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Brake problem


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1984 BMW RT 100

The front twin calipers lock up, it takes time so it's a gradual thing. When they do lock the only way to move the bike is to open both bleeds.

When I do that it's good for a few times using the brake, then it locks again.

Where is the problem?

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Probably the master cylinder piston not fully retracting.  Another possibility is the hose(s).  They can delaminate or swell internally acting like a check valve.

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And I don't know that bike specifically, if it has any valving between the master and calipers the problem could be there.

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Most likely front master cylinder. Both brake calipers would no go kaput at exactly same time.

 There is really, really small fluid pressure relief hole at the bottom of reservoir. There is one more, much bigger feed hole. 5 bucks says relief hole is clogged. 

What happens fluid can't return freely to master cylinder, brakes drag, heat builds up, fluid expands, pressure builds up, brakes lock, etc.

 

Anyway, that would be my first line of attack.

Edited by tomek
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Thanks to both.

It drives about 30 miles before lock up.

I'll pull the master first.

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One way to determine if it's the master or hoses is to crack the banjo bolt at the master when it has started dragging.  If that relieves it it's the master, if not it's the hose.

 

Actually; I just re-read your original post where you say you have to crack both bleeders, I had missed that.  If that's true then the problem is both hoses going to the calipers have gone bad, unless there's some kind of valve between the master and calipers.

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All good stuff above, but I'll add another personal experience, in case it might apply.  On my 2010 Triumph, I installed an aftermarket brake lever, and had the same thing happen, but I only had to crack one bleed screw.  The new lever wasn't quite allowing the master piston to fully return.  The "dry end" of the master piston goes into a hole in the lever, and it needed to be just a few thousandth's deeper.  A drill bit fixed the problem.

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All good advice.

The Master is the culprit.

So corroded the return bleed needed to drilled out so, tomek is spot on.

 

20210713_200653.jpg

20210713_204009.jpg

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Makes the case for regular maintenance.

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57 minutes ago, OMG said:

Makes the case for regular maintenance.

 

Every time I flushed the brake/clutch system, I always drained out the master reservoir and swabbed out any debris with a paper towel and then q-tips to ensure the gunk never built up.

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9 hours ago, OMG said:

 tomek is spot on.

 

 

Of course. 😋 I don't understand why some here sometimes have a nerve to question my infinite wisdom.😉

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1 minute ago, Zero Knievel said:

 

Every time I flushed the brake/clutch system, I always drained out the master reservoir and swabbed out any debris with a paper towel and then q-tips to ensure the gunk never built up.

Any gunk might re-dissolve in new fluid, the paper and cotton lint probably won't so I try to avoid doing that if I can.

 

I typically suction out the old fluid along with any build-up before adding fluid.

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6 minutes ago, superhawk996 said:

 

 

I typically suction out the old fluid along with any build-up before adding fluid.

+1 

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5 hours ago, superhawk996 said:

Any gunk might re-dissolve in new fluid, the paper and cotton lint probably won't so I try to avoid doing that if I can.

 

I typically suction out the old fluid along with any build-up before adding fluid.

In Zero's defense, if one is OCD enough to thoroughly swab out the reservoir, they would make sure there is no lint left behind.  Don't ask me how I know.😉

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1 hour ago, jon haney said:

In Zero's defense, if one is OCD enough to thoroughly swab out the reservoir, they would make sure there is no lint left behind.  Don't ask me how I know.😉

 

Thanks.  Indeed...my work was so spotless you could eat off of it. :D 

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