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Hello all- I'm new to the forum and new to an '85 r80 rt. Sorry if this is a well-trodden topic, but I couldn't find reference to my problem: I removed the front tire/rim to fix a flat, then reassembled it (axle assembly, etc.,) no problems. But when moving the bike found that there was NO front braking, none. The brake lever felt very firm, little movement and when I cracked the bleeder screw could get no fluid out even when really squeezing hard. I was careful to not touch the brakes while the wheel was out, and the disc slipped right back into the calipers with no problem, suggesting they did not accidentally get forced out too much. Is there some mechanism that locks up both front calipers upon wheel removal??? Any suggestions/theories appreciated!
Bruce.

Follow the brake line back up to the reservoir and see if there is a kink.

A bit more background please...

You say you are new to the bike. Have you ridden the bike since you got it... I guess what I'm trying to say is do you know whether this is a new problem or one that may have been there when you got the bike?

Does the bike have one or two discs (they came either way)?

In answer to your question, there is no automatic mechanism to stop the pistons moving when the calipers are off the disc. In fact, it's always a good move to wedge or fix something between the pads in case you or someone else inadvertantly grabs the lever... but that isn't relevant here.

The front brake is a very simple hydraulic system. It consists of a pump (master cylinder) a length of pipe and a caliper (or two). If you pull the lever and nothing moves then either the master cylinder is stuck (not uncommon with a bike that has been standing... hence my first question), the pads or pistons are stuck in the caliper ... unlikely since there are two (or even four) of each so you'd have to be unfortunate if they both or all got stuck at the same time, or the pipe is blocked.

In your shoes, I would first check my recent work on the caliper simply on the grounds that a fault that is found immediately after work often results from something not done right. Then I would disconnect the line at the caliper and see if I could pump fluid through the line... If so, then the issues is with the caliper(s). Next I'd disconnect it at the master to see if I could pump fluid there... If so, the issue is in the line. If not then it's the master cylinder.

Known issues:

1) Master cylinder getting corroded from water in the system. Normally on a bike that has been standing for some time...

2) Lever pivot getting stuck... Ditto

3) Kinked rubber brake pipe... will probably need replacing

4) Rubber flaking off the inside of the pipe and blocking it... hose will defintely need replacing

5 Pistons corroding and getting stuck in the caliper... If they've all stuck you will have been pretty unlucky but unless they have, you'd get some braking.

6) Pads incorrectly fitted or stuck in the caliper(s)

Hope that is some help...

Rob

Thanks All for the ideas on this. Rob --all good questions and possibles... Bike is new to me, but I did ride it before removing/refitting the front wheel. Brakes worked perfectly. I cracked open each connection starting with the master cylinder and got oozing fluid at each point downstream and finally found no fluid at the connection at the end of the rubber flex hose starting from within the fairing (i.e. some block in the final flex hose). I read up that internal "aneurisms" can occur in brake lines and cause blockages. So removal and refit will hopefully solve the problem Cheers again--

bruce.

rbthompson said " I did ride it before removing/refitting the front wheel. Brakes worked perfectly. I read up that internal "aneurisms" can occur in brake lines and cause blockages."

"Internal "aneurisms" don't happen instantly in both brake lines.

You accidentally pinched a brake line somewhere.
Don't ask why I'm so sure about this one.

gspd wrote: ↑Thu Dec 03, 2020 11:54 pm rbthompson said " I did ride it before removing/refitting the front wheel. Brakes worked perfectly. I read up that internal "aneurisms" can occur in brake lines and cause blockages."

"Internal "aneurisms" don't happen instantly in both brake lines.

You accidentally pinched a brake line somewhere.
Don't ask why I'm so sure about this one.

The Monoshock bikes only have one brake line, even when there are twin calipers... and not all R80s had twin calipers... On the monshock with twin discs, the single line goes from the master cylinder to either the right or the left caliper (it differed from year to year... don't ask me why) then a second line leaves the caliper and goes over the wheel to the other caliper. I don't know why BMW chose to do it this way... but it can make the brake a sod to bleed. Getting back to the point though, if the 'aneurism' is in the line leading from the first caliper to the master cylinder, it will affect both calipers.

Rob

Rob Frankham said: if the 'aneurism' is in the line leading from the first caliper to the master cylinder, it will affect both calipers.

I'm assuming in this case that the front wheel still rotates freely, which would rule out improper re-installation.

I wasn't sure of the exact setup of this particular bike, but I STRONGLY suggest that the flexible line from the master cylinder to the first caliper (or to a splitter on some models) was somehow pinched, squashed, or kinked, probably somewhere around the lower triple clamp/front frame/steering stop/fork brace/fender mount area. Many models also have short sections of rigid metal brake piping that can become totally blocked by a simple sharp bend.

Let's face it, would anybody here be gullible enough to actually swallow that "instant brake line aneurism" hypothesis after bringing their perfectly functioning bike in to someone for a simple tire change and getting it back an hour later with a non-functioning front brake, with a rock hard lever?
I doubt it.

I'm not saying internal brake line or hose blockage cannot happen, what I'm saying is that it is a gradual process that happens over time and its first symptom will be a stuck or dragging brake and there was no mention of that.

I'm trying not to 'assume' anything... but Bruce stated, in his first post, the this occurred when he removed the wheel to fix a flat. There's no need to disturb the lines much to do that. That being the case, I think a kinked line or pinched flexible line is not very likely. That being said, I didn't rule it out.

If moving the piston to get the wheel back into place has disturbed debris in the line, it can quite easily get into a position where it blocks the line suddenly and without warning. Again, I'm not saying that's definitely the cause but it is a possibility.

There are a number of potential causes for the symptoms and they need to be investigated one at a time to find the solution...

Rpb

Rob Frankham said: "If moving the piston to get the wheel back into place has disturbed debris in the line, it can quite easily get into a position where it blocks the line suddenly and without warning.

It's nice to speculate, but have you ever actually experienced what you just said? I don't think anyone ever has. Theorizing is one thing, but that scenario NEVER happens in real life.
Yes, prying or pushing the piston in may have displaced a bit of loose debris, but NEVER to the extent that the (2000 lbs?) pressure from the master would not push it right back. The hard as rock lever implies a solid blockage, not a bit of loose crap in a line.
I'm sure the brake circuit will show some sign of trauma when it is carefully inspected.