Guys,

I recently purchased a neglected 86 K75 (44,000mls) and am working my through it. The front brakes didn't work and on taking the reservoir apart, I found a load of gunk inside and also inside the cylinder. I cleaned it up with using a piece of wire, brake cleaner, blasting air and finally an ultrasonic bath. It is now spotless and both holes are open as can be seen in the pictures below. Also, the piston, seals, and cylinder bore all look in excellent condition. 

So why the post? When I fill the reservoir with fluid I get no output to the brake line when I pump the brake lever! There is no brake fluid escaping anywhere either. This does not make sense to me and can only guess that the rubber piston seals are not as good as they look. Do I need to buy a rebuild kit (new piston and seals) ? 

Cheers
Ken  

Normally if you fill the master cylinder with fluid and block the outlet you should get pressure when the lever is pumped. WARNING don't get brake fluid on your paintwork. Even with slightly scored cups you should get some pressure build up. Picture shows cups correctly orientated you haven't changed them since I presume??
Regards Martin.

My guess is the seals are duff, but you might also have an air bubble in the line that's taking the p!ss. A reasonable (but not foolproof) check is to push the caliper pistons hard home, then try bleeding. What that does is to reduce the amount of air space in the hydraulic system.

Have you removed the lines at the calipers?  I would test for flow at every joint past the reservoir until you locate where the blockage might be.  Bleeding the front brakes is a pain but be patient and eventually you will get everything worked out.

Thanks for the replies, Here’s what happened. I tried the usual bleeding procedure which I have done many times and no fluid came out of the nipple. I removed the brake line at the reservoir and still nothing came out when I pumped the brake lever. I removed the reservoir and that’s when I found all the crud. Cleaned it as described and put it back together and still get the same result. I can only think it’s the seals on the piston but they “appear” fine. Looks like I’ll just have to buy the rebuild kit today.

kenrams wrote:I tried the usual bleeding procedure which I have done many times and no fluid came out of the nipple. I removed the brake line at the reservoir and still nothing came out when I pumped the brake lever. I removed the reservoir and that’s when I found all the crud. 

Have you checked the brake lines for crud?

The first part of the video involves the master cylinder; the second part—at 1:29—involves one of the brake lines.

At this point in the procedure, the lines are disconnected and I am getting no fluid out of the bottom of the brake cylinder when I pump the lever.

Are you absolutely sure both holes are open?  I only saw the large one in the photos. 

Do any air bubbles rise into the reservoir when you squeeze the lever or tap the reservoir?

With no piston in the master cylinder, will the fluid flow down from the reservoir? 

Is any fluid leaking from the lever end of the piston?

Did you lubricate the piston seals with brake fluid before you put it together?  I soak mine in a small pan of fluid for a minute or so to make sure there are no dry spots between the seals and the piston.  Some guys even put some fluid in the bore when they assemble the master cylinder.

I took my K100's reservoir apart and removed the known good piston (and seals). I fitted it into the K75 and after two pumps of the lever the brakes came back to life. 

So the lesson is that although the old piston and seals looked "good" they actually were not. 

Thanks for the input guys! I am sure I will be back with questions as I work my way further through the bike this winter