Topic: brake fluid leaking from master cylinder reservoir

[rob]

Hi everybody,

I have always had trouble with the master cylinder reservoir: taking it off, and putting it back on.  It seems like quite a bit of pressure is required to remove it and put it back on.  After having taken it off to clean out that really little hole that sucks brake fluid into the chamber, I put the reservoir back on, but now it is slowly leaking from around the edges.  I have the two screws that hold it on really tight.

Is there a trick to getting that seal to set properly?  Or to easily get the reservoir to set easily into that hole?

Thanks,

Rob

[Bob Wessner]

Specifically what edges are leaking? Do you mean around the cap edge?

[bryanj]

what bike. Most of the sohc had screw on caps but you mention two screws

[hymodyne]

I put a master cylinder rebuild kit in a while ago and was puzzled by the fact that it still leaked when I was finished. It took a dentist -type -pic to clean out the groove where the circlip seats. third try was a charm

hym

[rob]

Hey eveyrone, sorry about that, I should have been more clear.

This is an 81 CB 650 Custom.  The reservoir has a cap that is held on with two screws, and has a rubber gasgket.  This part is fine.  The leak is occuring at the bottom of the reservoir, where the reservoir meets the metal brake lever.  The part where the reservoir joins with the brake lever is a circle coming from it, and it pressure fits into the a hole in the brake lever, and has a rubber O-Ring.  The leak must be occuring from around this O-Ring, as the fluid is escaping through the bottom, between the brake lever housing and the reservoir.

I think the problem is that it took a lot of pressure to get the reservoir into the hole the hole in the brake lever housing.

Rob

[Bob Wessner]

Rob,

If the inards are anything like the 750's, what you see at the lever end is really a dust boot, won't seal fluid. I think the leak is further into the assembly. Has this M/C been rebuilt recently? If so, a seal may be mounted incorrectly. If it hasn't been rebuilt recently, it may need to be.

[rob]

Hi Bob,

The leak isn't coming out of the master cylinder, or the dust boot, but a rebuild is probably in order, just need to get some circlip pliers.  The leak is coming from where the plastic reservoir that stores the brake fluid sits on top of the brake lever.

Rob

[Robert]

If I got you right, the leak is here:
(It's the 4-screw-version mbc, but should look similar)



I know it's a pita, but I fear you have to take off the reservoir again. There's an o-ring that is propably not seating correctly in its groove.
Also check the o-ring for damage and make sure the groove is clean.
Prelubing the o-ring with brake fluid helps to get it all together again.
I use a bench vise (sp?) to build up the pressure in combination with some logs of wood to distribute the pressure evenly to the reservoir.

[rob]

Yep.  Where did you find that image?  Or did you scan it in?

Rob

[Robert]

That's a scan. But there are numerous online part sellers where you can look up diagrams for all kind of bikes. e.g powersportspro.