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Discussion Starter · #1 ·
I've been having trouble with my front brakes dragging, so I thought I'd disassemble the master cylinder, give it a good cleaning and check for bad parts. I had previously done the same with the calipers, but no improvement. I got everything apart with no trouble, and everything looks pretty good although plenty grimey.

The problem came when I tried to reassemble. There doesn't seem to be anything to keep the piston in place in the cylinder. Is there supposed to be some sort of a keeper in the piston groove? I was very careful taking things apart and I don't think I dropped anything, but something seems to be missing. The pictures show the pieces in the order they came out. Let me know if I've lost something or if I'm just not putting something in right.
 

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I've been having trouble with my front brakes dragging, so I thought I'd disassemble the master cylinder, give it a good cleaning and check for bad parts. I had previously done the same with the calipers, but no improvement. I got everything apart with no trouble, and everything looks pretty good although plenty grimey.
The C-clip should be all that keeps it together, it doesn't look you are missing anything. Also... Let me try to address the brake drag thing. my experience:

My bike had bad front drag. Maybe one turn before stopping.

I kept completely disassembling the front and putting it together to make sure everything was aligned. I cleaned the MC and made sure the brake pistons were clean too. I finally threw it together with everything right and fresh pads. STILL tons of drag.

I decided to ride it. I got the brakes and the bike hot and when I stopped the bike it was SMOOTH as butter! So check the drag after a short ride (only if you know everything is aligned).
 

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Discussion Starter · #3 ·
Thanks for the response. I was starting to think I'd either stumped the experts or had just asked a dumb question.

I'm still not convinced it's correct, but I went ahead and reassembled the MC. The piston diameter is the same end to end as the inner diameter of the white nylon bushing and o-ring it slides through. The C-clip holds the bushing and o-ring in place, but doesn't do anything to stop the piston. The only thing to keep the spring from shooting the piston out is the lever (currently). I'm sure there was something to hold it in before because I've removed the lever more than once and the MC seemed to be a self contained unit. At any rate, it's back together and holding fluid. I just need to finish bleeding, and then we'll see how it works.

I didn't mention it earlier but I had identified the reason my brakes dragged. There are two holes in the bottom of the MC reservoir. One allows fluid into the cylinder and the second smaller hole lets pressure from thermal expansion bleed back into the reservoir. If I popped a caliper bleeder before a ride, I could go about 10 miles before my brakes started to act up. As it turns out, the second hole was plugged solid. I used a micro drill bit to open it back up, so hopefully problem solved.
 

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Usually the brakes dragging is caused by incorrect adjustment, either freeplay on the rear lever, or the little adjuster screw on the front lever. I hadn't heard of the second hole becoming plugged, good information to know!

As to the lack of something holding your MC together; change your O-ring. Unfortunately, easier said than done. Can't buy rebuild kits for most MC's anymore. But you can buy o rings...

The C clip holds the white bushing in place. The O ring is sized such that it shouldn't fit thru the bushing. If the O ring becomes worn, it will slip thru, and damage the rubber boot in the process.

I have yet to determine the "exact" correct O ring size. My Paso (which I have a set of Goldline MC's on) I used a size 111 EPDM O ring. It's a touch too big. It works, but it's damned tight to get assembled. The next size smaller should be perfect. Unsure if this O ring is the same on yours. I've got 50 of them, let me know if you want to try some!

Definitely use EPDM; most other types are not compatible with brake fluid. I tried Buna-N, and Viton, before researching what compositions are compatible. The other compounds swell and eventually make the piston hard to move.

EPDM is the only one that is recommended for brake fluid.

Michael

EDIT: I see you don't show the boot in your photos? Usually there's a rubber boot, held in place by the circlip, that the little Tylenol-shaped pill sticks thru. Keeps the piston clean. Is yours missing?

I'm referring to the rubber piece next to the circlip and pliers in this photo:



Edit2: Oh, and that little white bushing in my photo is in the wrong direction in this photo, should have the narrow end pointing into the bore.
 

