Hot Disk Brake Issues.

[Tom B]

I have recently put some Oxford brand pads in my rear brake, simply because I had them handy when my last set wore out.

Other than being a bit squeeky and not having quite as good a feel the did the job for my hybrid.

However yesterday I was going down one of the bigger local descents (back of Sharnyford towards Todmordem for those that know it) where unbeknownst to me there are currently four sets of temp lights, of course each was on red resulting in 4 big stops from approaching 40mph in the first and over 30mph thereafter. After the first couple of stops the rear lever went hard and the brake was binding. After a mile or two at slow speed on the flat I stopped and the lever feel became normal and the pads freed off.

Now its bugging me as to what has gone wrong. If the fluid had boiled I'd expect soft squishy lever and no or little braking effort. I'm thinking the piston has overheated and jammed until cooled when it was able to retract.

No other issues for the next 40miles including on descents from Heptonstall and into Littleborough from crag.

Either way I'm going to put some shimano pads in and see if that makes a difference.

Calipers and Levers are Shimano m615 with 160mm rotors.

 

[bikingdad90]

Did you drag the brakes or did you use lots of on/off pulls, it might help explain what happened as some pads fade and overheat if you drag them?

I find new pad and brake combinations are really sensitive to riding style especially if the pads are different (sintered, organic etc) to what was last in there till they bed in. Sometimes I need to adapt my riding style to suit.

 

[Tom B]

Each application was constant and pretty hard.

I'm guessing the Oxford brand are sintered but they don't say.

 

[lpretro1]

What make of brake (Giant mph3 by any chance) - if it is the old-fashioned 'closed' type of system then that can easily happen as when brake gets very hot there is no room for expansion of the fluid inside the mc - the Giants have a thumbwheel on which you can slack off (except that seizes up early in it's life and never works). |More modern brakes have expansion room thanks to a diaphragm in the master cylinder

 

[Yellow Saddle]

I have recently put some Oxford brand pads in my rear brake, simply because I had them handy when my last set wore out.

Other than being a bit squeeky and not having quite as good a feel the did the job for my hybrid.

However yesterday I was going down one of the bigger local descents (back of Sharnyford towards Todmordem for those that know it) where unbeknownst to me there are currently four sets of temp lights, of course each was on red resulting in 4 big stops from approaching 40mph in the first and over 30mph thereafter. After the first couple of stops the rear lever went hard and the brake was binding. After a mile or two at slow speed on the flat I stopped and the lever feel became normal and the pads freed off.

Now its bugging me as to what has gone wrong. If the fluid had boiled I'd expect soft squishy lever and no or little braking effort. I'm thinking the piston has overheated and jammed until cooled when it was able to retract.

No other issues for the next 40miles including on descents from Heptonstall and into Littleborough from crag.

Either way I'm going to put some shimano pads in and see if that makes a difference.

Calipers and Levers are Shimano m615 with 160mm rotors.

You had hydraulic lock. This happens when the fluid heats, expands and has nowhere to go other than to push the pistons out against the brake pads.
This is very easy to do by dragging the rear brake for long distances instead of doing a quick job by using the front brake harder, but over a shorter distance. Rear brakes only do about 15% of the slowing down and if used as the primary brake, will overheat.
Further, just slapping this or that pad without knowing what it is, isn't good practice. New pads ideally need to be matched to the disc and to the previous pads. In other words, if you had resin pads in there, use resin again. Also, some discs are not metal compatible and only work with resin pads. There's more. When fitting new pads, you need to bed them in properly in a controlled but deliberate series of slow-downs (never stops), up to 20 of them increasing in aggression. Only then will the brakes work properly. However, when you have switched from resin to metal, they will still operate noisily even though they work.

These things make a big difference since when the brakes don't work as well as they do, you apply them harder and they get hotter and hotter.

 

[Drago]

Hydros don't always react in seemingly logical ways. For example, boil the fluid in Giant MPH calipers and by some quirk of the internal valving lock on and won't release.

 

[Tom B]

You had hydraulic lock. This happens when the fluid heats, expands and has nowhere to go other than to push the pistons out against the brake pads.
This is very easy to do by dragging the rear brake for long distances instead of doing a quick job by using the front brake harder, but over a shorter distance. Rear brakes only do about 15% of the slowing down and if used as the primary brake, will overheat.
Further, just slapping this or that pad without knowing what it is, isn't good practice. New pads ideally need to be matched to the disc and to the previous pads. In other words, if you had resin pads in there, use resin again. Also, some discs are not metal compatible and only work with resin pads. There's more. When fitting new pads, you need to bed them in properly in a controlled but deliberate series of slow-downs (never stops), up to 20 of them increasing in aggression. Only then will the brakes work properly. However, when you have switched from resin to metal, they will still operate noisily even though they work.

These things make a big difference since when the brakes don't work as well as they do, you apply them harder and they get hotter and hotter.

That makes sense, I should add the pads have been in a few weeks now and were bedded largely as described.

I was dragging and favouring the rear as there was some ice about with water on the road and my view is a rear skid is much more recoverable/controlable/gives the option to choose the scene. Of course once it begun to bind it was always generating more heat.

 

[Levo-Lon]

As said fluid lock..its a lot of heat for small brakes..40mph down hill..
id have 180mm rotors and maybe some bigger pads if i was doing that type of stopping often.
shimano XT are quite good for high heat stopping..

also you may have over filled the system? Often brakes are bled with pads in and this leads to overfill..

 

[Yellow Saddle]

As said fluid lock..its a lot of heat for small brakes..40mph down hill..
id have 180mm rotors and maybe some bigger pads if i was doing that type of stopping often.
shimano XT are quite good for high heat stopping..

also you may have over filled the system? Often brakes are bled with pads in and this leads to overfill..

Just one point. Choosing bigger pads is not an option. The caliper takes what it takes.

 

[Levo-Lon]

Just one point. Choosing bigger pads is not an option. The caliper takes what it takes.

xt brakes...not xt pads in his caliper..new xt brakes ..but yes my post was poorly worded