GL1000 rear brake master cylinder mounting bolt distance

[toomanybikes]

Could someone with a GL1000 please tell me the ctr-ctr distance for the mounting bolts of the rear brake master cylinder?

I'm working on a CB750F rescue with no luck finding a usable m/c. I have two, the original and one from a salvage bike, but both are currently frozen or otherwise damaged. The GL part "looks" like it may be physically compatible, which would open up options on ebay or here.

Pic for reference. I measure the mounting holes at 1-7/8 or ~48mm ctr-ctr

PXL_20250717_124232923.jpg

[Fred Camper]

1 and 7/8th on my GL1000. But my bet is it metric 48 mm.

[Whiskerfish]

IIRC the 750F had a separate reservoir. The wing reservoir is part of the M/C

[toomanybikes]

1 and 7/8th on my GL1000. But my bet is it metric 48 mm.

Thanks. That helps broaden my search if I can't get my existing ones unstuck

[toomanybikes]

IIRC the 750F had a separate reservoir. The wing reservoir is part of the M/C

Yes, but if I have my 3D orientations correct, there is room for the GL's wing reservoir on the CB

PXL_20250717_102105999-EDIT.jpg

[Whiskerfish]

[redglbx]

Let us know how it works out..

[toomanybikes]

Let us know how it works out..

Others on the SOHC/4 forum have done it. Apparently you have to get a banjo-flare nut adapter since the GL1000 used a flare nut. Seems like an easy solution, but sourcing a functional one is still a challenge, short of paying $350 for a NOS unit on ebay.

[Sidecar Bob]

The GL1100 rear master has a remote reservoir. I can't get at mine to measure right now but the one on my '83 originally came from an earlier 1100 and I had it on the '79 GL1000 for several years before I mounted it on the '83 so I'm pretty sure that the centres on all GL1000 & 1100 are the same.

I have had a single (stainless braided) line from the master to the caliper since i put it together in '97. I seem to recall having to pull something out of the port where the steel line originally went but I'm not sure if I'm remembering correctly (I didn't keep such good records then and it might have been another project). I'm pretty sure that I did have to find a banjo bolt with the correct threads to fit in place of the original flare nut; I know for sure that I did not use a banjo to flare nut adapter because I never heard of such a thing until I read this thread.

[redglbx]

Wealth of knowledge Bob !

I agree, personally I won’t ever use anything but the braided stainless lines on any of my bikes unless it’s a total stock show bike because so many people get their bowels in an uproar if it’s not just STOCK ! But I think my days of showing are over.

[Fred Camper]

There are lots of GL's in Museums now, they will not take any more. So better to ride them and that means braided stainless for all.

[redglbx]

Fred,,

[toomanybikes]

The GL1100 rear master has a remote reservoir. I can't get at mine to measure right now but the one on my '83 originally came from an earlier 1100 and I had it on the '79 GL1000 for several years before I mounted it on the '83 so I'm pretty sure that the centres on all GL1000 & 1100 are the same.

I have had a single (stainless braided) line from the master to the caliper since i put it together in '97. I seem to recall having to pull something out of the port where the steel line originally went but I'm not sure if I'm remembering correctly (I didn't keep such good records then and it might have been another project). I'm pretty sure that I did have to find a banjo bolt with the correct threads to fit in place of the original flare nut; I know for sure that I did not use a banjo to flare nut adapter because I never heard of such a thing until I read this thread.

Good idea. From my 1100 pics, the mounting distance is probably the same, but the m/c are rotated 180deg. See pics, GL1100 on top, followed by CB750. Notice the line to the reservoir exits out the right (NE) of the GL m/c and the left (NW) of the CB,

Re: the later discussion on brake lines, I (regrettably) do try to re-use existing rubber lines when possible. But the CB750's were toast so I bought a full set of Galfer lines.

gl1100_rearMC.jpg

PXL_20250722_120111506.jpg

[Sidecar Bob]

Rubber brake lines should be replaced every 2 or 3 fluid changes (= 5 or 6 years). Modern stainless braided ones last practically forever and double the life of the fluid).

Rubber lines commonly become soft after decades of contact with brake fluid too. If you wrap one hand around a rubber line and squeeze the lever with the other you will feel the tubing expand. This is fluid moved by the master cylinder that doesn't contribute to the movement of the caliper piston(s). In other words: lost braking power.
If you wrap your hand around a braided ss line and apply the brake you will feel the line stiffen, but it will not expand.

Have you ever wondered why brake fluid turns red/brown after a couple of years? it's a combination of oxidization caused by the moisture that finds it's way through the porous rubber and rubber particles suspended in the fluid. I think it is obvious that this reduces the incomprehensibility of the fluid, and thus impairs the function of the brake. Not to mention what can happen if the water boils because the caliper gets hot.

[pidjones]

I've not tried it on a GL, but had success using a Chinabay generic rear master on a CB750F. Just disassemble, clean assemble correctly with fresh brake fluid lubricating the parts. They will be filthy inside when you receive them.