77 GS750 Rear master cylinder alternatives?

[93Bandit]

I "rebuilt" my rear MC by disassembling and thoroughly cleaning it. All the rubber seals and boots appeared to be in good condition, they were pliable, no nicks or tears, so I reused them. MC bled and holds pressure just fine, but the other day I noticed brake fluid is filling the piston rod dust boot on the bottom, so clearly it's leaking. The external portion of the piston rod is heavily pitted along with the washer and ball joint that's inside. The bore looked OK for the most part, but had some discoloration.

Bottom line, I think I may need to replace it to be safe. I don't know if new seals would solve the issue? The MC is not longer available new, and I found some used MC's from newer GS's that aren't exactly the same but may work. But I'm wondering if there are any brand new affordable MC's that people use that I can try that would bolt on? Either aftermarket or from newer model bikes? I will go the older used route if I have too, and that's probably the most affordable option, but I fear I may deal with continual leaks and/or at worst, a failed brake system.

Any thoughts?

[salty_monk]

Given the frame similarities you might be able to take something from my thread: https://www.thegsresources.com/_foru...ern&highlight=

I also think the DL650 unit might work without a rod extension...

[93Bandit]

Hey, thanks for the information. I read your thread and that doesn't look too difficult. I have yet to take measurements, so I'll need to do that before committing to anything.

I think the biggest challenge will be if the mount hole spacing is different and if I need to do the modifications you did, because my MC mounts to the backside of the frame downtube. Yours has a bracket that extends out from the frame tube, so you can easily access the holes and drill them out. Mine are harder to access... See images below.





However, when searching for a ninja 300 MC on fleabay, I came across this generic china MC that looks like it copies the ninja MC design, but has elongated mounting holes that may allow mounting to my existing mount. But I wonder if it'd be any good? China made afterall... https://www.ebay.com/itm/31359593506...pid=8049069129

[salty_monk]

I'd be wary of that personally... I wasn't able to find anything that had the same hole spacing but there are some candidates but I didn't want to end up buying a bunch of MC's to find out. Yours does look a bit harder to work with than mine for sure.... but maybe you can get a long drill in from the other side followed by a bottoming tap.... Good luck!

[rphillips]

A few yrs back, we put one of those China, $25 masters on a buddy's Honda CB1100F, his orig. was too corroded to rebuild. It was amazing how many different bikes it was advertised to fit, but it worked fine with only a couple of extra washers, for spacers, to make it fit flush against the frame. Still working fine for him. I didn't see it on the link above, but many of those China ones, on ebay, will have a list of bikes they will work on, or sometimes a place to put in your bike model & it will tell will fit or won't fit...We were surprised at the quality it seemed to be.

[93Bandit]

A few yrs back, we put one of those China, $25 masters on a buddy's Honda CB1100F, his orig. was too corroded to rebuild. It was amazing how many different bikes it was advertised to fit, but it worked fine with only a couple of extra washers, for spacers, to make it fit flush against the frame. Still working fine for him. I didn't see it on the link above, but many of those China ones, on ebay, will have a list of bikes they will work on, or sometimes a place to put in your bike model & it will tell will fit or won't fit...We were surprised at the quality it seemed to be.

OK cool, thanks for the information. There's a lot of chinese clone products I won't buy because it's such a complex component, but as for these old master cylinders, they're not too complicated. I'll search on my model and see if I can find one with similar/same specs.

[salty_monk]

Yeah the worst I can see happening is that the internal O rings swell and it locks up... (or it leaks). Either way you should find out pretty quickly!

Maybe use it with DOT 5 silicon instead of DOT 4 or 5.1 as that would protect against the swelling issue at least.

[salty_monk]

https://www.z1enterprises.com/search?q=rear%20brake%20master&product_list_dir=asc&product_list_order=name

https://www.mikesxs.net/yamaha-xs650-master-cylinder-assembly-rear.html

Same company these days... I did think about doing it the other way... Making an adapter plate to fit on the master cylinder rather than drilling the bracket on the frame. It could be done I think (if you can find one with the holes close enough together).

[93Bandit]

Yeah the worst I can see happening is that the internal O rings swell and it locks up... (or it leaks). Either way you should find out pretty quickly!

Maybe use it with DOT 5 silicon instead of DOT 4 or 5.1 as that would protect against the swelling issue at least.

Oh yeah, I'll find out real quick. At least with a bike you get independent brake circuits, so if there's an issue with one you still get the other. However that doesn't really help if the one that fails locks up. That would make a guy pinch a hole right through the seat in a hurry!

https://www.z1enterprises.com/search...ist_order=name

https://www.mikesxs.net/yamaha-xs650-master-cylinder-assembly-rear.html

Same company these days... I did think about doing it the other way... Making an adapter plate to fit on the master cylinder rather than drilling the bracket on the frame. It could be done I think (if you can find one with the holes close enough together).

The only issue with an adaptor plate is that depending on how crazy you have to get, the pedal won't align properly. I'm really hoping I can find something that I can mount to the existing mounting points, or as close as possible.

[mustangflyr]

There is an alternative to replacing the MC. Providing that you do have the internal components to over rebuild it consider having your MC sleeved. Did it on mine. No more needing the entire MC to be replaced.