Boxerworks.com

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.

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.

barryh wrote: ↑Mon May 13, 2024 10:58 amI 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.

SteveD wrote: ↑Mon May 13, 2024 9:30 pm
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.

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

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.

jackonz wrote: ↑Thu May 23, 2024 10:31 pm 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.