No Rear Brakes after Rebuild

[toomanytoys]

This is a pretty interesting issue that I can't seem to figure out. Before I bite the bullet and buy a whole new master cyl or get a new rebuild kit I figured I'd inquire here.

While rebuilding my 78 XS1100E, I rebuilt the rear caliper and master cyl (along with just about everything else on the bike). The rear brake worked for maybe 1500 miles, although it did always have a pretty long throw before it would start to engage. Suddenly one day it stopped working, and I noticed that every time you pushed the lever, fluid would shoot out of the plunger hole.

At this point I figured one of the new plunger seals (probably the rear one) must have blown out somehow. I took it apart and everything looked normal, no crud down in the cyl hole, no rips or tears that I could see, spooge hole was clear...everything looked just as it should. I put it all back together, back on the bike and now it just seems to pump air. It's not pissing fluid out of the plunger hole any longer, but I can hear it squish squish every time I push the pedal and while I'm bleeding it there's just contantly air bubbles being pushed out. If I shut the bleeder and pump up the brakes I can get a tiny big of pressure, but I can literally hear the squishing noise of air being sucked into the master and of course more air bubbles as I continue bleeding. As far as I can tell...there's only one way air can get into this rig, and that's by going around the plunger where the brake pedal lever pushes on it. There are two things I noticed that did seem like maybe they weren't correct, but I didn't know if they could cause such dramatic issues:

1) The 14mm bolt that covers up the spooge hole doesn't have any type of copper gasket or anything on it, does it need one? Could I be sucking air in from there?

2) In most rebuild kits I notice there's a small thin washer that's supposed to go behind the first plunger. I don't have one of those little washers, but I have a hard time believing that little washer can make such a huge difference.

Before I drop another $30 on another rebuild kit, does anyone have any other recommendations for me? Is it possible to have the front and rear plungers mixed up (they look the same to me)? I'm worried that the next rebuild kit will just yield the same results. I could try to source a used master cyl off ebay, but these things are pretty simple so I have a hard time believing there's anything wrong with the one I have.

[DiverRay]

The rear on the '78 has a plastic cover/res. on top, and the 14mm is the HP line to the caliper. The 14mm is a banjo bolt, in that it has a hole in the center, and one near the head the fluid goes through. If this is the bolt you are talking about, the yes, it MUST have TWO copper washers, on on each side.
The next thing is once you rebuild the MC, you need to "bench bleed" it. This is just put it in a vise so it's upright, add a little clean brake fluid, and while holding your finger over the hole the banjo bolt goes into, push in on the plunger about 3/8" and let go. When you push, it should drive air past your finger, and then you keep it sealed when you release it. Do this until brake fluid starts to squirt out, then re-install everything back on the bike.
Some fluid WILL shoot out of the res. when you work the brake, this is normal.

[toomanytoys]

The 14mm bolt I was referring too that doesn't have a copper washer is actually the 14mm bolt under the reservoir that gives you access to the spooge hole. This is not high pressure, but I was thinking maybe it's possible to suck air past this bolt.

The high pressure line to the caliper does have copper gaskets on both sides of the banjo bolt.

I did bench bleed it as well, but it never pumps 100% fluid...it will just pump a combination of air and brake fluid past my finger for all of eternity. I'm going to try giving it one last really thorough cleaning and then swapping the rubber plungers before I give up on it. If anyone has any spare rear master cyl they would be willing to sell me I will probably be in the market here shortly.

[toomanytoys]

Ok, took it back apart, and I was able to confirm that the two rubber gaskets are actually different sizes. Currently, I have the smaller of the two at the very front of the plunger facing the high pressure line.

Can anyone confirm this is correct? The pictures I've been able to find online are mostly of the front plunger, and most aren't very high quality so it's hard to tell for sure. I also noticed that there is actually a very very small nick/tear in the smaller gasket. It's very small, but it's on the outer edge and my guess is that it probably doesn't take much for these things to act up. I ordered a new kit, but I still would like to confirm the correct position of the two gaskets.

[DiverRay]

OK, you are looking at the bleed screw, and yes, it needs a copper washer as well.,
Looking at my manual, section 5-10, the photo of the rear MC shows the Yamaha kit containing a new spring, two seals, the end or top one has the lip on the end near the spring, the second has a lip just back from the end, and it's about the middle of the piston. The back of the piston has a packman shaped washer and then the "C" clip. If you go to an online site for parts, they usually have a diagram like the book that should help in assembly.
The small tear WILL cause problems, so replacing seals is a good thing. When your done, you can bench bleed, and then use the bleed screw to do the final bleed of the master. I find tapping on the hose/pipe between the master and caliper helps move the air bubbles from the middle to each end.
Hope this helps a bit. I may look for my spare MC later this weekend so I can take a photo or two for you. (IF I have time!!)

[toomanytoys]

SUCCESS!!! Thank you Diver. The new kit arrived around lunch and I was able to get it back together and get it back on the bike. I could immediately tell it was better, but man is it a pain to get these damn things bled right. I spent probably 20 minutes and a full big bottle of fluid bleeding the brakes and trying different methods before I finally got an acceptable amount of pressure in the pedal. It's way better than it ever was before. I think the trick with these things is really just getting them bled right, it's obvious now that I did not have the rear master sufficiently bled last go around.