twin rotor upgrade: reuse old master cylinder?

[DimitriT]

I'm upgrading the 750T to twin rotors from an 82 650G. So far
I've replaced the right fork lower and reconditioned the new
calipers. Now I'm looking at the master cylinder which came off
the donor bike. The part is good but it will need a new piston
cup set and some major dress up. I've got a recently rebuilt and
perfectly working master from the original single rotor. So I'm
thinking.. what's wrong with reusing the old master? The two
parts look very much alike (but different part #s for everything on
the fiche).

Has anyone tried it?

[TheCafeKid]

I have never done it, but read a bit about it here when i was thinking of doing the same myself. According to most, the MC is fine to use, and you can even run two lines off of it, one for each caliper, instead of running a spliter. You'll need a longer banjo bolt to do so, but they are easily found.

[dardoonk]

I did this upgrade, but found that I could buy a newer mc off a modern sportbike on Ebay for less than it was going to cost me to rebuild the original.

[earlfor]

I would replace the master cylinder with one intended for a dual caliper system. It takes a specific amount of fluid displacement per stroke of the brake lever to operate the brakes normally. If too little fluid volume is displaced, then too long a stroke is required. In the worst case scenario, it could take two full pumps of the brake lever to result in any contact of the pads to the rotor. :-) Put a dual cylinder on a single disk, and the brakes become too touchy resulting in an all or nothing situation. Master cylinders are matched to braking systems.

I'm not saying you cant do what you're considering, but you will get a better braking system by not doing it. :-)

Earl

[bwringer]

What's the piston diameter on the donor MC compared to the old one? It should be marked somewhere on the MC.

The only stat that really makes any difference is the piston diameter.

If they're the same, go for it.

If the old one (the one you want to use) is smaller, expect more lever travel but a bit less effort.

[chef1366]

If you're using the M/C that I sent you a rebuild kit for it is intended for single caliper units. It did not fit my double caliper M/C as it was too small.
Rebuild the other one or get a newer one that is probably better as dardoonk previously stated.

[DimitriT]

Chef, I got the parts from another guy as a set. Good info on the bore diameter. I'll check it. I think the donor part has a bigger bore. Even if I get a newer part off ebay I'll still want a new cup&piston in there as I don't trust old parts right there so I may just finish the job with the M/C which came with the parts.

Now.. can someone remind me how I'm supposed to get the dust cover around the piston after I've circlipped it? I've done it once but can't remember

[spyug]

It is very unlikely that the stock master will be the right size for the twin disk installation. The easiest fix is getting a master from a sportbike that came with dual disks.

I did the conversion on my XS and picked up a master cyclinder from an FJ1100 sportbike. Worked like a charm.

As mentioned, the big problem with using your stocker is that it will be undersize for the application and will not be able to push enough fluid. You may not be able to get the brakes to actuate which ,of course, may have dire consequences.

As you know good brakes are critical so in this area don't skrimp.

Good luck with it and let us know how it works for you.

Cheers,
Spyug.

[tone]

Very good info on master cylinder piston sizes here but i just thought i'd mention again certain early gs's used the same cylinder on both single & twin disc models so its worth checking :-D

Having said that i'd always prefer to go for a later model master if only for the benefit of an ajustable lever :-D

[8trackmind]

The single disk GS's use a master with a 14mm bore. The dual disk bikes use a 5/8 bore. (believe it or not)
If I remember correctly, some of the 1100's may have used a 16mm piston. (to help feed the anti-dive units) As mentioned before, the piston size is cast into the body of the master cylinder on the underside.

Also, you mentioned that you only changed one fork lower. You have to swap both sides. The single disk models used a larger rotor, so the caliper is mounted higher on the fork leg.

[DimitriT]

The single disk GS's use a master with a 14mm bore. The dual disk bikes use a 5/8 bore. (believe it or not)
If I remember correctly, some of the 1100's may have used a 16mm piston. (to help feed the anti-dive units) As mentioned before, the piston size is cast into the body of the master cylinder on the underside.

Also, you mentioned that you only changed one fork leg. You have to swap both sides. The single disk models used a larger rotor, so the caliper is mounted higher on the fork leg.

Wow thanks for the info on the caliper position. I will check that tonight. I didn't swap the left lower because it looked right from eyeballing it. But it's likely a few mm out of position. I used the original fork tubes and springs, BTW. All these parts appear interchangeable with the dual-rotor lowers. I did need a new right bearing spacer but the left spacing appeared correct with the original speedo drive. It looks like the dual-rotor right lower is slightly thicker where the wheel axle passes through. Any other things to consider?

I will check the bore size on my existing MC and the new one.

I guess the hydraulic leverage ratio would be affected so you'd have more travel with less force for a narrower bore. I wonder if it makes a difference with braided steel lines (as I plan on using). With my existing MC, the lever goes stiff after only about 1/2" of travel so even twice that wouldn't be too bad. And the difference in volume per travel is only about 13% between 14mm and 5/8in (if my math is correct = (5/8in / 2)^2 / (14mm / 2)^2).

I think I might do some experimenting with the existing MC and if the lever travels too much, I'll use a larger bore MC.

Learned alot from this thread. Thanks!

[8trackmind]

Any other things to consider?

Yep. You also need to check the caliper bracket clearance on the right side. In most cases, the triple tree on bikes with a single disk is a bit narrower. (only applies to bikes with 35mm fork tubes) It only affects the right side because they both use the same speedo drive.
You may have to take a couple mm's off the inside of the mounting ears so the caliper bracket won't come in contact with the rotor.

You said you changed the wheel spacer on the right side. It was about 2mm to 3mm narrower right? If so, that's how much metal you'll have to take off the inside of the caliper ears on the fork.

[DimitriT]

Yep. You also need to check the caliper bracket clearance on the right side. In most cases, the triple tree on bikes with a single disk is a bit narrower. (only applies to bikes with 35mm fork tubes) It only affects the right side because they both use the same speedo drive.
You may have to take a couple mm's off the inside of the mounting ears so the caliper bracket won't come in contact with the rotor.

You said you changed the wheel spacer on the right side. It was about 2mm to 3mm narrower right? If so, that's how much metal you'll have to take off the inside of the caliper ears on the fork.

The 82 750T and 650G share the same triple tee so this wasn't an issue. The rotors lined up fine and didn't come into contact with anything but the pads. The only thing I had to change was the right spacer because the right fork lower has a wider axle pass-through point than on the stock.

I didn't consider the rotor diameter though and I will swap out the left fork lower to account for that. Thanks for pointing that out.

All the best!