Front Master Cylinder - Rebuild or Replace?

[tkent02]

Anyone know what piston bore I should be getting for a single caliper brake? Would I need a different bore if I later upgrade to a twin-pot brake (single disk, just twin-pots)?

Sometimes they work, sometimes they don't. Not sure why. On a few bikes like the '79 750 I did with the 1000 engine, dual brakes worked perfectly with the single disc master cylinder. On the 550 I'm working with now, it didn't work very well at all. I don't have any idea why. You got some braking, but it didn't really come on all that hard.

That's OK, my brakes are sweet now!

[Grimly]

Anyone know what piston bore I should be getting for a single caliper brake? Would I need a different bore if I later upgrade to a twin-pot brake (single disk, just twin-pots)?

If you want to be dead accurate about it, you need to work out the hydraulic leverage involved, but generally, a 16mm / 5/8th" will happily do a pair of 1980-on calipers and for the older, '79 smaller calipers, a 14mm or even 13mm will do fine.
However - there's one big proviso - that's not taking the wear and tear of old caliper mounts into account. High-mileage worn mounts will sometimes knock back the pistons a touch if the front rattles on bumps and if you have to do any sudden braking, you find yourself with some lost motion on the lever before anything happens. A situation best avoided.

If you go too small, you get a good feeling lever that runs out of room before you brake enough to avoid hitting something, and if you go too big, you get a wooden lever that you have to heave on to stop any time soon.
Google for brake hydraulic leverage - it'll tell you all you need to know.

[srg]

Thank you Grimly, that's exactly the information I was looking for. I like to be dead accurate about it ;-) I'll do some research on Google.

[Nessism]

For dual caliper bikes Suzuki used a 5/8" master cylinder. I'd look for one of those 16mm jobbies off ebay. If you go with a 14mm master the hand lever will feel spongy compared to larger master, and more hand lever movement will be required. Don't be surprised if the lever comes almost all the way back to the grip during heavy braking.

[Grimly]

This is a fairly good guide... Useful starting point to help avoid combinations which simply don't work too well or could be dangerous.

http://www.vintagebrake.com/mastercylinder.htm

[850 Combat]

I wish that I would have read this earlier. I just spent $204 on a new OEM. The one I'm working on is pretty rough.

[Somamp3]

why does it always say that my 1985 gs550esf is incompatible with the 16mm or 5/8 master cylinders on ebay?

[Triam]

why does it always say that my 1985 gs550esf is incompatible with the 16mm or 5/8 master cylinders on ebay?

They posters have to specify what vehicles it fits.

[Nessism]

why does it always say that my 1985 gs550esf is incompatible with the 16mm or 5/8 master cylinders on ebay?

Don't worry about what they say. Your bike is supposed to have a master that size so just get one. Some have a mirror lug and others don't. Also take note of the angle of the master. Some are angled to account of mounting on bars that angle downward.

[Somamp3]

Thanks for the help guys! I'll put an order this week because I can't tell how much brake fluid is in the reservoir due to a frosted site glass.