Leaky Brake Master Cylinder

Repair A Leaky Brake Master Cylinder

Many times, but not always, leaking brake master cylinders, front or rear, can be cured by just taking them apart and cleaning them. Over time, brake fluid can dry up and turn crusty and when this crusty brake fluid get in the seals around the piston it will cause leaks.

Regardless, even if you end up needing to rebuild it or replace it, the first thing to do is take it apart to asses its condition. All K bike master cylinders have a 3mm Allen head machine screw will allow you to remove the piston from the cylinder. For example, here's a K bike rear master cylinder:

Once you have it apart, gently clean the rubber seals on the piston with some water and a paper towel. Then inspect both seals for nicks or other damage. If the seals are damaged then you'll at least need a rebuild kit.

Next, clean out the bore of the master cylinder with some 0000 grade steel wool. Then inspect the bore for pitting. If the bore is pitted then you'll need to replace the entire master cylinder.

If neither the seals nor the bore appear to be damaged then re-assemble the master cylinder and install it. Keep an eye on it to see if it's leaking anymore.

If you're ordering a rear master rebuild kit, then you need to know if your master cylinder is 12 or 13 mm. The diameter is cast into the side of it:

Here's a table of relevant BMW part numbers:

Part	Diameter (mm)	non-ABS	ABS	Part #
Rear Master Cylinder	12	Y	Y	34311458142
Rear Master Repair Kit	12	Y	Y	34312311064
Rear Master Cylinder	13	Y		34311454351
Rear Master Repair Kit	13	Y		34311454438
K75/K100 Front Master Cylinder	13	Y		32722310742
K75/K100 Front Master Piston	13	Y		32722302356
K75/K100 Front Master Cylinder	13		Y	32722310741
4V Front Master Cylinder	20	Y	Y	32722352190
4V Front Master Repair Kit	20	Y	Y	32722332037

Here's how to clean/rebuild a 4V front master cylinder without having to take it off of the bike:

1. Put the bike on the center stand.
2. Place some folded paper towels on the knee panel and front right of the gas tank just in case you spill any brake fluid.
3. Remove the four screws the hold the master cylinder reservoir cover on. Be sure to use a good screwdriver and adequate downward pressure so as not to strip the heads of the screws.
4. Use some paper towels to soak up and remove all of the brake fluid from the master cylinder reservoir.
5. Remove the screw at the base of the right combination switch. Again, use a good screwdriver and pressure to avoid stripping the head of the screw.
6. Loosen the countersunk Allen bolt that holds the right perch in position on the handlebars.
7. Rotate the top of the master cylinder up towards the rear of the bike.
8. Remove the screw that holds the plastic cover on the front of the master cylinder.
9. Remove the 5mm Allen headed bolt that the brake lever pivots on. It usually doesn't fall right out when it gets to the end of its threads. Using a good Allen wrench (like the one in the factory toolkit) seat the Allen wrench fully in the bolt cap head and apply some angular force to try to pull it out the rest of the way.
10. Remove the brake lever and rubber boot.
11. Remove the 3mm set screw (retaining screw) that holds the piston in the master cylinder. It’s a good idea to keep your thumb over the master cylinder piston. It has a spring behind it and it may fly across the room if you don’t.
12. In my case there was enough old crystallized brake fluid at the front of the master cylinder bore that the piston didn’t just pop right out. What I’ve found works to get it out is to insert an 8mm Allen wrench and apply some angular pressure while turning it to get the piston to rotate and come out. (Have some paper towels handy to clean up any brake fluid that might spill.)
13. You’ll probably see some dried up crystallized brake fluid near the front of the bore. This is the crap that gets in the piston’s front rubber seal and causes leakage.
14. Using some steel wool, clean out all of the dried up brake fluid. Some very small remnants of the steel wool will fall into the master cylinder when you do this. Once you’ve finished using the steel wool, use some paper towels to clean that out of the master cylinder.
15. Clean any brake fluid crud off of the rubber seals on the piston. (Don’t use steel wool for this, use paper towels or a microfiber cloth.)
16. Inspect the lips of the seals on the piston. If those have any nicks or cuts in them then you’ll need a master cylinder rebuild kit.
17. Inspect the bore (master cylinder wall) for any pitting. If enough water has sat in the master cylinder long enough to cause pitting then a rebuild won’t work and you’ll need to replace the whole master cylinder. Sometimes a good disassembly and cleaning (clean the bore with 0000 steel wool) will restore the master cylinder without the need for a rebuild kit.
18. Put the spring and piston back into the master cylinder and, while holding the piston in, replace the set screw.
19. Reinstall the rubber boot.
20. Reinstall the brake lever and it’s pivot bolt, making sure that the brake switch lever is in the right position between the brake lever and switch.
21. Reinstall the back plastic cover.
22. Rotate the master cylinder back into position (I sit on the seat and eyeball it so that it’s at the same angle as the clutch lever) and tighten the countersunk Allen bolt that holds the right perch in position on the handlebars. Do not overtighten this bolt or it will break the clamp off of the master cylinder and you’ll need to buy a whole new master cylinder for $350+. Just get it nice and snug.
23. Turn the handlebars all of the way to the left so that the master cylinder is level.
24. Put enough brake fluid is the master cylinder reservoir so that it comes about half of the way up the view window on the right side of the reservoir.
25. S-L-O-W-L-Y and repeatedly squeeze the brake lever until you stop seeing little tiny bubbles come up from the hole inside the reservoir near the rear.
26. Fill the master cylinder reservoir until it’s about halfway up the view window in the right side.
27. Replace the cover with the rubber boot and reinstall the four screws that hold it down. You want to get those screws fairly tight to prevent brake fluid from leaking out of the top of the reservoir.
28. Go for a cautious test ride to make sure that your brakes are working properly.