Fandomii

Hi folks,

I have a Magura MT4 pair of hydraulic disc brakes. They've done about 2500km in original pads. A couple months ago I noticed that the left (front) brake had to be pressed in more than the right brake to actuate. A mechanic told me that this is due to brake pad wear, but the pads were still good and not needing replacement.
Q1: Is this true, that as the brake pads wear out, the lever needs to be pressed more to actuate?)

Now last week I brought the bike to another mechanic, with a complaint that the back brake pads (right handle) were touching the rotor a bit. After I received the bike, the right handle now needs to be squeezed much more than before.
Q2: Can someone explain to me what happened after the visit to the second mechanic, why is the lever more "squishy" now? Did they maybe mess something up?

Darth Lefty 

Q1 - this was true of mechanical disc brakes but not true for hydraulic brakes. The brake pistons normally don't slide in the seals except due to wear or maintenance. Rather their seals roll a bit, and the seals pull the pads back when pressure is released. (Imagine trying to start rolling a rubber band off a cylinder, but not quite getting it to roll over - then letting go and it returns to where it started.). That distance is more or less constant unless something there is worn out. The pistons do slide very gradually in the seals as the pads wear down, so the pull back distance is maintained. When the pads wear and the pistons slide, the volume in the caliper increases, and more oil comes down from the master cylinder reservoir, allowed past the master piston by the timing ports So the bite point is also maintained.

Q2 - yes, likely bled improperly and now there is some air in the system. Which had probably already happened on the left side.

Bleed instructions. If you really wanted to take this further you (or the mechanic) might rebuild the brakes with new seals.

MAGURA MT: Tech Videos - YouTube

Fandomii

Thank you. I actually noticed the left pull back distance had suddenly been "increased", however the bike had not had any visits to the bike shop and I had not tampered with the brakes - so no bleeding. Could it be some dirt or something else that made the left brake more squishy?

andrewclaus

Without getting hands on it, it's hard to tell exactly what you mean by "squishy." If once the pads hit the rotor the lever is firm, I wouldn't call that squishy. Squishy is when there's air in the system and the pads contact the rotor but not firmly.

Excessive brake lever travel may not be squishy. It sounds like that's the case from your first post. It sounds like the piston seals are "stuck" in their original position and are not sliding out as designed. That's probably from dirt and corrosion. As mentioned above, the calipers may need to be serviced/rebuilt.

I saw a Park Tools video about cleaning the seals and exercising them to get them to move and I did that successfully once, but I'm not an expert on that. It takes some fiddling around and maybe your local shops can't take the time to do that, or you just haven't found the right mechanic.

Fandomii

Yes this is exactly what I mean, excessive brake lever travel but with good brake performance. My guess is dirt and corrosion as you said. I'll try sand look for that video you mentioned. Many thanks.

Darth Lefty 

From another recent thread.

This is not guaranteed to be your problem, but it's some things to do that make the brakes better. Also a good visualization of the pistons not-really-sliding when braking but really-sliding when wearing or during maintenance

Kapusta

Does the lever actually travel farther, or is it just set closer to the bar to start with? If it is the latter, there may be a lever reach adjustment that needs tweaking.

Fandomii

It travels farther unfortunately. The bike mechanic I talked to (who actually made my right lever also travel farther - without me asking him to) said that " I could tighten the screw on the brake lever to "push" the liquid in and tune the lever". But I'm not sure I trust this guy. Can someone explain this a bit better?

Kapusta

What screw is he talking about? Pic would help, here.

If he is talking about the reach adjustment screw, then no, it does not push the fluid anywhere nor does it change how far the lever travels. What it does do is change how far from the bar the lever starts and stops. You can easily see what this does by adjusting it yourself.

Daniel S

Hi, Did you ever get this resolved? My right brake lever is all of a sudden getting much close to my bars than my left lever when applying the brake.

Fandomii

I didn't get it resolved yet, I've been living with it, but I'm pretty sure it has to do with bleeding of the lines. I suggest you bring yours to an authorized repair shop to bleed it and remove the air from the system. Alternatively you can do it yourself but it requires some tools and steady hands.

jgwilliams

I had Magura Louise brakes on a bike I had many years ago and had exactly the problem you describe, where one piston travel was longer than the other. I found instructions on Magura's site on how to deal with 'sticked' pistons that solved the problem for me.