Question about front brake master cylinder Oring

[kshansen]

This is actually for my 1982 XS650 but as it uses the same master cylinder(3G1-25850-00-00) as some of the XS400's thought I'd run it by the people over here too.

The oring I'm wanting to replace is the one that the plastic reservoir snap into on top of the master cylinder. For some reason the people at Yamaha decided that this was to technical a part to sell as a separate part, the oring and the plastic reservoir.

I have at least two or three of these master cylinders and all could benefit from new orings. They work just fine but due to the age and minor corrosion the orings will seep a drop or two a week. Anyone probably knows that is a bad thing as brake fluid likes to pull moisture out of the air and that causes more corrosion.

So far I have gotten a few replies but they seem to be very contradicting as to the size of these orings. Just in the cross section alone I have been told anywhere from 3.5 mm down to 1.5 mm

 

[BBS360]

Measuring the old o-ring in mine I get about a 3.75mm diameter.
Seems to be 48mm diameter from edge to edge on the whole ring.

I seem to recall my xs360's seal had a square cross-section, rather than a normal circular o-ring.

 

[kshansen]

Measuring the old o-ring in mine I get about a 3.75mm diameter.
Seems to be 48mm diameter from edge to edge on the whole ring.

I seem to recall my xs360's seal had a square cross-section, rather than a normal circular o-ring.

Well went to local hydraulic shop and Joe matched the oring to the reservoir and master cylinder for me and the size we came up with was 39.7 mm ID and a 3.5 mm cross section oring. With just a touch of lube on the oring and the taper of the reservoir a moderate amount of pressure while turning slightly and it snapped together nice and takes a medium amount of torque to turn it once assembled. So the oring seems to have the correct amount of "crush" to seal just fine.

 

[xschris]

A lot of time cleaning the area between the cap and reservoir then soaking the old o-ring will do the trick. I did this on a old one that sat for 30yrs and after two years still no leaks. But if you find a good place to get them post a link as it can help others.

 

[kshansen]

A lot of time cleaning the area between the cap and reservoir then soaking the old o-ring will do the trick. I did this on a old one that sat for 30yrs and after two years still no leaks. But if you find a good place to get them post a link as it can help others.

I had already tried that route and thought the oring looked okay. It took a few days sitting before I noticed the shinny film on the master cylinder. I then cleaned it off real good thinking I may have spilled some while filling it the last time after bleeding. Well two or three days latter there was a shinny film on the master again.

Just a word of caution here. I was just told on another forum that there might be an incompatibility problem with the oring material in the orings I got today. The recommended oring material would be a EPDM not the nitrile or Buna like I believe mine are will keep an eye on it for possible leaking and try locating some in the correct material.

Well if you are near East Syracuse, NY I got mine at Nu-Way Hydralics. $0.46 each plus tax!

I someone wants to stock up on them there is always McMaster-Carr:

$6.87 plus tax and shipping for a pkg of 25, that should take care of half the XS400's with front discs on the forum!

 

[fR3ZNO]

I'm having this same exact problem. The first time I rebuilt my MC, I used a Buna N o ring that I found in a kit that I had laying around. It's started weeping after I rebuilt my MC again a few months ago. I think my issue is the ID of the o ring is too big, even though the reservoir snaps into the MC with a decent amount of force. Still too much of a gap for it to seal I guess. Either that or the o ring gets distorted once it's pushed into the seal pocket.

Thanks for the info. Super helpful. Gotta love forums!

Also, if you don't want to buy a pack of 25 from McMaster Carr, you can buy individual o rings from http://www.theoringstore.com

They have pretty much any size, cross section shape, and material you could ever need.

I've used them before for other projects of mine