Magura MT4 pad replacement due to contamination?

[JT!]

After buying a used Fourplay I think it's clear to me now that the rear pads and rotor are contaminated. My front break is stupid grabby and will hold solid. The rear has nothing close. I originally thought it was a bad bleed but the lever feels great, I just have zero power. I suspect this is the case because when I bought the bike, the rear brake had been replaced and there was about 2 feet of hose wrapped around the headtube. I think the brake must have leaked onto the rotor and wasn't cleaned properly before installing the new brake improperly.

Should I buy new pads or try and decontaminate the old ones? I do have a torch lighter which uses regular lighter fluid, could I use that to burn off contaminates? Or would I need a butane torch?

If I should buy new pads, should I stick with Magura ones, or are off-brand as good? If I should stick with magura, should I get P, C or R?

Should I use automotive brake cleaner, acetone or ISO alcohol on the rotor?

Thanks!

 

[PeterH]

All of the above.

What I would do is first up order some new pads, you'll need them eventually anyways, but in the mean time go at the current ones and see what you can do. Isopropyl alcohol is my go to for this. Clean everything, the rotor, soak the pads in it, and with a q-tip get in the caliper a bit (the contamination had to come from somewhere, hopefully its not a slow leak from the pistons). Then I take a fine grit sandpaper and resurface the pads, then isoprop again. Wipe it all down with a clean cloth and do the bedding in by dragging them down a hill (Duluth should have you covered for hills) and dosing it with water. Ride it around for a bit and if you still have the "squeal" and lack of bit from contamination then go the next step and torch it. I get mine hot enough that they burn on their own when the torch is removed, to me that is any oil/fluid that got in there burning off. Then sand and isoprop again and repeat the above steps. If none of that works put the new pads in (and again hope for no small leak).

[JT!]

1 hour ago, PeterH said:

(Duluth should have you covered for hills)

Oh boy does it!

Ok I'll give that all that a shot. Pads will take a while to be delivered so I can be doing all this in the meantime.

[Swoofty]

I figured you would already know all that JT! Yeah ditto for what PeterH said. Clean the rotor with REAL isopropyl alcohol (99% or better, make sure it has no other additives). Clean it with a cotton cloth (old tshirt) and NOT paper towels. Burn the pads on a gas stove or with a torch. I used to sand the pads after that to remove the glaze, but the typical water bedding process will resurface the pads too (get the pads nice and wet, ride up to a nice speed and brake fairly hard for 3 seconds, but don't let the wheel stop; repeat until they stop squealing).  I've had good results with maguras 9.p stock pads, but in the rear I usually go for trialtech, BBB sintered metalic or trickstuff pads. The 9.p is best for modulation, the rest are 'trials' pads. Good luck!

 

Just for fun, check the alignment of the caliper if you haven't already.

[JT!]

Ok here's what I did. I only have acetone handy right now so I removed the pads and soaked them in acetone for about 15 mins, scrubbed them down with a toothbrush and soaked them again. Removed them and sanded them down on some fine grit sand paper, one last soak.

Wiped the rotor down with acetone for about 20 mins. Wiped out the inside of the caliper until the cotton buds came out clean.

Put it all back together and pumped the brakes a few times till the pads came in.

Immediately, significantly worse than before. We're talking pedaling uphill with the brake held down with ease kind of bad.

Tried to bed them in with water and dragging the brake downhill a few times with maybe a slight improvement to bring them up to about the same kind of hold as they were before. We're talking two finger breaking and still doesn't put the bike into a skid.

So my options are, 1. Assume they're still contaminated and use fire to decontaminate this time around. 2. Assume I have a defective caliper and it's leaking onto the pads. 3. Although I've bled the breaks half a dozen times, I'm still not getting a good enough bleed on them.

Can anyone suggest which of those 3 is the most likely?

Edit: Fixed! (Hopefully). Someone sent me @Ali C's "My New Van Is Just What I Needed!" video. Took the pads back out, found my butane lighter and used it on both pads, not ideal, I need a proper torch, but one pad didn't do anything, the other set on fire. I eventually burned out everything and sanded them back down again. Acetoned the rotor, cleaned out the caliper again and even without any bedding in it's already in a different league to where it used to be. I'd imagine once they get bedded in they're gonna be perfect.

