XS2 master cylinder parts interchange and rebuild

My 72 master cylinder was full of what looked like powdered Parmesan cheese when I got it. I have the MC off the bike right now. The plunger will go in and out freely, but it will not push any fluid. So I just pulled the plunger out to see what I could see. While there's nothing torn, it's a bit crusty and rusty in there. So I figure a rebuild is in order. I have everything soaking in Evapo-Rust to clean it up. It's not really that bad though.

So I have another MC off of another bike. Unknown year. It's reservoir is a lot taller than mine. So I'm guessing it's from a dual disc set up. The caliper and hose do however look identical to my 72.

So on my 72, on the bottom, it is marked 5/8. On the unknown, it is marked 11/16. Is this the bore diameter?? I guess I could pull the unknown apart and see if it fit's, but I'm kinda guessing it won't. I just hate to tear it apart since it seems like a good unit and is part of a fully connected system with fluid in it.

So let's say the plunger will fit. Would you guy's trust it to a clean up and reinstall. Or should I maybe go for a mikes,,or even nos rebuild kit. I guess I could try to just clean up my original too. Please advise. Thanks

Those markings are I believe the bore sizes. So don't pull the spare apart. A mikes kit or a new master C. I don't think there is much difference in price. I would not use NOS for brake rubber parts.

Just an annoying question; how do you not know for sure if it is a single or double caliper set up if it is still in a connected system????

gggGary said:

Those markings are I believe the bore sizes. So don't pull the spare apart. A mikes kit or a new master C. I don't think there is much difference in price. I would not use NOS for brake rubber parts.

Just an annoying question; how do you not know for sure if it is a single or double caliper set up if it is still in a connected system????

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I'm just guessing it's from a dual set up because it's a taller reservoir. I don't know for sure. I think someone swapped it out. I got it from a guy who got it from a guy,,,,lol.

When did they start putting writing on the cap's. Of course I guess that could be the wrong cap too.

So I tore apart the 72 mc. I'm sure it would be best to replace it because there is some pitting in the bore. How ever. That's not really on my short list. I need way to many other item's just to get this thing on the road. I will how ever put it on the long list.

So the way I see it, I have 2 choice's.

1. The 72 mc cleaned up pretty good,,,other than the minor pitting. Do you guy's think I could put it back together and run it. What would be the worst thing that could happen. Could I have the mc blow out and crash. Or would I just maybe have not so great brake's. Would it be ok to try as long as the thing doesn't leak.

2. I could try and see if the other mc will work. I do believe it is from a dual disc set up. Would that be too much pressure, and send me over the bar's.

So other than getting a new mc,,,wich I will in the future if one of these doesn't work out,,, wich of the 2 evil's would I be better off with.

Improperly matched master cylinder bore to caliper ratio will result in poor braking performance. An improperly functioning unit; well that becomes a safety issue. I purchased a brand new unit from mikes for only 55 bucks. Much cheaper than rebuild kit, diaphragm, and cap. There are other ways to save money, I just don't think skimping on good brakes is one of 'em.

turn left said:

Improperly matched master cylinder bore to caliper ratio will result in poor braking performance. An improperly functioning unit; well that becomes a safety issue. I purchased a brand new unit from mikes for only 55 bucks. Much cheaper than rebuild kit, diaphragm, and cap. There are other ways to save money, I just don't think skimping on good brakes is one of 'em.

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Is that the exact unit you purchased. Will it work with my 72 early style brake switch. I don't suppose you'd just happen to have a pic of the bottom of it.

CalsXS2 said:

Is that the exact unit you purchased. Will it work with my 72 early style brake switch. I don't suppose you'd just happen to have a pic of the bottom of it.

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mikes p/n 08-6009
5/8" bore (16mm)
no provisions for a brake switch but this will work

EDIT (previous post): regarding your existing front brake lamp switch, it may work.. I don't have the same switch to fit it to the master cyl for verification

Hey, thanks for the pics.

