Magura bleed tip from bike shop mechanic

[RebornRider]

I was chatting with the bike mechanic at Mammoth Mountain bike park about bleeding Magura MT brakes. I wanted an "earlier" bite point than what I could get with the MT7 adjusters and HC3 lever adjusters.

The normal bleed procedure calls for removing the pads and using the Transport Device (Magura's name, not mine) to hold the pistons in position. The mechanic said I could get a earlier bite point (less lever movement before the pads contact the rotor) by overfilling (his word, not mine) the system. So I paid the man to do his thing on my brakes.

The big difference in his procedure is to keep the pads installed and use a 2mm spacer between the pads to limit piston extension rather than no pads and the transport device. In theory, his method allows the pistons to push out further from the caliper, which makes room for more fluid.

Obviously, there is a risk of getting mineral oil on the pads, so you must be extra extra careful.

His method did work. My bite point is now at 15mm of lever travel (measured at the end of the lever), and a hard squeeze gives about 25mm of travel. The Magura Modulation is still there, and the short travel really helped with my confidence when trying to go a little bit faster than the previous lap.

It may not be for everyone, but I'll keep using this "overfilling" method.

Edit to add, before anyone brings it up, that this overfilling thing did not cause any brake drag. None. I tested by the usual method of lifting the front tire off the ground and giving it a spin. No drag.

 

[bazxa]

Thanks good info

 

[Planemo]

I'd be interested to understand how this would work. Assuming no air in the system, piston retraction distance is governed by the overall design volume of the master cylinder/lines/calipers Vs lever travel. This is why you should get the same bite point whether your pads are brand new or totally worn. It's how most hydraulic systems work as they are effectively self-adjusting. In short I can't see how adding more fluid changes bite point.

There shouldn't be an extra risk of oil on pads either, as there shouldn't be any leaks. Unless you are talking about overfilling the reservoir but that would apply in all circumstances.

You clearly believe it works and I am not doubting your real life experience, I'm just intrigued.

 

[RebornRider]

I think you answered your own question

I'd be interested to understand how this would work. Assuming no air in the system, piston retraction distance is governed by the overall design volume of the master cylinder/lines/calipers Vs lever travel.

Here is what I think is happening. If the system is designed for a fluid volume of 50ml (for example) with the caliper pistons set for the thickness of new pads and you force in 60ml, where is the extra 10ml going to go? It will push the pistons out.

Another way to think about it is to imagine what would happen if you bled and filled the system with the pads installed and no spacer between them. You allow the pads to touch. When you've removed the syringes and closed the system, you won't be able to push the pads far enough apart to get the disk between them. You have seriously overfilled the system.

The system adapts to pad wear by transferring fluid from the master cylinder on the handlebar to the caliper cylinders. The master is no longer as full as it can be, but that's how the system is designed. You could add fluid to the master cylinder in this case, which means the system is now overfilled, and it will work fine until you install new pads. You won't be able to retract the pistons far enough to get new pads to fit.

There shouldn't be an extra risk of oil on pads either, as there shouldn't be any leaks. Unless you are talking about overfilling the reservoir but that would apply in all circumstances.

The oil on the pads concern applies to the bleeding process, not after the bleeding is done. You will get oil on the calipers as part of the bleeding process because the bleed screw is removed and you are installing and removed the syringe. If the pads are installed while you're doing this, you risk getting oil on the pads. The offical Magura process calls for removing the pads and using the Transport Device (as they call it) to maintain piston spacing.