Hydraulic disc brakes

[Alan O]

I have a couple of hydraulic disc brake issues that hopefully someone knowledgeable will be able to help with.

I have a GT MTB with Shimano Deore LX brakes - new pads when I got it a few months ago, very good stopping and 'bite'.

What I want is to have two sets of wheels set up, one with big knobbly tyres for the mud and one with tyres more suited to road/cycle path. I got all the parts, and I tried fitting the new front wheel the other day. The rotor position isn't quite the same and it rubs a bit - it's slightly too far in towards the wheel. I'm presuming I just need some shims to move the rotor out a fraction - does anyone run two sets of disc brake wheels and does this make sense?

The other issue is that after I put the original wheels back on I discovered that the front brake had gone quite spongey - more travel in the lever and less bite. I'd had the bike upside down while I was doing all this (I don't have a stand), and it suddenly occurred to me that that might not be such a good idea with a brake system that relies on fluids. I also went out today just to check how the brakes work, and the front brake has almost no stopping power now - the rear one that I haven't touched is still fine.

I'm wondering, therefore, if I've interrupted the brake fluid system and now need to bleed the brakes (which will be another learning experience, I guess). I'm also wondering if maybe I've managed to get some fluid to leak out on to the pads and stop them gripping.

Any thoughts on what an idiot I might be? Might I need to bleed the brakes and fit new pads?

Alan

 

[Deleted member 23692]

I run two seats of wheels and got round the slight variation problem by using the same model hubs on both wheels. However when running different model hubs I just reset the callipers on the frame/forks each time I swapped wheels. I probably only swapped a dozen or so times a year and it wasn't really a hardship.

Turning a bike upside down shouldn't affect the hydraulics as it's a sealed system, assuming you had no air in there beforehand. Did you push the pistons back in the caliper? If so they'll take a few good pumps to move them back in contact with the rotor

 

[Cubist]

Easy bit first. You don't need to shim anything, all you need to do is slacken off the bolts holding the calliper to the frame or fork and recentre it.

They may need bleeding, but it's unlikely unless you have actually lost fluid. Get the bike upright and pump the lever. You may be able to regain pressure. Another option is to tap the calliper and hose with a screwdriver handle and then tape the lever in the closed position for an hour or two. If this improves things then consider bleeding them as there is air in the system

 

[Yellow Saddle]

As others have said re adjustment.

If your bike was upside down AND you manipulated the brake lever, then air has gotten into the system and you will have to bleed it.

Diagnosing air in the system is easy. Pump the lever and see if it "hardens" up. Take careful note of how the first lever depression felt because the change can be subtle. If you feel a change, let it stand for an hour or two and pull the lever. If it had gone soft again, there is air and bleeding is your only option.

 

[Custom24]

As others have said re adjustment.

If your bike was upside down AND you manipulated the brake lever, then air has gotten into the system and you will have to bleed it.

How does this introduce air into a sealed system? This is a genuine question from me, not Socratic irony.

 

[Alan O]

Thanks folks, that's a great help.

Yes, I did operate the lever when the bike was upside down. I've just tried pumping the lever now, and it did harden a little. I'll check again in a couple of hours, but it does appear I need to bleed - still, it's good to learn how to do it.

I was hoping to be able to swap wheels without adjusting the brake callipers, as I'll probably swap quite often. But it does seem to be quick and easy to adjust, so a few minutes every time I change will be no hardship - and I won't bother with the shim idea.

Alan

 

[Yellow Saddle]

How does this introduce air into a sealed system? This is a genuine question from me, not Socratic irony.

