Very poor rear brake performance

joestella28

Hi all, I've recently started getting into mountain biking and so bought a second hand starter bike in the form of a Voodoo Bantu. All in all I'm very happy with the bike (only paid £180 for it) but for some reason the back brake is awful. Both calipers are Shimano BR-M445's and the front is spot on, admittedly the front disk is 180mm and the rear a 160mm but the difference between the two really is night and day.

When I say poor I mean I can barely lock it up on the soft stuff even when really giving the lever a handful.

It also makes a hell of a lot of screeching when using. I've changed the pads (though the old original Shimano ones still had plenty of meat on them) and I also changed the brake oil and bleed them. Though the disk appeared to be in great condition I swapped it for the front to see if that would change this but the same symptoms remained.

Any advice would be much appreciated as I've run out of ideas (short of upgrading!)

Many thanks,

Joe

Greer_

The brakes should work so somethings just gone wrong. A bleed wasn't needed though.

The pads AND rotor may have been contaminated so when you changed the pads, you may have just contaminated the new ones. You may just need to change the pads again and clean the rotor and calliper with isopropyl alcohol.

Darkstalker420

Lightly sand down pads with wet n dry and give the rotor a good wipe down with isopropanol alcohol. Then follow the standard bedding in practice (some say it works some don't). But i found it did help me.

Thanks.

EDIT: Lol good advice in two's

Greer_

Darkstalker420 wrote:

Lightly sand down pads with wet n dry and give the rotor a good wipe down with isopropanol alcohol. Then follow the standard bedding in practice (some say it works some don't). But i found it did help me.

Thanks.

EDIT: Lol good advice in two's

That will only work if the pads were glazed. If they were contaminated it won't have much effect.

joestella28

Thanks guys, without sounding stupid, what would they be contaminated with? I removed the pads from the caliper and used a spacer when I bled them so no contamination from the oil. I'll give the disk a clean tomorrow and will try gently sanding the pads with wet & dry tomorrow.

Thanks again,

Joe

cooldad

Ever spray lube or anything near the back end? How do you lube your chain?

allthegearnoidea

pop the pads out and see if they have a shiny surface on them, if so then light rub on some sand paper to remove the sheen, lightly rub the disc with sand paper too and clean off with some isopropyl alcohol as mentioned, that should give you a clean brakign surface, also check that your pistons are moving equally, check one sides not moving more and pushing the disc over! and also try and make sure they are straight on the disc as that can cause awful squeel,
finally i'd look at the lever adjustment, could be that the rear lever is too far in and that your not getting enough movement on it to allow good pressure, a friends bike was like it, front was great rear wouldnt hold at all, would the lever out half a dozen turns on the screw and as good as front!!

contamination can come from anywhere, lube, spay oils, frame protection sprays etc, good way to se how far it travels is to spray some upwards out the can infront of a window in bright light, the spray mist travels Meters!! not a lot of bike shops will advise using sprays on bikes for this purpose!!

Plyphon

Do you pressure wash your bike off after a ride? This is a massive no-no for many reasons - but it can also spray grease/oil places where it shouldn't

Sometimes you're just unlucky.

I had it once, just went for a ride - was very muddy so I guess something nasty just splashed in there - perhaps oil from a tractor or something. Pad wasn't ruined but performance was, as you say, night and day.

It just happens.

As people have said you need to clean rotor when you change pads. Take it off if you have the right heads and give it a good going over both sides

joestella28

Hi Guys,

Thanks for the replies, I have cleaned the rotor with Isopropyl alcohol and also gave pads some wet and dry treatment, as mentioned they certainly look less shiny now.

I do use wet lube but always apply with a rag to minimise any fling on any bike, and I haven't washed it yet (literately only been out for about an hour of dry use). Seeing as it was like it when I got it I'm guessing contamination happened pre-my ownership.

I'll give the bike a whirl tomorrow and see if it's worked.

Thanks again all,

Joe

Greer_

You'll have to bed them in again. About 20 hard stops with each brake then pour water on them. If the water sizzles, the pads are hot enough and they have been properly bedded in.

cooldad

Or just use them normally, which works just as well.

YeehaaMcgee

Just checking, but, you know Shimano brakes use Mineral oil, not DOT fluid, right?

Chunkers1980

Good point. People should just stop messing.

Greer_

YeehaaMcgee wrote:

Just checking, but, you know Shimano brakes use Mineral oil, not DOT fluid, right?

That thought crossed my mind too!

YeehaaMcgee

It never ceases to amuse me, the amount of black magic voodoo people spout about "bedding in" their brakes.
i have to wonder, do they do these things with their cars as well? And Clutch plates?

joestella28

Haha, yes I am using the correct mineral oil, we'll see tomorrow if the problem has been fixed

Cheers,

Joe

Greer_

YeehaaMcgee wrote:

It never ceases to amuse me, the amount of black magic voodoo people spout about "bedding in" their brakes.
i have to wonder, do they do these things with their cars as well? And Clutch plates?

Bedding in I don't mind. It's the ones who talk about baking their pads in the oven for an hour and the likes that gets me! If a pad is contaminated, it's wrecked! It can't be fixed!

YeehaaMcgee

Greer_, your'e one of the medicine men...

Greer_

Meh, was ignoring that! I've heard many different things (most of which appeared on my thread a few weeks ago) and that was the only reasonable one I could find!