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Front Master Cylinder...about that hole....

R

robinelwood

Forum Apprentice
In various threads about rebuilding the front master cylinder, there is a common refrain of 'the second return hole'....a very small hole that lets fluid flow back and not cause the brakes to bind/lock over repeated use.

My '80 850 has an issue...the PO (a forum guru and a real stand-up guy) rebuilt the calipers and the Master Cylinder, and put stainless lines on.

When I picked up the bike, he gave me an 8mm wrench because the front brakes had an unexplained tendency to bind once in a while, and I would need it to bleed a little fluid off.

Generally, the brakes work great... good firm pull, release ok. But, as he warned, it sometimes starts to bind...rolling the bike around gets harder, starts to squeal and if you ride any distance, the disc gets smoky hot and the wheel wants to lock up. Not the best feeling.

Well.... I finally got around to tearing down the master cylinder. As Bob said, it has a nice clean bore and fresh rubber boots. But... I can only find one hole. There's a divot on top that looks like it SHOULD be a second hole, but it doesn't go through, and the bore is smooth and unmarked in that spot inside.

Before I give up and reassemble it as it is... can anyone point out where the return port should be or is? (or...alternate reasons the brakes start to bind!) 100_1624.jpg
 
Your "divot" is your second hole. Use some very fine wire to open it up.

Most have found that a single bristle from a wire brush will do the job.

.
 
Thanks steve..I'll work on that more. A bristle seems to not want to go into it, and the fact that the inside of the bore seems so unmarked there...maybe my previous attempts to open it schmeared it over. Thabks for quick response!
 
Yup. looks like previous attempts with blunter objects had peened the aluminum around until there was no hole. A wire bristle wouldn't touch it, but cleaning up the hole followed by judicious use of a skinny point as a punch has opened up a tiny little hole, just big enough for a guitar high e string or a grill brush bristle. Tomorrow, we'll see if it all works better, but in the mean time, thank you for good advice!
 
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