The system isn't really sealed, it just has a lid on it so that the fluid can't fall out when turned upside down. Imagine the fluid reservoir at the lever and the level of fluid in it. The fluid doesn't reach all the way to the top of the lid because then you can't push the pads back if needed. Now, if you turn the bike upside down, the air above the fluid is now where the reservoir's feed hole to the master cylinder sits. Normally the fluid's surface tension and minute size of the hole will keep the hole closed but if you pump the lever you will pop the "seal" (surface tension) of the brake fluid and air will enter a few mm into the system. Now if you turn the bike back up, the fluid returns to where it is supposed to lie but traps the bubble inside the line. The bubble won't float up, again because of surface tension and the small size of the capillary. Pumping the lever then drives it down or simply creates a sponge in the system. By sponge I mean a rubbery effect.

Some reservoirs have a rubber bellows in the lid that extends into the fluid and technically prevents air between fluid and bellows, yet still allows you to push the pads back and raise the bellows level. Even so, there may be air in there if the mechanic didn't make it overflow when he fitted the lid/bellows.

Other systems like SRAM Elixir don't have a reservoir, just an supposedly-full flexible bladder. But that's just a bubble trap. Bubbles make their way into the system particularly well with those brakes. I consider those to be utter rubbish or at best, a huge PITA.

 

[Yellow Saddle]

Thanks folks, that's a great help.

Yes, I did operate the lever when the bike was upside down. I've just tried pumping the lever now, and it did harden a little. I'll check again in a couple of hours, but it does appear I need to bleed - still, it's good to learn how to do it.

I was hoping to be able to swap wheels without adjusting the brake callipers, as I'll probably swap quite often. But it does seem to be quick and easy to adjust, so a few minutes every time I change will be no hardship - and I won't bother with the shim idea.

Alan

The bleed will be easy and surprisingly quick, if you are set up properly. The bubble is so tiny you won't even see it coming out. Fortunately it is right at the top, so you'll only have to inject a few CCs of fluid from the bottom up to expel it.

 

[Custom24]

The system isn't really sealed, it just has a lid on it so that the fluid can't fall out when turned upside down. Imagine the fluid reservoir at the lever and the level of fluid in it. The fluid doesn't reach all the way to the top of the lid because then you can't push the pads back if needed. Now, if you turn the bike upside down, the air above the fluid is now where the reservoir's feed hole to the master cylinder sits. Normally the fluid's surface tension and minute size of the hole will keep the hole closed but if you pump the lever you will pop the "seal" (surface tension) of the brake fluid and air will enter a few mm into the system. Now if you turn the bike back up, the fluid returns to where it is supposed to lie but traps the bubble inside the line. The bubble won't float up, again because of surface tension and the small size of the capillary. Pumping the lever then drives it down or simply creates a sponge in the system. By sponge I mean a rubbery effect.

Some reservoirs have a rubber bellows in the lid that extends into the fluid and technically prevents air between fluid and bellows, yet still allows you to push the pads back and raise the bellows level. Even so, there may be air in there if the mechanic didn't make it overflow when he fitted the lid/bellows.

Other systems like SRAM Elixir don't have a reservoir, just an supposedly-full flexible bladder. But that's just a bubble trap. Bubbles make their way into the system particularly well with those brakes. I consider those to be utter rubbish or at best, a huge PITA.

Thank you. I never considered there needed to be air or at least flexibility in the system, although it's obvious when explained. Could the OP then try pushing the pads back in once more with bike right side up? Could that force the bubble back up far enough to float back up to the lid?

I also think your response above needs to be stickied as a FAQ somehow. There is a misconception I think among cyclists that you must not turn a bike with hydros upside down. Based on your explanation, I think that should be "avoid turning it upside down, but if you have to, do not operate the brake levers"

 

[keithmac]

You should be able to "burp" the air out wih a few pumps of the lever with the bike upright again.

When my fluid was slightly low in my Shimano resovoirs I used to have to pump the lever a couple of times after moving it around on the back wheel (wheely style).

