K75s Front Master Cylinder Has No pressure

This bike sat for 15 years. The front master cylinder was really gunned up, even the black rubber gasket had melted. I thoroughly cleaned it, even having to hone the cylinder with a drill press, wood dowel, and some 600 through 1500 wet/dry emery paper. The rubber cups needed cleaning too. I put it all back together and it seemed fine. But I bled the system and still there is no pressure in it. It will pump fluid out the calipers but not with any force. My questions are: 1) any source for those two cups besides the $65 that Beemer Boneyard charges for the rebuild kit? 2) Anything else I should try/clean to fix this? I can't believe the honing I did would enlarge the diameter enough to cause this severe of a pressure loss. There is just nothing at the handle. I am using a suction bleeder on it, and I don't see any air in the lines. 3) Which would be the better route: try re-bulding with the kit; buy a used one off Ebay and hope for the best; or pay the $186 for a new unit? (As usual, incredible price on these BMW parts.) Thanks.

You might want to try Re-Psycle BMW in Lithopolis, Ohio (a nice day's ride there and back from Akron if you take the back roads, less if on the super). I would buy used from these guys rather than trust ebay. At Re-Psycle, Tony has a K75 and is pretty knowledgeable.

https://re-psycle.com/

On my 77 R100RS the master cylinder (located beneath the tank) was leaking, but still provided pressure to the front brakes. Rebuilding the master cylinder was effective.

On my 94 R1100RS the master cylinder started leaking during a return trip from a rally. Pressure was still available to the front brakes, but fluid was being lost. A rag was tied about the master cylinder and the fluid was topped up a couple of times back home. A new master cylinder was fitted. End of problems.

I think in your case I would consider replacement of the master cylinder.

We're talking about the front brake here, a very important component. If this were my bike, I'd spring for a new m/c.

I just took the assembly off and removed the plunger. This time I looked closer and I can see that there is a small pit down low in the bore, probably where the rubber sat against it all those years with bad fluid. It's slight, but probably enough to both eat at the rubber cup edge, and let fluid past. So it's looks like the cylinder itself is shot. People want $100 for a used one, with no guarantee that it wouldn't need a $65 rebuild kit. So you're at the price of a new one. I agree, the front brake is really THE brake on motorcycles. I've never seen a rear one that did much. I'll probably order a new one. Any suggestions on the best source?

I agree that it should probably be replaced or rebuilt with a new kit (piston and seals).

But the problem you are having bleeding it probably is because you do not have the handlebars turned all the way to the left. With the handlebars going straight or turned to the right, the rear (lever end) of the master cylinder is lower than the front of the master cylinder. Since the hose exiting the master is at the bottom front of the cylinder, there will be a very large air pocket at the front top of the cylinder. The only way to eliminate this pocket is to turn the bars all the way to the left. This will make the lever end (and reservoir end) higher than the hose end. The air pocket will either rise and go out the reservoir or be pumped/sucked out through the calipers. After the system is sufficiently bled, leave the bars to the left and with the reservoir cap off, pull in on the lever and let the lever snap back to it's normal position. You will notice a couple of micro bubbles come up into the reservoir. Keep doing this until no more bubbles appear. For the next few days, when you park the bike, park with bars all the way to the left. When you get back to the bike each time, do the snap back thing with the lever a half dozen times (with the cap ON). This will help get any last micro bubbles out, if there are any left.

Make SURE to put a towel on the fairing below the master after installing any master ( used, rebuilt, new). Put a paper towel on top of the towel (it will highlight any drips). Check the lever end of the cylinder for any seepage. Check often for a week or two. Put the towels back on after rides until you're SURE that the rear seal is good. A drip on the fairing will cost a fortune in paint that a little precaution will prevent.

After 25 years I went ahead and bought a new MC assembly. I thought about the rebuild kit, but realized if that did not solve it, it was wasted money, and there were pits. The MC is pricey, but the bike has cost so little since I bought it in 1988, it was well worth it.

A tip: When I installed the new MC I could not get pressure. Air that would not bleed out no mater how much I pumped. So, with the reservoir half full, stuck a 1 cm diameter syringe in tightly the hole in the bottom and pulled the plunger up. That sucked the air right out and I got a firm lever.

Scott

tinytrains;1130834 [B said:

A tip:[/B] When I installed the new MC I could not get pressure. Air that would not bleed out no mater how much I pumped. So, with the reservoir half full, stuck a 1 cm diameter syringe in tightly the hole in the bottom and pulled the plunger up. That sucked the air right out and I got a firm lever.

Scott

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Or you could have turned the bars to the left, like described in my earlier post.

My 2¢ suggests that IF you didn't use a vacuum syle of bleeder whilst preforming the bleed procedure, try that first. . Secondly I would double check to ensure the orientation of the piston is correct as the skirted seals will only work in one direction. Third I would inspect the integrity of the brake lines to ensure this is not a causitive factor. If all of these things are or have been done and still it's a no-go I would buy a new master cylinder. Good luck.

98lee said:

Or you could have turned the bars to the left, like described in my earlier post.

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I should read more of the posts. Good idea.