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Clutch bleeding problem on k12RS

4K views 13 replies 4 participants last post by  H96669 
#1 ·
Hi all,

I am new here but have a serious problem. Last week I noticed a crack on the hydraulic line going from the clutch master cylinder to the slave cylinder. I replaced the line and have spent the last 2 days trying to bleed the line. I have tried everything from the traditional method of using a few squeezes and opening the bleed valve off the slave cylinder to using my mity-vac to create pressure. Nothing is working. I fear that I may have killed her... Anyone have any suggestions?
 
#2 ·
I doubt you have broken anything. Was the clutch working before you spotted the crack?

Most likely you have got an air lock in the master cylinder piston chamber.

Close the bleeder at the slave. Put polythene over the front fender for protection. Crack open slightly the banjo on the master cyl. and with something underneath to catch the fluid, slowly pull in the clutch lever (fluid should come out). Hold in theclutch lever and tighten the banjo nut. Repeat a couple of times then blead at the slave bleeder.

Until you get fluid coming out at the master cyl. banjo you will get nowhere.

Keep some water handy to wash any spilt fluid. It kills paintwork!



 
#4 ·
No luck. I tried to push fluid out of the master cylinder using the method you described and it still does not want to work. Nothing is coming out other than a little air. I backed the banjo bolt out all the way to see if any fluid was getting pushed out and nothing. created some suction on my finger, though. Looks like I may need a new master cylinder. Thoughts?
 
#5 ·
Depends how far in you want to go. You could remove the MC to the work bench and dismantle it, looking for gunge or anything obvious wrong. Some really small holes to blow through with an air line too.

I forgot to mention there's a thumbwheel adjuster to set the lever in 3 positions. That should be on the number which sets the lever as far forwards as possible for big hands during bleeding. That then gives max stroke on the MC piston.

I bought a BMW repair kit for my brake side (I think both are the same). What comes is the piston and seals. It is cheaper to try and refurb than buy a complete MC. Refurb only works if the MC bore in the body is clean without scoring.

Depending on how competent you are, if you find the MC seals were bad, I would refurb the brake side afterwards.



 
#8 ·
From memory, you do end up removing small parts and screws. Photograph or take notes as you remove parts so you put them back correct.

You adopt the same precautions and cleanliness as when dealing with hydraulic brake parts.

If you have not got one already, you should really buy the Clymer shop manual which will pay you back many times for the many expensive mistakes you do not make.



 
#9 ·
I think I will get a manual. I took the cylinder apart last night and cleaned it. Did not see anything major wrong with it so I put it back together and put some fluid in it. I am getting a lot of air bubbles out of it now but I guess my question is does new parts in the cylinder make that much of a difference? I will bleed some more air out of it today to see where I get.
 
#10 · (Edited)
You have to know what to look for.

You get a magnifying eyeglass on the wiping surfaces of both seals and look for scoring marks, roughness and wear on the seal lip edges. But not an exact science I'm afraid. A new seal is nice and springy and you feel some resistance as you push the assembly into the bore. An old seal can be floppy and lost its natural spring to seal agains the sides of the bore. Once you can get some back pressure the old seals might still work. But if they are old, why try to save a few cents?

You can see how small the piston assembly is and any fine scoring on the seals will stop them sealing. You should do the same thing on the bore too. As I said, if the bore is rough and scored the MC is trash. On the brake side you can hold in the front brake lever hard for 10 minutes and see if the lever gets back to the throttle grip. If it does those seals are failed. You cannot do the same leak past check on the MC clutch seal.

When the MC is working normall, you should see a litte 'squirt' of fluid coming from that small hole in the bottom which usually leaps up ready to drop over on to your fender paintwork.



 
#11 ·
Got the new rebuild kit and installed it. I got all the air out and it works! The old one had worn rubber seals and now I understand why my brakes feel a little sluggish. They are next. Thanks for all the help. I never knew you could replace the piston inside the master Cylinder Housing!
 
#12 ·
Good... :clap:

I'll give you a tip if you keep working on MCs. Most procedures recommend using brake flluid as a lube to insert the pistons. Nothing really wrong with that but you have to remove all the residuals from the outside of the piston, the part that goes against the circlip and the rubber boot.

If you leave any brake fluid there it will attract moisture and corrode the external parts. There are proper lubes for brake parts,I use "Red Rubber Grease" you can use that all over as it mixes with brake fluids. Don't use sillicone based lubes, they may look like they mix at first, a couple days in my test jar and I saw big greasy clumps. :wtf:

Best cleaner for brake parts and the one recommended by BMW is Methyl Hydrate. Won't swell seals and does remove all the brake fluid residues.
 
#13 ·
#14 ·
sailor said:
Canadian Tire in their online web site says: "Methyl Hydrate can be used as fondue fuel"
http://www.canadiantire.ca/AST/brow...ers/PRD~0497127P/Methyl+Hydrate.jsp?locale=en

Question: if stuck, in an emergency, do you think fondue fuel would work to clean brake parts also ?
:yesnod: :teeth :rotf: Add some of the fuel line antifreezes to the list. Any good alcohol would do but I'll save my Ethanol. Isopropyl??? Hard to find over 70% U.S.P. and that's why the campers don't like burning it in their alcohol stoves they'd rather use fondue fuel/Heet fuel line antifreeze/Methyl hydrate.

Je pense que du "94" (Ethanol) ferait l'affaire mais c'est rendu cher je pense a la SAQ. :rotf:
 
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