Partsnmore Master Cylinder

[TopCatGr58]

Hey SD,

Several years ago, I replaced my front MC with a Yamaha unit, got it from a parts shop, didn't know as much about them as now. It apparently was off of a single disc bike, cause even though it will work, I have to essentially pull the lever almost all the way to the grip before it developes enough volume pushed thru the lines to actuate the calipers to enough of an effect for adequate braking!

So...your XJ MC will probably work, but you'll find you'll probably have to squeeze it a whole lot farther than you would one of the larger capacity MC's for the same amount of braking action!
T.C.

[lsippell]

I wonder if it's possible to bore the XJ ft. m/c larger and put a different models 'kit' in it?

Louis

[T748]

XJ master cylinder?

I have an 82 XJ11 that has the front brake master modified to both fronts. This was done by a previous owner. I don't know if this is the original front mastercylinder or not. If it is, it works fine going to both fronts. I get good handle feel and there is still plenty of travel left. If it's not the original, whats it off of? I didnt see a part number on it. The rear mastercylinder is NOT an XJ part as far as i can tell. XS maybe? Here are a few pic's- Tom.

[tommystinson]

The master cylinder issue is pretty straightforward. (I'm running the 16mm universal unit on my Special, but with different bars) The ratio of m/c piston area to caliper piston area is what matters. Double the area for for dual calipers to do the math. Using piston dimensions off the special: 14.3 to 1 I.E. It takes 14.3 mm of m/c piston travel to effect 1 mm of caliper piston travel (assuming 2 pistons). The smaller unit figures to be about 25 to 1. The m/c's often are embossed with 1/2 or similar. That isn't because they're SAE, it's because they are suitable for one OR two discs/calipers. Using a 1/2 m/c with only one front disc isn't unsafe, it simply means much less effort will be needed to generate the braking pressure to stop the bike. Whether the brakes will be as easy to control for the rider is a question of "style" as much as anything. As far as the reservoir position goes, as long as the suction/bleed holes are immersed in fluid at all times, you're all set. The angle of the reservoir is for convenience in checking the level and filling. If you've got a special, get new bars.

[superdave]

respond to T748

Take the master cyclinder off the bar, check out the numbers just below the sight glass. There is the manufacture and size number. I noticed in your photo, your not using braided lines But oem rubbers. Is that true all the way past the splitter to the calipers ? Too get the bike to stop is it a two finger effort now! ?? Thats what i'm striving for !

[T748]

superdave- I checked the numbers and heres what it says.
14, a circle with a line through it, 6, T, 1.
The brake hoses are original style rubbers all the way down. The brake effort is a two finger affair. I've never locked the fronts up, but I have made a few "panic" stops and they have worked great. Not as good as the Bandit I just got, but plenty of stop as far as I'm concerned. Lever travel for a quick hard stop is about 50%. It does have new pads in the front and the rear brake will lock up easy. I am planning to go the SS brake line route over the winter along with new fork seals and the infamous 2nd gear fix. -Tom.

[Hobbit]

mc

Tommystinson wrote that the 1/2 embossed on the master cylinder meant one or two calipers. Then why do my master cylinders have 5/8 on the strange one that the PO had put on , and the original has 11/16?
I replaced the 5/8 with the 11/16 and the brake feel is MUCH better. I actually have clearence between the lever and the throttle when fully applied.

[DiverRay]

The 14 with a circle, 1/2, 5/8, etc. are all the bore size. The single brake is a 14mm, and dual is 16mm. The 1/2, 5/8 are all inch measurements.
The larger bore size will push more fluid, and make it easier for dual brakes. A large bore in a single brake should make it easier to control, as the effort required will be higher.
This is all basic hydraulics from Arizona Auto Institute, back in '72.
Ray

[superdave]

t 748

That 14 is the cup diameter in mm. The rest is manufacturing jargin. The same master cyclinder that works duel calipers on the fz and fj's of the mid 80's. The 14mm m/c (for street use) for two calipers should be ample stopping power. The stainless lines will give a more linear feel. If you want after market/more stopping power than go for it! It's only money. I'am a cheep bassturd and going stock with the ss lines. This is my 2 cents worth! If I don't like it I can put it back the way it was !

[Shuriken]

So the "PNM 16mm dual disc MC" will work on the 79sf or not?

Ben

[TopCatGr58]

Hey Ben,
What Tommystinson said:

(I'm running the 16mm universal unit on my Special, but with different bars) As far as the reservoir position goes, as long as the suction/bleed holes are immersed in fluid at all times, you're all set. The angle of the reservoir is for convenience in checking the level and filling. If you've got a special, get new bars.

Ben, you can see the photos of the PNM MC, it's nice and deep, so once it's positioned, and you then rotate the bars far left to put the MC level, fill with fluid and then seal it, in normal operating position, air shouldn't be able to get to the piston/suction bleed holes. I've got a smaller MC, have Goldwing bars, so they don't bend down as much as the OEM Buckhorns, but they still bend down, it doesn't sit level, but still works fine!
T.C.