Did it come with that brake lever. The pic on the mikes shows it,,so I just thought it might. That lever will not work with my switch. It needs to have a pin sticking up on it. Also, the master needs to have a flat spot, and a hole in it to accept a pin that's on the switch.

I couldn't tell by your pic if it had the flat spot and hole,,,it was dark in that area. Can you post a pic that will show it more clearly. I just happen to have a friend who is a machinist that might be willing to hook me up,,,I hope.

What sucks is I just bought a NOS switch with the proper gray wire loom. So that will be wasted, and I will have to buy another switch. And what ever else I need to hook it up. That right there makes it double of what it would cost to rebuild mine.

On the plus side, mikes mc would give me a place to mount a mirror. I just hope it has the hole,,,or my friend can do something with it.





Here's the proper switch from mikes.

Yes it comes with the lever. No it does not have the pin for 70-73 brake switch. A banjo bolt switch is $11. Yes it has threads for a mirror. You should also consider new brake lines; the originals go bad from the inside out. Bottom line is you're gonna have to spend some cash to maintain the bike. I'm not one to immediately jump to replace over repair but in this case the price difference is relatively minimal.

turn left said:

Yes it comes with the lever. No it does not have the pin for 70-73 brake switch. A banjo bolt switch is $11. Yes it has threads for a mirror. You should also consider new brake lines; the originals go bad from the inside out. Bottom line is you're gonna have to spend some cash to maintain the bike. I'm not one to immediately jump to replace over repair but in this case the price difference is relatively minimal.

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I was all ready planning on new lines.

Does it have the small hole show by the red arrow on the left.

@post#11 no small hole for the 70-73 style switch. I suppose if you insist on using that switch (since you already purchased a new one) the master cyl & lever could be modified to accept it. just don't pay your buddy more than $11 b/c you could just get the banjo switch at that point

Just to through in my .02$ worth.The markings on the M/C's is the bore size. The early model years up to 76 used a 16 mm bore with the single two piston caliper. With a dual set up they used a 22 mm bore M/C.
A 14 mm bore M/C would improve your brakes. More power, better feel. One off a 77 or 78 will be ideal. Flat reservoir. The Specials had a sloped reservoir to match the high handle bars.
The pitting in your 72's bore will leak, enough to weaken the brakes, I wouldn't trust it.
On the unknown M/C has an 11/16 inch or 18 mm bore. That would be ok on a dual set up with two of the early two piston calipers. Not so good on your single set up. The lever will pull harder with worse braking and less feel. You will pull the lever and not much happens till the wheel suddenly locks, You won't feel this till it happens.
On the brake switch. Some of the later M/C's have the switch as part of the M/C. If the one you choose don't, Mike's brake switch banjo bolt item # 29-7002, is a good option. You just replace any banjo bolt in the brake system and hook the wires into where your stock switch plugs in. I have one in my rear brakes and it works great.
The braided line is a great upgrade. Most manufacturers reccomend changing out the rubber brake lines, like every 3 or 6 years. I can't recall just how often. Your stock lines have a plastic ring around them. It has the manufactured date on it. If yours are more than 10 years old they should be changed.
In a lot of ways I agree with turnleft about not over fixing things. Brakes are one exception. When it comes to brakes, how much is your life worth?
Leo

Thanks guy's.

Well about that banjo bolt switch. I wasn't clear on how it connected. I thought you had to put a tee in the line somehow. I thought it was going to look like a 3 Stooges plumbing job. I was set against it. But now I understand that you just replace the banjo bolt. OK then.

I guess the best thing to do would be to see if my friend can make the new mc work with my 72 switch. If not, then I'll go with the banjo bolt switch. I could always try to sell the expensive gray wire loomed switch I bought.

Like I said before, I will also have a place for a mirror now. Which means I can get a threaded perch for the clutch side and have matching mirror's. This will the allow me to sell my original clamp on mirror. Heck, that would fund the whole project. This might turn into a money making deal.

Thanks again.