|   The dreaded "pump"! | Friddell
Mar 31, 2002 11:21 AM | | For background, I am an expert trials rider that has been using (and bleeding) Magura rim brakes since 1993. Last fall I decided to try disc brakes so I bought a set of 2001 Louise, front and rear. Like most, I had to shorten the rear brake line during installation. Ever since, I've had problems with the brakes fading periodically during my rides. As a trials rider, the fade cannot be due to heat because I never get moving more than about five miles an hour. Assuming the problem must be air in the system, I did a quick bleed. Apparently, I wasn't quite careful enough and I made the situation worse. So, I tried several times more, being increasingly more careful each time to avoid getting air in the system. Each time, it just got worse and worse. Finally, I decided to drain the whole system and start over. This worked...well, mostly.
Now my brakes have gone from fading to the dreaded pumping. I have read the section on the Magura Cult webpage concerning pumping so I now understand that it is very unlikely that there is air in the system causing the pump, as I initially thought. I understand that the pumping comes purely from me quickly actuating the lever such that the fluid is unable to to retreat back into the reservoir. However, quick movements of the lever is something that is required in trials. So, by the time I get a few moves into a section, my lever is almost all the way out and my hand is cramping from the strain.
Basically, my question is this: Could the pumping be caused by the apparently more viscous Magura Blood that I have since used to replace the original green Shell mineral oil? Has anyone else noticed a difference in viscosity between the green Shell oil versus the "blood?" If this is the problem, what can I do to either thin the Blood, find the green Shell oil or find something equivalent? |
| Some answers. | Mike T.
Mar 31, 2002 1:05 PM | | Earlier Louise were notorious for having relatively (relative to their front brake anyway) slow return. Part of it was due to the diameter of the line, part to do with the tiny hole in the line insert, part due to fluid friction and part due to oil viscosity.
This "pump" when the lever was cycled quickly was due to the oil not being able to make it back to the master cylinder quickly enough and it was vastly amplified if the pads were not adjusted as close to the rotor as possible.
I'm not sure if there was any viscosity difference between the two compatible oils.
What I would suggest is this - replace the oil with Finish Line 2.5 weight shock oil and adjust the pads as close to the rotor as possible. If the lever blade is then too far from the bars for your liking, use the blade reach adjuster to move it closer to the bars.
Magura has issued new bleeding instructions for all their disc brakes both old and new. These instructions are new and are now on their website -
www.magura.de/english/frameset/default.htm
Most of them (but probably not all) are on my Cult site in the Self Adjusting section (yep I'm aware that your brake isn't self adjusting).
If all this doesn't satisfy you the next thing to do is to replace the rear line with one of the Magura steel braided lines (make sure it's the Magura brand) as that line does not use the insert which greatly reduces the bore of the line.
So, in order of simplicity and cost I'd do this in this order -
1. Re-adjust the brakes carefully.
2. Re-bleed using the new method.
3. Replace the oil with 2.5w.
4. Replace the rear line with a braided one.
5. Swap the brake for an Avid cable disc - I'll guarantee you'll get no pump with that one! :o)
Mike T. (mcm # 717) |
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