it until you know why. Test at a walking pace in a safe area, not in traffic or on a descent.
Quick checks: pads, rotor, lever feel
Three quick checks help narrow the cause:
- Pad wear: Look into the caliper. Very thin pads, or any visible backing plate, mean it is time to replace them. Check your brake maker’s guidance on minimum pad thickness.
- Rotor condition: Look for oil, shiny glazed patches, deep scoring, or bends. Rotors also wear thinner, and many have a minimum thickness marked on them.
- Lever feel: Does the lever feel spongy, pull to the handlebar, or feel firm but give little stopping power? Each points to a different cause.
Contaminated pads and rotor
Oil, chain lube, grease, or some cleaning products on the pads or rotor are a very common cause of weak braking, often with squealing.
- Clean both sides of the rotor with isopropyl alcohol on a clean, lint-free rag.
- Inspect the pads. If they look dark, wet, or glossy, they may be soaked. Many manufacturers advise replacing contaminated pads, since cleaning may not fully remove oil.
- Find the source, such as chain lubrication (see using WD-40 on a bike chain for why sprays are risky near brakes) or sprays used near the wheel.
For related noise, see how to stop squeaky bike brakes.
Worn pads and rotors
As pads wear, lever travel can increase and braking can feel weaker. Replace worn pads, and check the rotor against its minimum thickness marking, if present. A worn or damaged rotor should be replaced. After fitting new parts, remember to bed them in.
Mechanical disc brakes: cable slack
On cable-actuated disc brakes, a lever that pulls far or feels soft often means the cable has stretched or the pads have worn.
- Use the barrel adjuster to bring the pads closer.
- If you run out of adjustment, loosen the cable pinch bolt, pull the cable tight, and re-tighten.
- Many calipers have pad adjusters that set the gap on one or both sides.
The full process is in how to adjust rear bike brakes, and the same ideas apply at the front.
Hydraulic disc brakes: spongy lever and bleeding
A soft or spongy lever on a hydraulic brake often points to air in the system, and sometimes a leak or low fluid. The usual remedy is a bleed using the correct fluid for your brake. Brakes use either mineral oil or DOT fluid depending on the brand, and mixing them can damage seals, so check the manufacturer’s specification.
Many riders take a bleed to a shop. If you do it yourself, follow your brake maker’s instructions exactly and keep fluid away from pads and rotors. Look for signs of leakage at the hose, lever and caliper, and treat a leak as a reason to stop riding.
Avoid squeezing a hydraulic lever with the wheel out, since the pistons can move out and make refitting difficult. If that happens, see brakes that will not spring back for how to deal with pistons.
New pads or rotors that need bedding in
Brand-new pads and rotors often feel weaker until they are bedded in, a process that lets friction material transfer evenly to the rotor. Many manufacturers describe a series of controlled brake applications at moderate speed. Follow their procedure for your pads, and avoid hard emergency braking until the brakes feel consistent.
Why disc brakes lock up or grab
Grabbing or sudden lock-up can come from contamination that bites unevenly, glazed pads, uneven bedding, or a caliper that is not centred. Hydraulic brakes can also be very powerful, so on a loose surface the front wheel can lock easily. If a brake grabs unpredictably, clean the rotor, check the pads, and see a shop if it persists.
Squeal and rub
Squealing even when you are not braking usually means something is rubbing. A misaligned caliper or a slightly bent rotor are typical culprits. Re-centre the caliper as described in fixing front brake rub.
When to go to a shop
Get professional help if the lever still feels spongy after pad and rotor checks, you suspect a leak, you are unsure how to bleed the brake, the rotor is bent or worn, or the brake has not improved after cleaning and adjustment.
FAQ
Why are my brakes not stopping?
Common causes are worn or contaminated pads, a contaminated or worn rotor, cable slack on mechanical discs, or air in hydraulic lines.
What would cause disc brakes to lock up?
Possible causes include contaminated or glazed pads, misalignment, or a very powerful brake on a low-traction surface.
Why do disc brakes squeal when I am not braking?
Usually something is rubbing, such as an off-centre caliper or a bent rotor.
What are the most common problems with disc brakes?
Contamination, worn pads, rub, spongy levers from air in the system, and pistons that stick.