How to: Free a stuck piston in a Magura HSxx lever/Thouroughly service a Magura rim brake

[CC12345678910]

I've just wrote this out in a topic and thought that since the question will no doubt be asked again that it should become a how to, I've no pictures as yet but next time I do this to a brake I will add some. Never done one of these before so here goes. Feel free to add amendments or question something I've written - I think I've covered everything.

How to free a stuck piston from a Magura HS11, HS22, HS33 master cylinder and derrivitives (NOT Echo)

 -Thoughly service Magura HS11, HS22, HS33 and derrivitives (for Echo only Section 2 applies)

Section 1

Prerequisites. You are going to need:

A working knowledge of how to bleed Magura HS series brakes, by doing a 'bath bleed' with water (personally I refer to this as a bucket bleed), and, if you chose to oil bleed, how to bleed with syringes.

A track pump with both schreader (car valve) and presta (road bike valve)

A spare shoud nut and piece of Magura hose with an olive pre tightened on it. A broken crossover with a barbed fitting will do, but it's really not the same due to the extra backpressure created by the smaller m6 fitting.

Penetrating Fluid. Ideally GT85. WD40 is passable.

3in1 style oil.

2 good size, clean containers in which to water bleed the brake. I cut two washed out 4 pint milk bottles in half with a craft knife or scissors & use the bottom sections, for example.

An open-ended 8mm spanner for the shroud nuts.

A proper set of good quality ball ended allen keys. A multi hex key set will not do!

In absoute worst case scenario (I've been here believe me), a small blow torch on low-medium heat. A good size kitchen gas hob could be a substitute.

SKILL LEVEL: I would describe this task as fairly advanced - it requires a dedicated workspace (even if only for a short time period), a reasonable amount of mechanical skill, tools and equipment. Ham-fisted newbies could quite easily trash what to others would be an otherwise easily repaired, seviceable brake, either though ignorance or using innadequate/totally the wrong tools.

METHOD

With the brake still all fully assembled, but removed from the bike;

1) Take the leverblade pivot bolt out. (5mm Allen key)

2) Wiggle the lever blade out.

3) Move the Tpa ball and rod round 90deg so that they are sat flat against the lever blade, facing away from the pivot.

4) Put the lever blade and pivot bolt back with everything like this. Tighten like normal.

5) Take out the grub screw in the master, the one where the top bleed hose goes.

6) Screw in your piece of hose & shroud nut. Nip it down with an good fitting 8mm spanner (NOT AN ADJUSTABLE!!!)

7) Take the other end of this hose and plug it into your track pump. In mine I have to unscrew the presta side, slide the fitting over the magura hose, wiggle the end into the little olive in the pump and then tighten the fitting up. Whatever you have to do this junction needs to be good & snug or else you're wasting your time.

8) Spray some GT85 down the bore of the master past the piston. Leave to soak for 5mins. Maybe repeat a couple of times.

9) Slowly pump some air in. A well lubed working piston should pop out with little fuss at 30ish PSI. A sticky one at anything up to 80 PSI. If It's being a real bitch, I've been there... See * below.

If it has worked;

10) Take the lever blade out again.

Find a corner of the garage/wherever you're doing this without a lot of stuff in, preferablely with some form of padding, the bottom of a cardbox works well. This is to stop the piston and spring richocheting off god knows where - that sh*t can honestly go miles, so for the love of all things holy point the master away from you .

11) Pump. HARD.

Hopefully that should have given off a satisfying POP sound as the piston was shot out at a fair ol' lick, and all is well. Find the piston and spring, lube the piston with silicon spray lube for a racing line, tensile, or generally any aftermaket metal piston.

For a standard plastic magura one you might want to lightly & evenly sand the skirt (the end nearest the handlebar when in use) with 400 grit wet and dry. Use trial and error methodology, or you'll not be riding 'til you source another piston, it doesn't need much sanding off. Then I recommend Petrolieum Jelly (vaseline) for lube.

Or submerge the piston in a cap full of magura oil in either case (this is the safest bet). Reassemble the master and skip steps 9.1-9.3.

* If it hasn't worked: Well, I did one with a mate once where it took an indicated 140psi to shift a swelled plastic piston off the bottom of the bore. Scary as f**k and I thought my pump was going to die... This is where the heat comes in.

9.1) With the system lightly pressurised spray some more GT85 down the bore. It might just pop at this stage. If not;

9.2) Grip the bar clamp with some pliers, and give the area if the master around the piston a lick of heat with a medium flame, ideally as little as is needed, and pump some more.

9.3) Repeat?? Then you might be in trouble but In 10yrs and counting of trials, often dealing with preowned parts of questionable condition I've not been defeated yet.

Section 2

SKILL LEVEL: P1$$ Easy

For the rest of the system.

When you've got the master working, bucket bleed the brake with the master in a separate container, and mix in a couple of drops of washing up liquid, just enough to make a layer of bubbles, no more. Seal the brake up, and chuck the water from the buckets. Pump the brake for 5mins. Now bucket bleed the brake again and (in my experience anyway) look at all the dislodged old oil residue and grot appear in the waste water bucket.

Chuck the water again, and bleed however you see fit, water in a bath/bucket/other or oil in a syringe. Never mix water and Magura oil, it seems to produce the grot you just flushed out.

After this hold one slave from moving with your thumb, pull the lever fully to make the slave fully extend, and pour some 3-in-1 type oil down past there into the gap. Work it in there by pumping the lever, repeat, then swap sides.

You may have to lube the piston and master again, as the washing up liquid may have washed away the lube you just put there, so pop the piston out, re-lube and bleed again.

Job done.

Edited March 19, 2016 by CC12345678910