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Phil Marvin's front master cylinder overhaul

 
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Jim
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Joined: 17 May 2003
Posts: 3840
Location: WHERETHEFUNNEVERENDS

PostPosted: Tue Jun 09, 2009 7:51 pm    Post subject: Phil Marvin's front master cylinder overhaul Reply with quote

HOW I REBUILT MY K1100LT FRONT BRAKE MASTER CYLINDER

1. Things needed: normal hand tools; 20mm rebuild kit BMW part number 32 72 2 332 037 or sourced from Motobins, Magura, etc.; 2 copper gaskets BMW part number 07 11 9 963 072; fresh DOT 4 brake fluid; alcohol; something to clean the cylinder - I used a brake cylinder hone (2 stone) and changed to fine grit stones; 2 wedges about as thick as the brake rotors.

2. Remove calipers from the sliders. Tie or bungee them so they are not hanging by the hoses. Insert the wedges between the brake pads and squeeze the lever to bring the pads against the wedges. The idea is to have as much fluid as possible in the calipers behind the pistons.

3. Cover the tank and any painted surfaces with towels, etc. AND have a spray bottle or other container filled with water within an arm's reach for when (not if) brake fluid does manage to get onto a painted part. (moderator note: brake fluid eats paint - real quick. You can also use a heavy plastic trash bag taped over the painted parts).

4. Remove the reservoir cap and suck out all the fluid with a syringe, turkey baster, enema bulb, etc. Replace the cap, but it doesn't need to be tightened.

5. Remove the brake lever and the cover in front of the piston.

6. Unscrew the banjo bolt, wrapping a rag around it to soak up (hopefully) all the brake fluid which will escape. Save the gaskets and anneal them for reuse, or toss them and use the new ones from (1). I have new ones but annealed and reused the old ones. (Murphy's Law - If I didn't have the new ones ready to use, I'd need them!)

7. Unscrew the screw on the side of the master cylinder which retains the piston. Remove the piston. Look at the grunge (technical term) in the cylinder. Look for pits and feel (with your little finger) the roughness. This is what killed the piston.

8. Lightly hone the cylinder, using the brake cylinder hone, fine sandpaper wrapped around a dowel, etc. When it feels smooth, stop. Check for pits. Mine was not pitted and felt smooth, although it was still discolo(u)red, when I finished honing.

9. Clean, clean, clean with an old rag and plenty of alcohol. You want a clean rag to come out of the bore clean! Flush it again with alcohol and do not wipe it dry; let it air dry, or dry it with compressed air. (I do not have a compressor.)

10. Lubricate the new piston with brake fluid and insert into the bore. NOTE: I don't remember when to tighten the set screw to retain the piston. Read your manual (Clymer or BMW) to get this info. It is important to do it correctly.

11. Reinstall the banjo bolt with new (or annealed) copper gaskets.

12. Reinstall the brake lever and the cover. DON'T SQUEEZE IT YET!

13. Remember getting as much fluid as possible into the calipers in (2)? Now, press the brake pads outward, forcing the fluid upward, (hopefully) filling the bore in front of the piston and forcing ALL the air out into the reservoir and the atmosphere. Mine came up 1/4, maybe 1/3 of the capacity of the reservoir. At this point, I had a good, firm lever with all the air out of the system.

14. Suction the fluid out of the reservoir and replace the fluid in the front brake system as you always (should) do during your annual brake fluid change/bleeding, bleeding both the ABS system and the brake calipers.
15. Clean up the mess.

16. Ride, keeping an eye on the m/c to make sure it doesn't leak.
NOTE: BMW does NOT make brake master cylinders. They buy them from Magura. If you need a new one, you may be able to buy it from Magura USA. If not, they should tell you their dealers. As I write this (June 9, 2009), BMW's recommended retail for a new master cylinder is $361.15 and for a rebuild kit is $70.31. The corresponding prices from Motobins are GBP115 and GBP25 or (at $1.60=GBP1) $184 and $40, plus shipping in either case.

I hope this helps. Remember, I am NOT a professional mechanic. I don't speak Legalese, but the above worked for me; I hope it will for you. Doing it my way, you do NOT have to disconnect and reconnect the heated grips wiring nor bleed the system to remove air.
_________________
Ride Safe,
Phil Marvin in El Paso, TX
'94 K75A/3
'95 K75RT
'95K75RTP
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Neiljohn



Joined: 04 Oct 2009
Posts: 24

PostPosted: Sun Oct 11, 2009 9:27 am    Post subject: Shorter method I used. Reply with quote

Having done this on an R80RT in the past I used this method:

1. Remove switch gear and rotate assembly on bars

2. Remove lever and rubber cover.

3. Place catch pot and rags under work area, holding pistion in place unscrew piston retaining screw.

4. Using fine blade screw drivers on opposite sides of piston, if it doesn't try to fly out by itself, easy piston from bore, don't loose the piston return spring.

5. Clean and inspect bore, flush and wipe clean with fresh brake fluid on clean paper towel, if required hone/scrape bore with soft copper scraper as required, pitting will need honing, deposits on surface scraping.

6. Rotate to normal orientation and loosen cover, allow old fluid to drain through bore, invert and wipe bore again, remove any residue with clean paper towel.

7. Flush and wipe bore again with fresh fluid, when clean and happy bore is good add 1/4" of fresh fluid to bottom of bore, lube new piston assembly with fresh fluid and insert with return spring into bore, hold in location and tighten retaining screw.

8. Replace rubber boot, lever etc, rotate to normal orientation and lock in place, open top of reservoir, using clean paper towel clean out reservoir, when clean add fluid to top of 'bump'.

9. Cover with clear plastic and gently operate the lever, a small jet of fluid from relief hole will travel feet if not covered, operate lever until only fluid emerges (no bubbles) topping up as required as you go, fially fill to 'nominal' full and refit lid.

10. Test brake lever for 'feel', if air is present it will need bleeding further, but as line hasn't been removed it should self bled from the cylinder through the relief hole already.

This worked for me, I don't like splitting lines if I can avoid doing so, but your mileage my vary and as ever it's at your own risk.

Whilst you've got the fluid can out check the rear brake reservoir too, mine had something growing in the bottom of it!
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