What's wrong with my brakes?

[Gad-Westy]

A couple of years ago, I bought a secondhand Giant mountain bike equipped with Shimano hydraulic disc brakes. The first bike I've ridden with that set up. I remember the seller warning me to be careful with the brakes to avoid going over the handlebars. But I found them pretty weak from the off. Seemed to need a lot of effort to stop quickly and generally seemed unimpressive.

I rode the bike for the first time in a few months recently and the brakes were awful. Totally ineffective. So I decided to do a bit of an overhaul. I bought some new brake pads for both ends (cheapish amazon ones but good reviews), some mineral oil and a bleeding kit. I also gave the discs a thorough clean with brake cleaner. My first attempt at bleeding them was a bit of a fail on my part as I didn't RTFM but the second time, I followed the instructions to the letter, pushing fluid up from the bleed valves and saw plenty of air escape.

However the brakes are worse than ever! I can pull the front brakes as hard as I can and can still move the wheel if I push hard enough.

So what am I doing wrong? ste pads? Though hard to believe they could be that bad. Poor bleed technique? Or could I have bad seals in hydraulic set up somewhere?

[ucb]

You've still got air in the system
Its probably easier to take the calipers, lines and levers off the bike and hand them up vertically to bleed them. And then do it again, and tap the lines and calipers to release any trapped bubbles.

Edited by ucb on Monday 20th February 19:01

[Lotobear]

TBH I've always found pulling the fluid through rather than pushing it from the caliper to be most effective method of bleeding on Shimano brakes.

[Gad-Westy]

Lotobear said:

TBH I've always found pulling the fluid through rather than pushing it from the caliper to be most effective method of bleeding on Shimano brakes.

Funnily enough that is what I did first time round before I read the instructions. I think the results were better than when I subsequently pushed.

[Gad-Westy]

ucb said:

You've still got air in the system
Its probably easier to take the calipers, lines and levers off the bike and hand them up vertically to bleed them. And then do it again, and tap the lines and calipers to release any trapped bubbles.

Edited by ucb on Monday 20th February 19:01

Cheers. I'll maybe give that a try then.

[OutInTheShed]

If it's a matter of bleeding, then the lever will run out of travel without generating any pressure.
Is that what's happening?

Other things that can go wrong:
Seized piston
Jamming pad
Various other things which mean the pressure in the fluid doesn't create force on the pads.

Pads contaminated with oil

Alignment. Pads don't contact disc evenly.

I have Shimano disc brakes on a pretty basic mountain bike.
They are the best feature of the bike.
I dare say there are several flavours of 'shimano disc brake' and I suspect mine might be the cheapest as it's a bike that I guess would be under £500 new?

The brakes are not 'over powerful' if I want to lock the back wheel and stand on the pedal in a fairly low gear, I do need to squeeze the lever pretty hard.
But they are nicely controllable and feel reassuring going down hills I can't pedal up.

So, if bleeding isn't obviously the issue, I'd suggest checking all the pistons move and everything else looks right.
Often when disc brakes on bikes aren't right, you can see the disc bend when you squeeze the brake.

I've never had to bleed these brakes so can't comment on technique for that.

[Gad-Westy]

I'm starting to think that the bleed kit I have isn't sealing correctly on the handlebars. When the bike leant over slightly, I notices some oil dripping off the lever. I'm thinking that if there is any negative pressure at all on the line, it could draw air back in at the top. So I'm going to try again with an assistant and maintain positive pressure on the syringe throughout.

I watched a guide about checking disc alignment too so I'll do that while I put it all back together.

[nickfrog]

You may simply have a slightly leaky seal that contaminates the disc/pads.

[TheLurker]

nickfrog said:

You may simply have a slightly leaky seal that contaminates the disc/pads.

This. And did you degrease the discs before replacing the pads?

The other thing, is have you bedded the pads in yet? I find quite a difference after a few hard stops. New pads are almost useless before this IMO.

[Gad-Westy]

TheLurker said:

nickfrog said:

You may simply have a slightly leaky seal that contaminates the disc/pads.

This. And did you degrease the discs before replacing the pads?

The other thing, is have you bedded the pads in yet? I find quite a difference after a few hard stops. New pads are almost useless before this IMO.

Discs were cleaned with brake cleaner prior to pad install and I also checked that the pistons were moving freely.

Since my last post, I meticulously bled the brakes again with the calipers off the bike and dangling with as straight a vertical line as possible to the levers. Kept positive pressure on the syringe at all times and tapped away at the calipers and lines as I pushed new fluid in. Took care to keep the pressure on as I tightened the bleed valve back up. When I reinstalled the calipers, I made sure that I aligned them to their discs.

