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so I am creating a few posts about bearings and wheels, (story so far):

how to reassemble mountain bike rear wheel hub and bearings

mountain bike freewheel hub stopped rotating

I have now got the wheels, bearings and freehub working. Started riding to work this morning and I almost couldn't stop at a junction. Thankfully wasn't going too fast. I physically can not get the wheel to lock. I have to use both brakes at full force to get the bike to roll to a stop. If I only use one brake I would not stop at any junction in time.

Could the cause be any of the following:

  • Over greased bearings?
  • Over greased axle?
  • not tightened cone hubs enough?
  • not tighted axle enough?

I do not have any grease on the disk brakes them selves. Rather nervous getting on the bike now. Don't seem to be doing too well with this bike home fixing thing.

over greased bearings?

EDIT

Forgot to mention that it is hydraulic disk brakes.

EDIT 2

So in the end I took it to the LBS as I didn't have much confidence in my self at this point. They had to replace the front pads but where able to clean the rear and cleaned the rotors. (I had tried to clean the rotors but obviously not vigorously enough). He said it was a bit of a pain to do. All is working again. They double checked my wheels and said I had put them back together correctly which is reassuring.

Thanks for all the help

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"hydraulic disk brakes" - Is the fluid level topped off? Any air in the line can reduce brake pressure by several orders of magnitude. Do they feel soft to engage? –  crasic Jun 21 '11 at 2:14
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4 Answers

up vote 1 down vote accepted

If the brakes appear to be engaging fully then it sounds like you have some contamination on the rotors/pads. I would clean the rotors throughly and replace the pads. I have heard it is possible to bake contaminants off pads in the oven, and if you want to give that a shot google for it.

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will give the rotors a good clean, can't get pads that quickly but will have a look at them tonight. Thanks –  Jon Jun 17 '11 at 15:58
    
Nice tip on the oven baking idea. –  ʍǝɥʇɐɯ Jun 18 '11 at 22:27
    
Brake parts cleaner does a good job of removing grease and oil most of the time, as pads are pretty dense and do not soak up most contaminates. Clean them liberally, let dry and then a light sanding of the pad surface should do it. Don't forget to clean the brake rotor. Buy it at any Auto parts store. –  Moab Jun 27 '11 at 2:24
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That would be my thought... Are you sure with all that work you've done you didn't get any grease on the pads or rotors?

I would pop out the pads and clean them thoroughly with something like acetone.... Likewise the rotors. Then make sure the cable adjustment is good and the discs are centered...

I am just working on the first bike I've handled with discs, so I'm doing a bit of quick research... These are the Shimano cable-pull models. The rear one is not centered properly and drags.

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will give the cleaning a good go, brakes are hydraulic (forgot to mention that, have updated post). Thanks –  Jon Jun 17 '11 at 16:00
    
Acetone can leave an oily residue, best to buy specific brake cleaner from any auto/hardware shop or rubbing alcohol. –  Mauro Jun 21 '11 at 12:09
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You may also need to re-center your disc in your caliper. I use Avid BB7's, and the owner's manual gives you the directions to re-center the disc. This is basically making it so that when the pads impact the disc, the pads on both sides pinch down on the disc. If the disc is too far off-center, only one pad will rub, which isn't really enough to stop the wheel.

This may or may not apply to your brakes, but I can't imagine it not being in the instructions.

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If the feel of the brakes (i.e. when you squeeze the lever) has changed since your maintenance then there may be air in the system, and you may have to bleed them. If this is the case then they will feel spongy, and the levers will probably move further back towards the handlebars than you're used to. This can sometimes happen if there is air in the reservoirs and you leave the bike upside down while you're working on it - the bubbles work up into the brake cables.

You can sometimes avoid having to bleed the brakes if you pump the levers repeatedly, but this is normally only a temporary fix.

Having said all that, I wouldn't expect braking to be as badly affected as yours seems to be, so it's more likely that there's some contamination on the discs or pads - maybe some spray lube got onto the rotor? Or maybe just greasy finger prints on the rotor? The best solution I've found is disc cleaner - just spray this on the rotor then apply the brakes and it will clean the pads as well.

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