Front master cylinder rebuild -- leak

[RandyRed]

Hi all,

Wondering if someone can clear this up for me. I recently rebuilt my front master cylinder, new brake lines, cleaned calipers etc. However, my master cylinder is leaking by the rubber boot at the lever.

I thought the boot was just to keep the dust/dirt out. Is it supposed to keep the fluid in? Or do I have another more internal problem with my rebuild?

Bike is a 1978 CB 750F.

-Randy

 

[RandyRed]

Here are some pictures of the boot and MC I'm referencing.

I'm curious if that boot actually holds brake fluid in. I doubt it should. I think it's just to keep the air/dirt out. But is it meant to seal the air as well. That should be for inner seal o-ring, right?

Anyone?

Trying to figure out if I have to order another kit and rebuilt it again.

Thanks,
R

 

[carnivorous chicken]

Yes, you're right: That boot isn't supposed to create a seal.

Should be sealed further down -- is the clip all the way in?

 

[MotoMadness]

The o-ring on the piston keeps it from leaking. Rubber boot is ust for dust.

 

[RandyRed]

Yeah, clip is all the way in. I'm guessing the bore wasn't in the best condition. Or I over tested the lever dry after I rebuilt it. It's just a slow leak but will generate a few drips pretty quickly.

Unfortunately, I'm leaning towards buying a new Nissin MC. I lost 40 bux on the rebuild but I don't really want to mess with having front brake troubles.

 

[Islandstar]

I recently rebuilt mine, and didn't have any issues, but I did have a lil issues with the lil black part that sits on the spring. I felt that if that wasn't seated properly there might be a leak, and there is a video on youtube with the rebuild and even that guy mentions the isuues with seating it. So before u buy another kit...

 

[RandyRed]

I recently rebuilt mine, and didn't have any issues, but I did have a lil issues with the lil black part that sits on the spring. I felt that if that wasn't seated properly there might be a leak, and there is a video on youtube with the rebuild and even that guy mentions the isuues with seating it. So before u buy another kit...

Cheers, thanks!

 

[Rich Ard]

If you have a Harbor Freight in the area, you can pick up a box full of o-rings for five bucks, and you'll be using it for years. If not, take it back apart and see if you can find a single o-ring at a hardware store or a bit NAPA or the like.

 

[ILoveThumpers]

Buy the kit with viton rings. They're the most chemical and heat-resistant HF offers.