Rear master cylinder priming issue.

[Dakota_Lew]

Do i have a faulty rear master cylinder or do i have a massive air bubble somewhere i need to go after?

First off this bike is a barn find and the master cylinder in the rear was smoked beyond rebuilding so i just bought a new one off of mikes (first new one i ever bought). I also put new lines on it but rebuilt the calipers from the ground up.
I installed everything, and then proceeded to use this post on brake bleeding (http://www.xs650.com/forum/showthread.php?t=11205). I figured out in a hurry, that with everything installed i couldn't get the master cylinder to prime or at least i thought. So i took the master cylinder out of the bike and the reservoir and held it up to get gravity to help me, while squeezing the plunger. After what felt like forever i got fluid to come out of the caliper but it was super super ridiculously slow.

I reinstalled everything and used the actual lever this time since my thumbs were about dead. I have probably cranked that pedal down 200+ times and still cant get much more then a milliliter of fluid out of the caliper... every other stroke. I can post a video tomorrow if needed to show the amount of fluid coming out(need a better internet). I am just stumped since I have got fluid coming out of the caliper, just no where needed at what i have seen before when i flushed new fluid into past XS 650's.

Oh also I have never rebuilt a master cylinders or had a new one out of the box for these bikes so maybe i needed to prime it in some way before install?

I think I either got a faulty master cylinder out of the box or I got a nasty air bubble somewhere in the cylinder it self or i have missed the boat (not the first time haha). Thoughts?

Thanks for your time!

 

[Dakota_Lew]

Here is a video of what I am getting out of the caliper when I pump the master cylinder.

http://s740.photobucket.com/user/lkuppler/media/VID_20140319_173020267.mp4.html

I tried to embed it but it came across so small on the forum that I just gave a direct link instead.

 

[timbeck]

Not sure but you may have to place the caliper below the level of the MC. Use a piece of wood or something between the brake pads so the caliper cly does not pop out. tim

 

[Dakota_Lew]

Opps, I tried that already, took the master cylinder out of the bike and held it up and pumped it with my thumbs. Maybe I need to try the opposite to get the air bubbles to flow to the caliper it self to get them out.

 

[5twins]

Back flushing or bleeding is a method I resort to on a difficult one. Sometimes it does the trick. If you don't have one, you may want to get one of those vacuum bleeder pumps. They work well to initially get the fluid through a new, dry system. After that, I usually resort back to the old tried and true "pump the lever/pedal" method.

 

[Dakota_Lew]

Yea, I added quick bleeders to it, i will rob one off another XS that i know has OEM to back bleed on. I have an oiler can with the correct brake fluid in it already that i can jimmy rig to do that. Worth a shot.

Just to confirm I can push fluid backwards threw the master cylinder when its at the zero position?

 

[5twins]

By "zero" position if you mean "at rest", not being applied, then yes. Watch the reservoir because it's going to fill more. You don't want it to spill fluid all over. Many times, a pesky air bubble will be stuck at the 90° junction of the MC and brake line. This will get that one out many times. It's easier and closer to push it back into the reservoir than getting it all the way down the line and out the bleed nipple. Tapping the area with the plastic handle of a screwdriver while you're pumping the fluid through can also aid in moving the air bubble along. Watch the fluid in the reservoir to see if any air bubbles are coming up and out.

 

[Dakota_Lew]

Correct at rest, thanks, yea I will pick up a small turkey baster then, I will try that and report back. Thanks for the advice.

 

[XSLeo]

Often on a new M/C you need to bleed the M/C before you put it on. With it off the bike I like to clamp it in my vise. This lets me use a screwdriver to push the plunger.
With the reservoir held above the M/C so there is a straight shot to the M/C, brake line off, put your finger tip over the hole where the brake line hooks.
Pump the M/C, As you push the plunger in fluid pushes out past your finger. As you release the plunger your finger seals the hole and it has to draw more fluid out of the reservoir. It should only take a few pumps to get good flow out past your finger.
I even had the lines and caliper of the bike. I then hooked the line to the M/C. I then bleed just the line. Once good flow out the line I hooked it to the caliper. I then bled the caliper. Once I got it all bled I reinstalled the whole thing with out taking anything apart. Once installed I bled it again on the bike just to be sure.
Leo

 

[retiredgentleman]

A very simple method is to pre-fill the brake line while you have it on the bench. Never start with a line full of air.

I hold the empty line in a "U" shape, and fill from one end using a syringe. When the fluid comes out the other end the line is full. Next pre-fill the caliper. Connect the full line to the full caliper, and to the empty master cylinder. Finally fill the M/C reservoir and bleed as required.

I had my front caliper, and brake line off the bike last week. I was also installing a new Brembo M/C. Following the above procedure, it was just a few minutes to bleed and get a hard M/C lever.

 

[Dakota_Lew]

Well it looks like I had other issues causing all this havoc. I am 90% sure I got a new master cylinder DOA. Below is a video of what I am seeing now, but first I tried leo's approach, worked for about 20 seconds then reverted back its original issue.

I then tried to back fill it like 5 twins, worked great but now its sucking air from where the rubber on the pump meets the metal, I can actual see it sucking air (couldnt get close enough to show on a video). When I back filled it the rubber on the cylinder got all wet with fluid, I thought it was from my gloves at first pumping it earlier but after looking at everything its pretty obvious.

Here is a video of the amount of air coming out of the caliper and its hardly taking any fluid from the reservoir... (cant clearly see it if I embeded the video, sorry)

Video Link: http://s740.photobucket.com/user/lkuppler/media/airleak.mp4.html

So now my question is it possible something is not seated right in the master cylinder, or is it just DOA? I am leery about dismantling since if its just S.O.L. I don't want to damage it any further so maybe I can get Mikes to replace it. Then again they may be difficult about the whole thing as seems to be the norm with them these days. Thoughts?