My front/rear master is leaking, should I rebuild/replace it?

[frankenduck]

Not necessarily.  Many times, especially on bikes that have sat for a while or not been serviced regularly, it's just some crud that has built up or been caught in the seal(s).

Regardless, the first thing you want to do is pull the master cylinder apart and assess it's current condition before deciding upon a course of action or spending money. 

Both the front and rear master cylinders have set (a.k.a retaining) screws in the side of them.  Removing this little 3mm Allen head machine screw will allow you to remove the piston from the cylinder.

Piston: Clean the piston and it's seals off carefully with some water and a paper towel.  Inspect the two rubber seals for abrasions or nicks.  If they look to be in a damaged state then a rebuild kit or finding a used working master cylinder might be in order.

Master cylinder: However, before you run off and buy a rebuild kit, it's also important to inspect the master cylinder bore(wall) for pitting.  I've found that using 0000 grade steel to clean the bore works wells without damaging the bore.  Once you've done that look closely in the bore to make sure that the wall is in good shape.  If it's pitted or otherwise damaged then a rebuild kit will not help and you'll need to replace the entire master cylinder assembly a with new or known good used assembly.

I've dealt with a lot of these leaking master cylinders and nine times out of ten a good cleaning is all that's required so clean it and inspect it first before throwing money at the problem.

UPDATE 3/13/12:  I attached a PDF to Reply #13 of this thread on cleaning or rebuilding a 4V front master cylinder.

[johnny]

so this silky motobrick came with a leaky front brake master cylinder and a rebuild kit... the seller was gonna fix it butts i asked him to give me the rebuild kit and i would tend to it...

this is what i gotts...



j o

[Inge K.]

That`s not a rebuild kit, but single parts. This is what a rebuild kit contains:

This is how I`ve done it three times so far, without problems:

1. Empty the reservior with a syringe.
2. Remove plastic cap and lever.
3. Loosen the clamp screw and tilt the pump/cylinder a bit upwards.
4. Push the piston a tiny bit inwards, remove the set screw.
(watch out for escaping fluid as the set screw comes loose).
5. Remove piston, clean and hone bore as needed.
6. Put some brake assembly grease (or clean brake fluid) on the new piston gaskets.
7. Put everything back in the reverse order.
8. Fill up the reservoir, very gently move the lever forth and back, untill no more air
is escaping from the holes in the bottom of the reservoir, also alter the position/angle
of the reservoir during this procedure.

Inge K.

[johnny]

its 30 degrees and a little windy... i gotts about 3 hours of sun thru the back door...





















i cant get the piston out... stuck for now...



j o

[frankenduck]

It's all that dried up crud that's keeping it in there if it doesn't pop out when you remove the set screw.  Stick an Allen wrench in there, put some angular force on it and try to pull the piston out.

Nice plaid:

[johnny]

it was pizzing me off... i wanted to save it... butts i had to ripp it on out...





hillbilly bore hone...









j o

[Inge K.]

I also have experienced that the piston also have been a bit difficult to remove,
due to corrosion in the outer end of the bore.

(which also have killed the outer gasket and started a leak, and the reason for three changes......yes, yes I know should have changed the whole assembly....but due to
the stingy person I am...I`ll wait untill I`m passed the worth of an assembly, in repair kits).

I have moved the piston a bit forth and back in the bore and twisting it, untill it have popped out by strength of the spring.

Inge K.

[frankenduck]

DOT 4, not 3:

[johnny]

just kidding... the dot 3 is what i use in my jeep...





i busted the front brake switch... butts im still rolling to indiana tomorrow...

j o

[Inge K.]

i busted the front brake switch...

Easily done........I`ve been close to.....................................two times.

Inge K.

[frankenduck]

So, Johnny, did you clean up and re-use the old piston or did you install the new one that you wasted $100 on? :loopy:

[johnny]

i didnt buy the new one because the silky came with a new one in a 8 ball baggy... i installed the new one because i f ed up the old one with my special removal tool... it was a beautiful experience... that dot 3 is so slippery i lubed my swing arm pivot bearings with it...

j o

[frankenduck]

Attached is a PDF file on cleaning or rebuilding the master cylinder on a 4V K bike front brake master cylinder.

[ckp]

Bringing up the dead here.

My bike (K1100RS) sat most of the winter. The warning indicator was staying on while I rode, along with the ABS lights flashing. Searched the forum for the problem and it led me here. The .pdf in Frankenduck's post was right on the money. I used some gray Scothbrite wrapped around a long standard screwdriver bit and electric screwdriver as a "hone" for the bore. All it really does is polish it as it removes the small amount of crystallization that was on the end. Flushed the system with a MityVac while I was at it. Wouldn't mind replacing the seals as I saw a tiny bit of etching on the outermost seal. It stops and goes without issue. Nice, nearly free repair.

Thanks

[djaysan]

sorry for digging old posts - but worth sharing:

after looking all around for a a cheap alternative to those expensive rebuild kit here is what i found:

the k100RS latest model shares the same master cylinder as the K1100 RS & LT but also the R1100's models

since the brand is magura i simply searched for 'magura repair kits' and guess what? I found it for half the price!

Magura ref. 2701122 on baybay https://www.ebay.com/itm/Magura-Master-Cylinder-Repair-Kit-2701122-0617-0258/383048110865?epid=245842009&hash=item592f725311:g:EcMAAOSwnehdKe0u#shpCntId

only 36USD instead of 75!

[Laitch]

I found it for half the price!  only 36USD instead of 75!

Post here and let us know how well it works after you've installed it. It seems like a great find.

[volador]

Great find! You verify the piston size with the seller?

[djaysan]

Great find! You verify the piston size with the seller?

20mm piston diameter 👌

[volador]

Outstanding. How's the fishing been in Mauritius? Any large GT or Tuna?

[djaysan]

Outstanding. How's the fishing been in Mauritius? Any large GT ?

Ahah! Huge GT there! My cousin is big into this game! let me know if you go there one day ;)
At the moment I'm freezing my a** in Łódź, Poland ;) 1st christmas in the snow! can't wait!!! this is my paradise now

[bocutter Ed]

in Łódź, Poland

My father is/was from there, b1920.

[volador]

Small world. I bring the Żywiec Beer from Greenpoint,Brooklyn when visiting a friends farm.