Another joyous hydraulic brake thread

[Jody]

Anyway, back on topic. Going to do a quick swap of the rear pads into the front but I don’t have any IPA to clean the discs. I have some electrical contact cleaner which is a slightly milder version of brake cleaner. Would this suffice? The more I think about it the more I question if the pads somehow have contaminates on them.

 

[Yellow Saddle]

I'm always ready to learn so please put me right on this. All I have read in the past pointed at outgassing so what is the true mechanism for the overheated pads to lose bite? By the way, I always understood the grooved discs where to remove dust and keep the friction interface clean but suspected that was also just sales BS! Never been tempted to fit nasty 'upgrade' parts when genuine or reputable OEM have always done me proud.

Organic/resin pads to outgas during the final curing. Final curing happens when the pads are bedded in after installation. The volume of gas that comes out is tiny. It can't be large, where will it come from? However, the "gas bearing" theory persists. It is myth.

Having said that, there are three types of brake fade.

1) Fluid fade. That happens when the fluid overheats and vaporises with the effects you understand and have been discussed above.
2) Pad fade. This happens when the pads overheat and change their frictional properties with heat. The best example of this is rubber brake pads on carbon rims. They quickly melt and this implies liquid, which is a lubricant. Other materials can fade without melting. The coefficient of friction of materials change with temperature and speed. Pad fade is when the intended temperature range of the pad is exceeded. Resin pads work at low temperature. Sintered pads work better at high temperatures.
3) Green fade. This is when a pad has not been bedded in properly had been overheated and deposited a too-thick layer of material onto the disc.
Green fade can be smelled and such a distressed pad gives off a stink reminiscent of a burning clutch. This smell is accompanied by a loss of braking force and excess pad material deposited on the disc.

On F1 cars the brakes and pads are made from carbon. The wear product is CO2. Yet even here the pads don't display gas fade. The holes drilled in the discs are purely for venting.

I will not be so sure that OEM is best. We've seen plenty of evidence here that OEM pads are often very unsatisfactory and after-market ones give better results will less noise and fade. I think this is one area where it is worth experimenting . @glasgowcyclist recently had a particularly difficult case to solve. I wonder how his new after-market pads turned out for him?

 

[Yellow Saddle]

Anyway, back on topic. Going to do a quick swap of the rear pads into the front but I don’t have any IPA to clean the discs. I have some electrical contact cleaner which is a slightly milder version of brake cleaner. Would this suffice? The more I think about it the more I question if the pads somehow have contaminates on them.

Brake cleaners bring very little job because the pad material that transfers to the disc is very hard and insoluble. It has to be sanded off. Hence me asking for photos of the disc and pad to see if that could be the problem.

Proof that brake cleaners don't work is that they don't dissolve pad material.

 

[Jody]

Brake cleaners bring very little job because the pad material that transfers to the disc is very hard and insoluble. It has to be sanded off. Hence me asking for photos of the disc and pad to see if that could be the problem.

Proof that brake cleaners don't work is that they don't dissolve pad material.

I have some red scotch pad at work that should do the trick or a mild emery paper.

 

[Yellow Saddle]

I have some red scotch pad at work that should do the trick or a mild emery paper.

You don't want mild anything. You need strong emery paper. Garnet paper is best. Grit 120 or so.

Remove the discs and sand both sides aggressively. You want to end up with a satin finish with no smooth parts.

If the pads are still OK like you say, you have to sand them as well. Here, be gentle. Lay fine grit paper on a very smooth top and move the pads in a figure 8 (to prevent chamfering) over the paper until the glaze is gone and they are matt in finish. Then brake the pads in all over again. We have discussed the method here a few times.

 

[I like Skol]

I recently cleaned up some bike discs with fine wet&dry (1200 I think) and some soapy water. Took the discs off the wheel and methodically did both faces. They were quite new rotors but had been getting bad noise issues when damp and wanted a fresh surface before fitting new pads. It worked really well.

