Front brake master cylinder.

[SteveD]

My R100RS is an 11/1981 build. Handlebar mounted master cylinder, 13mm, changed from 15mm many years ago with a great improvement to feel.

The lever travel had diminished recently, but still ok, so I removed the master cylinder to investigate. The master cylinder looked good, and I ensured the holes were clear with brake cleaner and a burst of compressed air.



It looks like my 13mm MC is missing a couple of bits, specifically the seal and the washer at the circlip end. The two seals are on the internal cylinder shaft. It's been like that for years.
With the master cylinder in hand, it looks like the circlip goes under the washer. However the fiche shows the circlip outside the washer. I'll take it as one of those anomalies we see occasionally with the fiche.

edit: sorted...see the following post.

To me, it looks like the circlip goes under the washer. The washer looks too big to fit into the cylinder (edit) and the one I found doesn't fit in..

Mine looked like this when it was removed. There is a circlip under there. Just no outer seal or washer. It cleaned up just fine.



So, pending further thoughts, I've reassembled the lot as it was. Adding the seal and washer won't take long when...
addition: I've found a washer amongst my parts stash (brake section) and will look for a seal that fits. I don't think I'm going to be too fussy on that, as long as it's snug enough and wide enough.

I'm still unsure why the travel has diminished recently. I don't think this explains it. I'm going to look at the ss hoses that are on the bike. Maybe the lining has deteriorated? The right side flushes ok from the top, but isn't accepting a retrograde flush.

[SteveD]

Brookes garage to the rescue. What a great resource his stuff is.
https://brook.reams.me/bmw-motorcycle-r ... cylinders/

There is a front dust seal and large flat washer that are installed in the throttle assembly. The seal fits over the end of the piston nose and the flat washer protects the seal. The brake lever presses on the end of the piston nose to pressurize the front brakes.

So I removed the master cylinder assembly with the reservoir (2 bolts/2 minutes) and lo and behold there was the washer and the seal d'oh! ...inside the throttle assembly. Removed, cleaned, a little rubber grease added and then reinstalled.

Now to the front hoses.

[barryh]

I have my master cylinder apart at the moment and yes the seal and washer were inside the throttle assembly.

I recognise the gunge in the end too. I wonder if that indicates a slight leakage past the seals. Whatever, everything seems in good condition once cleaned up so it's going back as it is.

[SteveD]

I recognise the gunge in the end too. I wonder if that indicates a slight leakage past the seals. Whatever, everything seems in good condition once cleaned up so it's going back as it is.

I was wondering the same thing. Replacing the seal would be simple enough. However it only seems available with the piston assembly at UKP45 (motorworks) or $US78 from EME...plus post.

Failing replacing just the seal, it's only 2 small bolts, a spritz of brake cleaner, rinse, dry, lube and reinstall to keep it from building up too much.

Re the hoses, I've previously performed a retrograde bleed successfully on these hoses, trying to solve a soft brake lever action when the SS hoses were initially installed. I can't do it anymore, no success with pushing the fluid to the open reservoir. The lever feels stiffer (not as wooden as the 15mm that it once had) but without the good travel it had. So now I'm wondering if there's any change to the lumen surface of the hose system, creating a limited non return valve? It bleeds just fine in the traditional way.
Next step would be to remove the hose from MC to splitter.

[barryh]

Re the hoses, I've previously performed a retrograde bleed successfully on these hoses, trying to solve a soft brake lever action when the SS hoses were initially installed. I can't do it anymore, no success with pushing the fluid to the open reservoir. The lever feels stiffer (not as wooden as the 15mm that it once had) but without the good travel it had. So now I'm wondering if there's any change to the lumen surface of the hose system, creating a limited non return valve? It bleeds just fine in the traditional way.
Next step would be to remove the hose from MC to splitter. [/color]

I wondered if I had a non return valve going on with the hose when I tried to change the fluid, which is is why I stripped the whole system down and blew through all the hoses and pipework. Everything seemed clear so I reassembled and filled from the bottom up using a large syringe of brake fluid. Except for one problem, that worked ok giving me some brake pressure straight away and after tying the lever back over night it's solid with no bleeding at all.

The problem was I left off the reservoir cap while refilling and brake fluid made a very good impression of a 2 ft high fountain out the top of the reservoir as I pushed it in too fast with the syringe. Surprisingly only a few spots on the paintwork and no harm done as I had a bucket of soapy water on hand.

Should I have fitted the cap and would that have worked with the cap not being vented ?

I 'm having trouble believing in hoses developing a one way valve. The bore seems very small perhaps no more than 2 or 3mm, I managed to poke some thin strimmer line right through to check that they were clear. One of this days I'm going to section an old hose to see what the construction is like.

[Rob Frankham]

If you do reverse filling ike that, fit the cap loosely without the rubber 'bellows'. If you fit the cap and bellows tightly, the rubber will be pressed against the top of the reservoir, the air in the reservoir will pressurise and simply push the fluid out again. You need to let the air escape but baffle the fountain effect.

Rob

[SteveD]

Yes due care to not splash it everywhere. My hoses are likely close to 20 years old, Venhill SS version. I'm unsure about the one way valve thing via lumen damage, more thinking it might be a little junk in the system.

[SteveD]

D'oh...bleed nipples were one way. I must've replaced them previously.
I had all the hoses apart and fluid was flowing both ways so I found some other nipples amongst my stuff, replaced them and all bled up beautifully. Good to go...err... stop!
The good thing about fiddling with the bike in the shed and having small issues is that it ensures you go looking for parts. I found a couple of boxes with all sorts of brake bits and pieces.

[jackonz]

I had issues bleeding my R80 with twin disks, no matter what I did it was spongy and this was with all new seals and hosed in calipers and master cylinder rebuild, then I read something about vacuum bleeding it so I used a machine from work and it seemed better it was still spongy, so on the advice of someone else they told me leave it overnight with the master cylinder cap on loosely but full, left it for 2 days and it came good, first time in 50 years have I come across something like that.

[SteveD]

I had issues bleeding my R80 with twin disks, no matter what I did it was spongy and this was with all new seals and hosed in calipers and master cylinder rebuild, then I read something about vacuum bleeding it so I used a machine from work and it seemed better it was still spongy, so on the advice of someone else they told me leave it overnight with the master cylinder cap on loosely but full, left it for 2 days and it came good, first time in 50 years have I come across something like that.

I use a 50ml syringe to push or pull the fluid, depending on what I'm trying to do. Helps to have a second person. Because I was trying to assess the state of the hoses without further dismantling I was trying to push uphill. Once I replaced the bleed nipples that worked well. I finished this task doing a quick traditional bleed. In retrospect it would've been quicker to just pull the whole lot apart which is what I eventually did. Added new fairing grommets in the process.

The old school trick is to tie the brake lever on o/n...not too tight. It also helps.