I'm bleeding

[XSokieSPECIAL]

Big John......

Will double check it all. When I was in on the puter I had left my syphoning rig on with the cannister off the vac. Welllllll, it syphoned it all out thru the system on the drive, DUH! anyway, that may be the easy way to bleed um'. Didn't solve the problem though.

I was watching when they were doing Dales rear brakes at MoM. He was having a VERY similar problem.

There are some holes in the piston behind the end cup that pull fluid from the M/C reservoir when the pedal is released. These were plugged on Dales causing it to create a vacuum situation in the line to the rear caliper.

JAT since you said you hadn't done anything with the rear M/C yet.

[Ivan]

If you are confident your cup seals have not inverted, then I am going to coincide with the other two posts. Either the piston isn't fully returning to uncover the relief port (spooge hole) or the piston isn't letting fluid through. They are set up to be somewhat of a check valve type pump. If the check isn't unseating at the proper time, it will just push fluid back and forth instead of moving it down the path. Clear as mud?

[jmnjrpa]

Took

the S.O.B. MC off and tore it apart. Front cup seal somehow has a chunk out of the lip. Figger that almost has to be the issue. Won't know for sure till my kit arives in a couple of days so I can change it out. Thanks to all.

[CatatonicBug]

the S.O.B. MC off and tore it apart. Front cup seal somehow has a chunk out of the lip. Figger that almost has to be the issue. Won't know for sure till my kit arives in a couple of days so I can change it out. Thanks to all.

What MC rebuild kit did you get?? The rebuild kit for the LH's rear MC does not exist. That's why I changed to an XJ rear MC.

[jmnjrpa]

From

Bike Bandit Item: 2675734
Ordered it 2 weeks ago - they shipped yesterday from Cali.
It better fit as it plainly states 1981 LH rear master cylinder kit.

[jmnjrpa]

Say Bug,

Do you still have the original cause mine's not in the best of shape?

[CatatonicBug]

Do you still have the original cause mine's not in the best of shape?

No, the wrecker I bought the XJ MC from wanted it in exchange.

[CatatonicBug]

Bike Bandit Item: 2675734
Ordered it 2 weeks ago - they shipped yesterday from Cali.
It better fit as it plainly states 1981 LH rear master cylinder kit.

That's strange... When I use their schematic to look at all parts available for the LH, that item shoes up as #15 on the "Brake" schematic, but it's not listed as an available part.

Do you have a link to where you found it?

[jmnjrpa]

Yeah

It's not listed as OEM

http://www.bikebandit.com/k-l-master...=8243&t=1&td=1

[jmnjrpa]

C-bug

I also located the kit here:

http://www.oldbikebarn.com/1981-Yama...dnight-Special

[draketrumpet]

IIRC from a week ago when I was working on mine, the old bike barn kit listed for the '81 MNS is exactly what it looked like.


I might be wrong, but if I'm not, then the rebuild kits are either the same, or this will not actually work for your MNS.

The Bike Bandit one (K&L) does look quite different from mine/oldbikebarn:


So, someone with a MNS may want to shoot oldbikebarn a note if it is actually not the right part.

[CatatonicBug]

I also located the kit here:

http://www.oldbikebarn.com/1981-Yama...dnight-Special

The problem with that one is that the LG and the LH use different Master Cylinders. The LG does not have linked brakes, and has a smaller bore. No other model uses the same size rear MC as the LH.

[CatatonicBug]

IIRC from a week ago when I was working on mine, the old bike barn kit listed for the '81 MNS is exactly what it looked like.

I know you can't always trust the pictures, since vendors often use a stock picture for several different items. That looks more like the font MC kit. Notice that the cup on the rubber seal has a hole in the middle? On the LH, that cup is solid, with no hole. The piston is flat on the end, and does not have a post that goes through the center of the cup.

[psycoreefer]

IIRC from a week ago when I was working on mine, the old bike barn kit listed for the '81 MNS is exactly what it looked like.

I might be wrong, but if I'm not, then the rebuild kits are either the same, or this will not actually work for your MNS.

The Bike Bandit one (K&L) does look quite different from mine/oldbikebarn:

So, someone with a MNS may want to shoot oldbikebarn a note if it is actually not the right part.

Just for those who may not know, please don't go by the "pictures" on websites, most will say that they are not pictures of the actual part, I know bikebandit doesn't show pictures of the part, just something similar. Most places use the same picture for all "brake rebuild kits", can make it hard sometimes.

[jmnjrpa]

Pics

The one on the left is an MNS. Right is an earlier rear piston. The 81 MNS uses a 11/16 bore in the rear and a 5/8(I believe) on the front. From the outside the earlier Master Cylinder looked the same.