 

[Crackle]

My vote is contamination too but I'm rather pessimistic that you'll be able to rescue the pads.

 

[Profpointy]

No, the one that says: "The lever is as normal at its usual bite point, no change in its feel but there is no bite to the pads."

Had there been air in the system, the lever would not have reached bite point and went all the way down to the handlebar with very little pressure.
Jody clearly says there is bite (feel/pressure/resistance) in the lever but it is the pads that are not biting i.e. not applying friction to the discs.

"Feels like when you have to bed a new set of pads in but every ride."

This clarifies it and shows an understanding of how new pads react before having been bedded in.




The test is simple and has been described on this forum. I'll repeat it. To test if there is air in your brake system do the following:

1) Approach the bike after it has been standing for some time - 20 minutes is enough. Position yourself at the bars and concentrate.
2) Pull both brake levers and hold. Notice how the levers feel viz a vie each other and with respect to how far they advance towards the bars.
3) Now pump them repeatedly and try and feel whether they/it are "pumping up". In other words, does the bite point improve and do the levers feel harder after pumping?
4) Let the bike stand for a while and do the exercise again. Has the levers gone "flat" in the interim?

There is air in the system if:
1) The lever feel hardens up with pumping.
2) The levers lose the feel after standing and require pumping again to get them to bite.

That's it. No other brake problem presents like this. No other brake problem can be fixed by bleeding. The need to bleed is rare.
The only reason you would bleed brakes that don't have air in the system is if the brakes use DOT fluid and a relatively short run using the brakes caused the fluid to boil and had the brakes fail on you due to vapour in the system. The diagnosis for this is the symptom described and the fact that the brakes returned to normal once they have cooled down.

Good explanation. I agree with that...

....however what we have from the op here is a rather less formal /less definitive version of the test based experienced in normal use. Also if there's only a little bit of air then spongyness might be subtle

 

[I like Skol]

Also if there's only a little bit of air then spongyness might be subtle

FFS it isn't air in the brake system, let it go! Besides, if it was that subtle the brakes would still work more or less normally.

 

[Jody]

....however what we have from the op here is a rather less formal /less definitive version of the test based experienced in normal use. Also if there's only a little bit of air then spongyness might be subtle

The brake in question had a new hose (due to longer forks) at Christmas. All fluid replaced and re-bled. No issues with the lever feel, travel or operation.

 

[Profpointy]

fair enough

 

[I like Skol]

The brake in question had a new hose (due to longer forks) at Christmas. All fluid replaced and re-bled. No issues with the lever feel, travel or operation.

That might be a clue actually, how much have you ridden the bike since then and when did the problem start? Maybe the rotor and pads got contaminated during the work?

 

[Yellow Saddle]

Good explanation. I agree with that...

....however what we have from the op here is a rather less formal /less definitive version of the test based experienced in normal use. Also if there's only a little bit of air then spongyness might be subtle

No, "little bit" doesn't get you off. The amount of air that creates sponginess is less than the size of one tenth of a champagne bubble. One such bubble. A pinprick of air. Air in the system is not subtle. It is binary - there or not.

 

[Jody]

That might be a clue actually, how much have you ridden the bike since then and when did the problem start? Maybe the rotor and pads got contaminated during the work?

Its had a few rides but not loads. Was OK after the new pipe was fitted but it could be washing that has transfered some unseen oil to the pads. Pads and wheels were off when the job was done to try and avoid contamination but anything is possible.

 

[Profpointy]

No, "little bit" doesn't get you off. The amount of air that creates sponginess is less than the size of one tenth of a champagne bubble. One such bubble. A pinprick of air. Air in the system is not subtle. It is binary - there or not.

OT now, but hang on that's just silly. Air the 10th of a size of a champage bubble can't compress more than, well a 10th of a champagne bubble if you compress it to zero. The difference in movement at the bar will be , what, a thou. . 5ml of air will compress down to a smaller volume and result in a fair bit of spongyness.