I'm wondering if it's possible that the caliper is leaking, I'm sure if it is, they'll go bad again, but if they do at least I know so I can replace it. If not, lesson learned, use fire on the pads, not chemicals.

Thanks for the help all, this was starting to drive me crazy.

 

[JT!]

Urgh, I'm slowly heading back to square one. Went and had a spin on it today and about halfway back to the worst it's ever been. Another thing I noticed is that one piston doesn't really want to retract from the disc while the other seems to move freely. The disc is perfectly centered in the caliper.

Defective caliper? Maybe leaking and contaminating the pads?

EDIT: Took the caliper off and on the mounts there was mineral oil, the whole underside of the caliper had a film of mineral oil on it. Tried to align the pads by using a method Magura suggested, but it seems the pads sit sooooo close to the disc there's always going to be come kind of light rubbing, I guess this might sort its self out as the pads bed in and level (I've been swapping the pads in and out / sanding etc). So the plan is to remove the caliper and the pads, torch the pads to decontaminate, then go over the caliper and disc with acetone and hope that the issue is is that I didn't do a good enough of a clean the first time around which is certainly possible. This would also explain why they work really well then slowly degrade with use.

Thoughts?

@Ali C I saw you do this in one of your vids recently, any ideas?

[Daan]

Check the bleed nipple and make sure it's done up really tight. I've had mine and those of a friend of mine leak from there. If you can do it up tighter, it's likely that's what it was leaking from. Though less susceptible, also check the banjo bolt while you're at it.

If you want to check the slave piston seals, make sure the brake is clean, put a bleed block in there (no pads) and squeeze the lever hard as often as you can for a minute or so. Then check if there's any wetness around the pistons.

Also in my experience, resurrecting pads works up to a degree, but never makes them as good again as fresh ones, especially so for organic pads like the Magura's.

[JT!]

1 hour ago, Daan said:

Check the bleed nipple and make sure it's done up really tight. I've had mine and those of a friend of mine leak from there. If you can do it up tighter, it's likely that's what it was leaking from. Though less susceptible, also check the banjo bolt while you're at it.

If you want to check the slave piston seals, make sure the brake is clean, put a bleed block in there (no pads) and squeeze the lever hard as often as you can for a minute or so. Then check if there's any wetness around the pistons.

Also in my experience, resurrecting pads works up to a degree, but never makes them as good again as fresh ones, especially so for organic pads like the Magura's.

Thanks! I'll double check and make sure everything is tight. The first time I torched the pads and put them back on they worked amazingly well with zero bedding in. So I'm going to do that to the pads again. I don't want to use new pads until I've figured out what the issue is because I'll just end up contaminating them too.

[JT!]

Urgh, I give up. Went through all the process again and still shit. Might just toss the thing in the shed and worry about it next year. Spent two weekends trying to get this sorted so I can ride it properly before winter, looks like that's not gonna happen.

This is what the rotor looks like after I toss on some water to try and see if it needs to bed in. Not sure if it's mineral oil, or just left over debris from sanding the pads.

[AdamR28]

That is pretty normal after wetting the rotor.

However, new pads AND rotor required to get the brake working properly again. No exceptions.

[JT!]

On 8/15/2022 at 11:16 AM, AdamR28 said:

That is pretty normal after wetting the rotor.

However, new pads AND rotor required to get the brake working properly again. No exceptions.

I've already ordered an entire new break. You think I have to replace the rotor too? Can't just acetone the crap out of it?

[Daan]

1 hour ago, JT! said:

I've already ordered an entire new break. You think I have to replace the rotor too? Can't just acetone the crap out of it?

Yeah, always does it for me. Acetone and a toothbrush.

[AdamR28]

2 hours ago, JT! said:

I've already ordered an entire new break. You think I have to replace the rotor too? Can't just acetone the crap out of it?

My experience is that you'll need a new rotor.

[MaderaMark]

I have been using these as long as I started learning.... Supa cheap yet I am very satisfied. I recently change rotors from avid g3(bent-slam)  to (knock-off) sram centerline. still bites  good enough for me.

 

[JT!]

19 hours ago, AdamR28 said:

My experience is that you'll need a new rotor.

Eh, in for a penny in for a pound. New rotor ordered.