Result: no improvement at all. I cannot lock either wheel up at stand still, no matter how much pressure I apply.

So that leaves me with nickfrog's suggestion of a leaks (but both of them at once?) or pads that need bedding in, I've done about 10 miles on them so far and made sure I've tried some big stops (though quite undramatic with current set up but my hands got a work out!). I'll persevere with trying to bed them in more but if that fails, I might try some more reputable pads. I really don't think there is anything else I can do on the bleeding side though open to ideas.

[nickfrog]

Yes unlikely to happen to both calipers at the same time.

[Lotobear]

Do you get a hard 'bite' on the brake levers and a high bite point, or do they feel spongy and start to bite close to the bars?

[Gad-Westy]

Lotobear said:

Do you get a hard 'bite' on the brake levers and a high bite point, or do they feel spongy and start to bite close to the bars?

The latter. They absolutely do not bite but resistance increases to something that can slow me down. I cannot pull the levers all the way to the bars but not that far off.

[Lotobear]

I would say you probably still have air in the system then, or possibly poor seals on the brake levers?

The latter description is exactly what I had before bleeding and then after bleeding by pushing the fluid up to the bleed reservoir (I then found that pulling it through from the top was more reliable). When they came good the bite point was high and firm - alsmot like hitting mechanical resistance.

Presumably you have an o ring on you bleed reservoir or have not over tightened it?

Worth checking the tube is tight on the syringe as well and not sucking any air in there.

[RoadToad84]

Tip I learnt on motorbikes was to bungee the lever back to the bar after bleeding and leave overnight. Helped force any air out

[Gad-Westy]

Lotobear said:

I would say you probably still have air in the system then, or possibly poor seals on the brake levers?

The latter description is exactly what I had before bleeding and then after bleeding by pushing the fluid up to the bleed reservoir (I then found that pulling it through from the top was more reliable). When they came good the bite point was high and firm - alsmot like hitting mechanical resistance.

Presumably you have an o ring on you bleed reservoir or have not over tightened it?

Worth checking the tube is tight on the syringe as well and not sucking any air in there.

Cheers. I think I've ruled out most of that out but I might see what I can rig up to try and pull fluid up rather than push. I do wonder about the brake lever seals though.

[Gad-Westy]

RoadToad84 said:

Tip I learnt on motorbikes was to bungee the lever back to the bar after bleeding and leave overnight. Helped force any air out

Thanks. I did wonder about doing that. I guess I just remove the fill screw on the lever and leave it open over night with lever pulled back?

[Lotobear]

Gad-Westy said:

RoadToad84 said:

Tip I learnt on motorbikes was to bungee the lever back to the bar after bleeding and leave overnight. Helped force any air out

Thanks. I did wonder about doing that. I guess I just remove the fill screw on the lever and leave it open over night with lever pulled back?

That's a good suggestion - I've bled troublesome hydraulic clutches that way by wedging the clutch pedal down over night with the cylinder cap left off - the bubbles slowly migrate up the system to the MC, and it does work.

But, TBH Shimano brakes are not usually throublesome to bleed (unike certain other ones) which makes me think you must have a fault issue somewhere.

[snotrag]

Your not bleeding them correctly.

The fact you mention something 'sealing' onto the master cylidner end is a giveaway - nothing needs to 'seal' here as these use are open brakes with a rubber bladder to compensate for fluid expansion, and are gravity bled.

Then the issue about keeping pressure on the syringe to 'force' fluid in - again, this is totallty wrong.


Shimano brakes, like Hope and some other bike brakes, and like the brakes on your car, are gravity bled, with an open reservoir at the top and using the master cylinder itself to push fluid through, ejecting air out of the bleed port at the other end until no more comes through.

[OutInTheShed]

I can't see a part number on my Shimano brakes.

The lever travel is adjustable.
The front brake has the end of the lever 3" from the grip at rest.
Nothing much happens until the lever is about 2.2" from the grip.
It's then progressive braking, by about 1.8" from the grip I'd be squeezing as hard as possible and worrying about going over the front.
Using both hands, it isn't getting within 1.5" of the grip.

If it's going nearly all the way to the grip, I think it probably is a bleeding issue (unless the pads are moving a very long way before touching the disk???) and the master cylinder is running out of travel.

The back brake seems to be adjusted with the lever initially ~0.3" closer to the grip but the progression is similar, the lever isn't going close to the grip even when gripped like it's an old Ducati clutch.