MTB disk brakes feeling a bit lacking...

[Black Sheep]

in 2006 I fitted deore hydraulic disk brakes to my mountain bike and the braking performance was pretty good.

They now feel to be quite lacking, even compared to my wife's cable pull disk brakes on a much newer bike.

admittedly the pads are the original ones from 2006, the fluid in the brakes is only a year or two newer.

Am I right in thinking the issue is likely the pads are a bit glazed and a quick scuff with some sandpaper is what's needed?

Worth replacing the fluid?
I know with motorbikes you're meant to replace the fluid at set intervals.

 

[DCBassman]

Brake fluid is hygroscopic, ie it absorbs moisture over time, and this degrades its performance, hence the change intervals specified for road vehicles. I don't know if there's the same need for bikes, but it would certainly be worth changing fluid and pads. I'd bet there was a good deal of improvement.

 

[Black Sheep]

I did wonder if that might be the case,

where do I start with finding the right stuff for my brakes?
CRC don't seem to stock the shimano stuff and I'm not sure what DOT it is

 

[Drago]

Shimano oil is a type of mineral oil, completely different stuff to normal DOT fluid. It isn't as bad as regular DOT fluid, but Mr Bassman is right, it will eventually suck nasties from the atmosphere and need replacing. Plenty cheap on the likes of ebay.

 

[Black Sheep]

Cheers, can do pretty much anything else on the bike (other than suspension service) but hydraulic brakes on the mountain bike are a bit 'new' to me.

any guesses how much fluid is needed to replace what's already in there?
I know it partially depends on the length of hose between lever and caliper but wanting to get enough to do the job without getting too much.
on ebay it seems to come in 50ml or 100ml...

 

[fossyant]

Buy an Epic bleed kit for shimano. They supply everything including easy to follow instructions.

Change your pads but clean up the discs first. Degrease them and lightly sand the rotor to ensure no contamination.

 

[Kajjal]

You should find the below useful :-

http://www.epicbleedsolutions.com/

 

[PeteXXX]

Make sure you use the correct specified brake fluid. The wrong type might damage the seals.
Check first, but I believe Shimano use mineral oil.

 

[Black Sheep]

if the bleed nipples are the same size as those used on motorbikes I'm sorted as I've got a kit for the Honda, just need the right fluid

I presume the easiest way to change the fluid is the same as a motorbike, open bleed valve, pull lever until level drops to the minimum, top up, repeat so that you've flushed the fluid through without putting air into the system?

 

[I like Skol]

The requirement to change the brake fluid as a 'service' item only serves 2 purposes, and surprisingly neither of them are feel or braking efficiency (well, not directly anyway). Bleeding to cure sponginess as a result of air in the system is another kettle of cod.....

Fluid is changed to remove contamination, usually water, which is absorbed by the fluid. DOT brake fluid is terrible for this while the mineral oil that Shimano specify and use is much more resistant to the water. Having moisture saturated brake fluid does not directly affect the brake performance in the way you might think. The two biggest problems caused by moisture contamination are;

1. Corrosion of brake components internally. Causing pistons to seize in calipers or fluid leaks where the brake seals are forced to run over corroded surfaces.
2. Reduced boiling point causing loss of braking when fluid gets hot. Normal DOT4 fluid boils at around 230°C but this drops to around 150°C when the moisture level increases. This is not a problem until it suddenly reveals itself, usually just when you most need your brakes, such as halfway down a steep twisty road/trail!

If your brake lever feels firm and the calipers are closing and retracting freely then changing the fluid will be unlikely to help improve your brake performance. A good clean of the rotors and a new set of pads is likely to be the solution.

 

[Black Sheep]

Thanks for that, the pads look to have a fair amount of wear left in them, but might be best to replace

 

[Jody]

This is not a problem until it suddenly reveals itself, usually just when you most need your brakes, such as halfway down a steep twisty road/trail!

Had this on the MTB when running Elixir brakes at almost 50mph and also in my old Mini Turbo whilst going quite quickly. Scared the sh8t out of me on both occasions.

 

[I like Skol]

Had this on the MTB when running Elixir brakes at almost 50mph and also in my old Mini Turbo whilst going quite quickly. Scared the sh8t out of me on both occasions.

Never experienced it despite enthusiastic efforts! The thought of hitting the brake pedal/lever and it sinking to the floor/bars is frightening. I have over cooked brakes on a car when descending Holme Moss towards Holmfirth quite quickly once. The pedal remained hard but the brakes suddenly had about as much stopping power as if someone had replaced the pads with blocks of ice! I have never fitted cheap pattern brake pads to any car since. All other vehicles have continued to work even with discs glowing red when fitted with genuine pads.