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The brakes on my 1974 R90S are not releasing. I searched the older posts and found out it is most likely the "vent hole" that needs to be cleaned out. The brake system has only a few thousand miles on it since having been rebuilt (by me). It has new hoses and the calipers were rebuilt. It stops well, but as stated above, the brakes don't release.

Do I have to take the under tank master cylinder off the bike to clean out the vent hole? Is the hole that needs cleaning right under the plastic reservoir? Do I clean it by running a wire through it? Where does the vent hole go?

Chuey

The hole vents the fluid in the tube back into the resevoir when the piston is retracted. If the vent is clogged the pressure in the tube cannot be released.

Your questions: Yes. You might be able to leave the MC on the bike.

Thanks, but do I push a wire down through the hole?

Chuey

Chuey wrote:Thanks, but do I push a wire down through the hole?

Chuey

You don't want to risk reaming the hole larger and creating a seal eating burr on the inside of the bore. I've used bamboo kebab skewers, gently, to good result. If there is a pile of debris down there blocking the hole, stir it up and suck it out first, before doing the pokie thing.

Chuey, although a clog in the return port may be your problem, if you flush & fill regularly it probably isn't...I thought the same thing myself when I had the same symptoms. A few pumps bleed would alleviate it only momentarily.




In the instance I describe the cylinder was actually hanging up on a torn dust cover(seal). Also a good time to check pad alignment via eccentric bolt-mine was far enough off to require replacing pads w/ less than 2500 mi on them after Maxbmw performed a deer collision "repair" without properly adjusting the caliper( among other things-rant over but I had to!). Their online fiche is most helpful though.

I cant recall the details of the cable adjustment on these things, but if it was too tight and the piston was not returning fully this would cause it too.

But generally if non venting is the problem the brake comes / stays on as the fluid heats up , and the longer you run the hotter, and worse, it gets.

At least it did when I maladjusted the brake pedal on the rear disk on my F650 GS-----.