I am always aware of, and grateful for, all those people who share their knowledge and experience via the internet. In this case I refer particularly to the Young Engineer, Martin, The Mighty Gryphon, and the others who have posted here.
I try to help when I’m able to, so here are my experiences. It’s a bit of a rambling story but someone might find it useful. Settle back and get comfortable.
My bike is a 1985 K100RS with NO ABS, suffering from Magura seal failure.
I stumbled on the Chinese master cylinder concept when searching Ebay for a BMW rear master cylinder. “Chinese master cylinder, fits pit bike and BMW K100” (!!!). Further investigation led me here.
The path I took involved using a 500mm braided teflon caravan brake line to replace the entire (non-ABS) brake line from the new Chinese master cylinder to the caliper. The line cost about $50 landed. 400mm would have been better, but the 500 was on offer. Initially I was going to use a 90 degree banjo fitting on the master cylinder, with a female section silver soldered onto it. The banjo bolt was to be cracked to aid in bleeding.
Then I changed my mind and decided to use a straight connection that replaced the banjo bolt (nominally m10 x 1.25) with a straight adaptor (M10x1 female to M10x1.25 male), thus coming straight out the back of the master cylinder as per original. If necessary, to aid bleeding the footrest plate can be dismounted and inverted (as detailed in someone’s post somewhere). The master cylinder arrived and it turned out that they used an odd thread for the banjo bolt. The diameter is 9.75mm, but the pitch is still 1.25mm, so a thread tap was used to open it out to M10x1.25. The banjo bolt came with a couple of Dowty-type washers, which was handy. A Dowty washer is a metal washer that incorporates a rubber o-ring bonded to the inside diameter. When the bolt is tightened the rubber deforms, but only to the thickness of the metal washer. This gives a great seal. It’s a simple and brilliant invention. Dowty was an hydraulics company, and among other things made air over oil suspension for motorcycles (Velocette and Panther come to mind).
Because the caliper and the new braided line use different flares I made an adaptor to mount the new pipe to the caliper. This consisted of a short length of brake pipe (ex-Holden Commodore) from the caliper ending in a female fitting (ex-Subaru),turned down and bored to take the pipe, to which was then silver soldered. This pipe was fitted to the caliper and then bent (in the fashion of the original pipe) so that it aimed at the centre of the hole in the “C” section bracket on the side of the swingarm. I bought the M10x1 to M10x1.25 master cylinder adaptor across the counter at the local brake and spare parts place. It was about $5. I used one of the Dowty washers between it and the master cylinder.
The easy solution is, of course, to use the original hose and pipe, screw the hose into an M10x1.0 to M10x1.25 adaptor, use a thread tap to open out the thread in the hole in the master cylinder and screw the whole thing straight into the back of the new master cylinder.... however..... a) I’d already bought the braided hose and b) this solution is not complex enough.
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If you’re in Australia you can use a Holden Commodore rubber front brake line to do the same job. You will need a joiner for the caliper end. This is a m10x1 double ended thingo that is hard to get. Or you could make or fabricate one (two sleeve nuts welded back to back might do it). The other end of this hose is a banjo type fitting.
The pin-in-a-dimple operation of the master cylinder pushrod seemed rather crude, and like The Mighty Gryphon I considered a heim -type joint (rose joint on the threaded rod and a saddle on the adjuster bolt on the pedal. Move the master cylinder back one hole and brace it with a bracket).
Then I tried a mock-up of the pin-and-dimple and decided that it was elegantly simple. I decided to put the dimple in the end of the master cylinder threaded rod so that I could retain the original rubber boot. I also put a nylock nut on the threaded rod until it just started to bind on the nylon. This helps to guide the pin into the dimple.
The master cylinder that I got had mounting holes at 40mm centres, not the advertised 45mm. I had to file one of the holes to make it fit, which was a bonus as I was able to file it at a bit of an angle in order to bring the threaded rod more central to the tit on the end of the brake lever adjusting screw. This is not critical, as the rod can be moved through a fair arc. This rod has a mushroom head which fits into a recess in the end of the piston. A washer fits over the rod and is retained in the bore by a circlip. The rod is thus spring loaded to the same axis as the bore, but will move from side to side with not too much effort. This assembly makes it easy to remove the rod to work on it (I used a lathe, but that’s probably overkill).
The 500mm hose was a bit hard to route neatly due to its excessive length, so I made a bracket which hangs down from the front mounting screw which attaches the tragkorb frame to the footrest plate. This makes the hose hang in a loop in front of the final drive, keeps it away from the centre stand, and also reduces the strain placed on the hose where it joins the master cylinder as a result of suspension movement.
I ran a new 8mm nitrile hose from the original reservoir to the new master cylinder. It was a reasonable fit on the barb of the 6mm reservoir pipe, and tightened up nicely with a hose clamp. Length 300mm, cost with clamps $7 from the local clutch and brake place.
The system was bled using a vacuum bleeder, a first for me. I can see now why Martin uses a rigid jar with his to catch the fluid. I used a plastic soft drink bottle and just about pulled it inside out with the vacuum. Bleeding was quick and easy and there was no need to invert the footrest plate.
Case closed.