Why is the Rear Brake Such a Pain to Bleed?

[DGXSER]

Well Ian, try my method for about ten minutes before you head to HF. That's all it took for me....this time. I will readily admit I have struggled for hours with the rear brake system in the past, even with a Mity-Vac. But I have never before "bench" bled the rear MC in position on the bike. MY best guess is that made all the difference.

And yes, you can even tighten the bleed screw closed and see air bubbles getting into the hose around the bleed nipple with a mity vac pulling vacuum on the hose. So, yes, wrap threads in Teflon. And also the bleed nipple as you shove the hose on.

[Schming]

Another point, if ya go with the vac, make sure the bleeder and vac hose are dry before fitting together and it'll seal better.

All great info above, and whatever way you decide to go will work.

GL

[clcorbin]

If you are willing to unbolt the master cylinder, you can also hook it up to a pressure bleeder. I use a Motive pressure bleeder. The hard part is figuring out how to attach the cap to the small motor cycle master cylinders, but once you do that, the rest is cake.

With a pressure bleeder, you hook up a small tank of clean brake fluid to the master cylinder and then pressurize it a small amount (10 to 12 psi usually). When you crack the bleed screw, pressure from the tank forces the fluid through the master cylinder, into the line, into the brake cylinder and then out the bleed screw and into a catch bottle attached to the bleed screw. There is no pumping, no priming, nothing. Just open the bleed and watch the fluid and air bubbles flow out. When there are no more bubbles and the fluid is flowing clear and clean, close the bleeder and depressurize and remove the pressure bleeder. Done. Just remember to depressurize FIRST.

The really nice thing is that the fluid velocity is much higher with this type of system than vacuum bleeding and is much more constant pump and hold. I can flush and bleed the brakes on any vehicle I own by myself in 30 minutes or less.

The only problem is getting it attached to the small motorcycle master cylinders. I've never used it on the rear, but I have used it on the front after rebuilding that master cylinder and then having a problem getting it primed and bled. After fighting for 20 minutes or so, I came up with a clamp that worked with the generic adapter (they make different adapters for different reservoirs) and made sure the reservoir couldn't come off the metal base. After that, it was done in just a few minutes.

[IanDMacDonald]

Yeah, I've watched some good Youtube videos of pressure-bleeding via pump sprayers, etc. Problem with our MC's is the fact that they are not screw-on caps like automotive or modern bike M/C's. I know HF sells a pressure bleeder, but I don't feel like spending a whole weekend fabricating a cap to seal for pressure, etc., when following Don's method is free and hopefully quick.

[IanDMacDonald]

I went out there and tried Don's method. I seemed to be getting fluid to the bleed valve, but I just could not get a firm pedal. Pulled the M/C off and brought it inside. Not sure if it's my problem or not, but the front seal does not want to go back into the cylinder. Maybe a bad seal? I'll wait for my kit to arrive to compare seals. Maybe the speed bleeders will also aid in my bleeding as well. Onto other projects . . .

[Schming]

Tip: when you go to rebuild the MC use a small dremel buff to clean the oxidation off of the bore.

[IanDMacDonald]

Oh, I already did all that. I just used the old seals, as I thought the kit I had would work.

[clcorbin]

I know HF sells a pressure bleeder, but I don't feel like spending a whole weekend fabricating a cap to seal for pressure, etc., when following Don's method is free and hopefully quick.

One, personally, I wouldn't trust anything from HF for anything important. The Motive bleeder is well built and will last. I've yet to see anything at HF that I would make that statement about...

Two, you don't need to "fabricate" anything. I just used a wood working clamp to hold the general purpose master cylinder adapter (the one designed to fit all those older master cylinders that used a metal cap held on with the metal clip) onto the reservoir. It was basically just a large, thick, rectangular piece of rubber with the hose in the center for the hose. Worked like a champ on the front, but you would have to remove the rear from the frame for room.

[IanDMacDonald]

Interesting. That was my fear. I heard good things about the motive pump.

[fredintoon]

I literally spent all evening on the rear brake, with no success. - - - Any suggestions or advice would be greatly appreciated.

Hi Ian,
I can't add to all the bleeding good tech tips but what I reckon is, it's all in the way you hold your tongue. Or whether or not the hydraulic faeries are mad at you.
2 riding seasons back my son and I got a bargain deal on s/st brake lines for our bikes; his XS750 Spl and my XS11 Spl (Identical brakes, right?)
He got all his lines installed and bled in an hour.
I got my rear brake line installed and bled in 20 minutes then fought with the fronts for 2 whole days before all the bubbles were gone.
It's good to be good but it's better to be lucky.

[Crusty Snippets]

Bleeding

When I did my front brakes on side would not pump up. That was the side I used brake fluid to lube the seal. On the other side I used the fancy grease.
I removed the non working caliper and this time used the grease and that caliper pumped up right away.

unkle Crusty

[Rasputin]

I just keep on using my little windshield washer pump to circulate all the air out of my brake system. Been working out well for me and a 81 MNS linked brake system is fully bled in 5 minutes tops. I fought with brake bleeding for years till I stumbled on this and never looked back. A universal pump will run you $20 max (I think I paid $14) plus a few feet of clear line (to watch the air bubbles come out) and hook it up to any battery (even the bike itself).
Of course all your system needs to be working or no amount of bleeding will help, as in if your M/C is not working correctly

[scalded dog]

had the same problem, finally I quit letting the pedal go all the way up, kept a little pressure on the pedal at all times, finally got it bled

[IanDMacDonald]

Ras- Could you elaborate on your pump idea? Sounds interesting.

[Rasputin]

There is a pic and some chat about it in http://www.xs11.com/forum/showthread.php?t=40099