 

[Alan O]

The system isn't really sealed, it just has a lid on it so that the fluid can't fall out when turned upside down. Imagine the fluid reservoir at the lever and the level of fluid in it. The fluid doesn't reach all the way to the top of the lid because then you can't push the pads back if needed. Now, if you turn the bike upside down, the air above the fluid is now where the reservoir's feed hole to the master cylinder sits. Normally the fluid's surface tension and minute size of the hole will keep the hole closed but if you pump the lever you will pop the "seal" (surface tension) of the brake fluid and air will enter a few mm into the system. Now if you turn the bike back up, the fluid returns to where it is supposed to lie but traps the bubble inside the line. The bubble won't float up, again because of surface tension and the small size of the capillary. Pumping the lever then drives it down or simply creates a sponge in the system. By sponge I mean a rubbery effect.

Some reservoirs have a rubber bellows in the lid that extends into the fluid and technically prevents air between fluid and bellows, yet still allows you to push the pads back and raise the bellows level. Even so, there may be air in there if the mechanic didn't make it overflow when he fitted the lid/bellows.

Other systems like SRAM Elixir don't have a reservoir, just an supposedly-full flexible bladder. But that's just a bubble trap. Bubbles make their way into the system particularly well with those brakes. I consider those to be utter rubbish or at best, a huge PITA.

That's a superb explanation - many thanks.

Alan

 

[Yellow Saddle]

Could the OP then try pushing the pads back in once more with bike right side up? Could that force the bubble back up far enough to float back up to the lid?

I've never tried it, but what you say sounds plausible.

I also think your response above needs to be stickied as a FAQ somehow.

I wish. There are so many things here that come up again and again. @MODERATOR!

There is a misconception I think among cyclists that you must not turn a bike with hydros upside down. Based on your explanation, I think that should be "avoid turning it upside down, but if you have to, do not operate the brake levers"

That sums up the situation nicely. Most people don't "operate" the levers when the bike is upside down on the floor, but could inadvertently kick the lever. However, bikes hanging front-wheel up on a wall always get the brake treatment by the tire-kickers that come into your garage.

 

[Custom24]

Could the OP then try pushing the pads back in once more with bike right side up? Could that force the bubble back up far enough to float back up to the lid?

I've never tried it, but what you say sounds plausible.



I wish. There are so many things here that come up again and again. @MODERATOR!




That sums up the situation nicely. Most people don't "operate" the levers when the bike is upside down on the floor, but could inadvertently kick the lever. However, bikes hanging front-wheel up on a wall always get the brake treatment by the tire-kickers that come into your garage.

Good point. And unfortunately my road bike with sram rival hydros (dropped bars) needs to be hung up on a wall in the bike store at work. Would hanging it from the rear wheel be better? Might discourage the tyre kickers from operating the brakes I guess at least

 

[Jody]

does anyone run two sets of disc brake wheels and does this make sense?

I run two set of wheels. The fronts are identical and the rear are different brands. The rear pads are out by a fraction of a mm but I just realign the caliper as needed. It's not a big job and will surfice until I find the appropriate size shim washers.

 

[Alan O]

Hi folks,

Thanks for the further suggestions - I'll try the idea of pushing the pads to see if that helps.

To update, after I'd pumped the levers a few times last night and detected some stiffening, I left it for a few hours and it did soften a little again. I did the pumping thing a few more times, leaving it to rest in between, and I've been doing the same thing again this morning. What's happening is the travel in the lever is getting shorter and closer to normal, but the bite is still a bit rubbery (yes, rubbery is a good word for it).

Going on what people are suggesting, I suspect I've pushed the pistons back when I was fiddling around with adjustments, in addition to almost certainly getting an air bubble.

I've ordered some fluid (Shimano mineral oil stuff), and I have plenty of bits of plastic tubing and syringes here*, so I'll do a bleed. In fact, I'm looking forward to doing it, as my obsessive nature means I don't like having bike parts that I'm not competent to adjust/repair.

Next thing to learn is indexed gear adjustment - never had it before I got this MTB (oh, and a Grisley I got for £30 that's still awaiting attention).

Thanks again everyone, this is a great forum.

Alan

*Syringes - I'm a film photography buff and I use them for measuring developing chemicals, honestly officer.