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Discussion Starter · #5 ·
Thanks for the input delagem. In your edit #2, you said to install the bushing with the lip toward the MC rather than toward the lever as your picture shows. I have mine installed with the lip toward the MC and it's leaking (wasn't leaking before I messed with it). With it installed as you have noted, the o-ring rides on the outer diameter of the bushing lip and the only seal against the piston is the bushing. I just wanted to confirm that this is correct. I was pretty sure that's how it came out but as I mentioned, I have a leak now and was hoping I had installed it backward.

My boot has a small rip. I think it was like that before, but I might have done it during disassembly. I don't suppose you can buy those separately? I may be in the market for a rebuild kit after all. Was hoping to avoid that.
 

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Thanks for the input delagem. In your edit #2, you said to install the bushing with the lip toward the MC rather than toward the lever as your picture shows. I have mine installed with the lip toward the MC and it's leaking (wasn't leaking before I messed with it). With it installed as you have noted, the o-ring rides on the outer diameter of the bushing lip and the only seal against the piston is the bushing. I just wanted to confirm that this is correct. I was pretty sure that's how it came out but as I mentioned, I have a leak now and was hoping I had installed it backward.

My boot has a small rip. I think it was like that before, but I might have done it during disassembly. I don't suppose you can buy those separately? I may be in the market for a rebuild kit after all. Was hoping to avoid that.
Fit just the bushing into the MC; try it both ways. You will probably see that there is a step in the bore, and that the white bushing only fits in properly one way. At least that's how it is with my MC's.

There really shouldn't be any brake fluid outboard of the O ring. At rest, the oil port should be open to the chamber between the piston seal and where the banjo bolt threads in for the hoses.

As you apply the brake, the piston seal passes the oil port, and from that point forward, all oil should be forced down the hoses to the calipers.

The O ring stops any oil coming out of the oil port from leaking out of the MC. The O ring should never move past the oil port. If it does, oil will indeed drip out the back side of the MC.

If your O ring is passing the oil port, you should have a screw on the lever to adjust the actuation point; try backing that out.

The other possibility is the O ring isn't passing the oil port, it's just not a good fit and is leaking.

I haven't found the exact rubber boot for sale anywhere. Mine has a tear in it too. It may be the same part as for the older Brembo rebuild kits, some of which are still available at places like Bevel Heaven. I haven't looked into it too much, as it isn't really for anything but keeping the back end of the piston clean, it's not a seal or anything.

I struggled with MC's for the past few months, due to Brembo not selling rebuild kits for the Goldline MC's, and my stubbornness to not throw out a good part over the lack of an O ring! That's where I discovered that most rubber compounds react with brake fluid, and swell, which makes your MC sticky.

Please note, despite owning a 900ss for over 21 years, I have little experience with their MC's. So I can't be of much help on the specifics of your exact MC. I broke the lever boss on my first crash in 1992, and junked the OEM MC's. Currently running Nissins on the ss, but run Goldline's on everything else. Hate the Nissin's, too... They're going in the trash this spring, and I'm going to pick up another set of the Goldlines.
 

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Discussion Starter · #7 ·
It's taken me a while to get back to the SS. I've had the rear end (suspension and all) torn out of my wife's Jag replacing bearings and seals, rebuilding calipers, etc., and it's taken all my time over the last few weeks. But it's finally done and back on the road (whew!!) so I can spend some time with the bikes again.

I think I have solved the leaky MC. When I assembled the MC the first time, I put the o-ring onto the white bushing over the outer diameter of the little lip. This keeps the o-ring from contacting the piston. Last night I took it apart again and reassembled, only this time I installed the o-ring first and then slid the bushing on. So far so good. It hasn't leaked over night, but I haven't had a chance to give it a road test. It's been in the 20's here for a couple of weeks and it's supposed to snow today and tomorrow. It may be spring before I get to take it on a shakedown cruise.
 
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