Bicycle Mechanics - magura gustav m help

hi!

new user; big problem. i purchased a set of magura gustav's in the spring and while installing the rear caliper on my frame i repositioned the brake line slightly which loosened it a little. a few hours into the first ride with these brakes the mineral oil (10w) pissed out of the line and all over the caliper/rear wheel.

i was close to my lbs so i rode over and asked if they could rebleed, or knew what i should do. they suggested i go to a drugstore and purchase mineral oil to rebleed with; at the time i was unfimiliar with the differences in oil and it turns out the drugstore oil is 30-50w. i got a little on the caliper since it was my first bleed and it's worked it's way to the 2 pistons in the caliper over the summer.

i replaced the pads tuesday night, and my first ride with them yesterday brought a problem to my attention. the oil is causing the pistons to seize and they're not backing off the rotor now; creating a drag that requires me to remove the caliper/wheel from the frame to push the pistons back in.

i believe the wax in the oil is causing the pistons to seize; magura apparently voids warranties due to using drugstore mineral oil (or anything that isn't their branded 10w) so i need to get this oil off the pistons somehow. i was thinking of using rubbing alcohol but i believe there's an o-ring at the base of the pistons inside the caliper (welded block) which i do not want to damage.

can anyone think of a method to clean the pistons?

I have not done the Gustav so this is pretty general. You need to remove the caliper and pop the pistons out. If this is a monobloc caliper(which it looks like from the one picture I saw on their site) I would do it this way: I use air pressure(Use eye protection, point the caliper so the piston will not hit you) You will probably need to hold one piston down in the bore while you pop the other out. To get the second one out, block the port of the removed piston and apply pressure again. If you can soplit the caliper do so and remove the pistons the same way, don't lose the little oring between the two halves. Once the pistons are out remove the seals and clean them with the correct mineral oil. Clean the bores with rubbing alcohol. Lube it up with mineral oil and reassemble. Then hook the caliper up and bleed out tthe system completely actioning the lever while doing it.


An easier method you could try is bleed the system out til you get fresh oil, then with the pads out(You should take em out to bleed anyway)using a wooden dowel or some other kinda soft stick like object, push the pistons back in their bore, then press the lever to expand the pistons, then cycle them back in with the stick. Do it a bunch, bleed to fresh one more time and you should be good to go.

Technical reference manual for 2005 model Magura Gustav M disc brakes (English) (http://www.magura.de/english/download/05/gustavm_05_e.pdf)

Something well worth flipping through before popping your caliper open (even has pictures!) :D