I'm bleeding

[jmnjrpa]

the brakes on my bike (81'MNS). I went thru the entire system and replaced the lines, installed MC kit on the front (cleaned the back but haven't replaced kit yet), and caliper kits all around. Using the mighty vac the front pumped out fairly quick. The back was another story. To extend the reservoir capacity I installed a clear tube in the fill hole. I pumped a quart of fluid thru the system! Went from front to back several times. The back has no air in it but not absolutely sure about the front. The pedal moves the pistons but they return to position. I also noticed when depressing the pedal the fluid would move higher on the clear tube???? About pumped out!!! Pic #1 shows fluid level before applying brake. Pic #2 is holding the pedal down. Any suggestions???

[jimbyjimb]

If it's not holding pressure it sounds to me like an internal leak. I don't know how these MC's are screwed together but it sounds like an internal seal in the MC is leaking somewhere, allowing the fluid past the piston and back toward the MC. Usually if it's an air problem the brakes won't work at all or will only work after being pumped a couple times. Forgive me if this is a stupid question, but is the tube you installed supplied pressure after the piston or is it hooked up only to the reservoir? It doesn't look like it is, but it does sound like something I would do. Not to say you're as dumb as me...

[jmnjrpa]

No

It is screwed into the fill plug threads. The 81 MNS is different from all other XS's in that the rear brake (pedal) has a porportioning valve that applies braking to the rear and one of the front discs. This lends to a lot of air in the lines to the front. I have checked for external leaks and could not find any. Only reason I did not change the kit on rear MC is because it is hard to get and I have one comeing but when??? It did not make sense to me that the level would increase when the brakes were applied??? Hope someone with a lot of brake experience will chime in as I am at my wits end.

[CatatonicBug]

As nice as that tube idea sounds, you can't do it that way. I can't explain it, but the MC creates pressure in the reservoir, and you have to SEAL that fill hole while bleeding. The main issue is that metal tube that goes up under the tank, to the union on the forks. That tube is the high point in the system, and becomes a collecion point for bubbles. Use the mighty-vac, connected to the caliper, and push the fluid toward the MC.

Did you disassemble the proportioning valve on the MC when you cleaned the system? I know most people don't but it is a potential point of gunk-collection.

[jmnjrpa]

Yeah

That baby was totally cleaned. Did not think about reversing the flow. Thanks, will try that. I had 25 inches of vacuum applied from the caliper and was getting a very good flow, so much as it was difficult keeping the reservoir filled even with the tube.

[randy]

Use the mighty-vac, connected to the caliper, and push the fluid toward the MC.

Are you saying work the mighty-vac in reverse? How do you do that?

[Ivan]

Sealing the reservoir against pressure? The little ribs in the rubber seal are to allow for expansion and contraction of the fluid at different temperatures and to allow the fluid level to drop as the pads wear. Sealing the reservoir has nothing to do with bleeding brakes. If it were sealed against variances in pressure, it would not work. The only reason the rubber is there is to keep out dirt and water. The reservoir needs to be vented to atmosphere, or its problems waiting to happen.

Usually a MC that pushes large amounts of fluid out the reservoir is the result of a frozen caliper trying to be remidied by pushing harder and harder on the pedal until one of the cup seals inverts, thereby making it pump fluid the wrong direction. During normal operation, a small amount will revert, but not near what the pictures show.

John, it would probably be a good time to disassemble the MC and check that the cups all open toward the outlet. I don't recall if the XS MC has two or just one. Quick take up types usually have two.

[draketrumpet]

Are you saying work the mighty-vac in reverse? How do you do that?

I think cBug is thinking of another mity vac thing that John in Hurst has. You fill it up with fluid and then pump it from the caliper to the MC. That way, gravity works for you instead of against you. With the 81 MNS having the high point in the middle, I'm not quite sure the best way to get it all out though, unless they thought about putting a bleed valve there too.

[ManitoulinLad]

Hey John: Your bike looks imaculate! I love the extra gold touches of the oil cap, bolts, even the end of your pegs. Would you be able sometime to put up a full pic of the bike. Coming from a guy who can't do the pic thing himself - Dan

[jmnjrpa]

Ivan

Have it right but I am using the old piston as I still have not taken delivery of the one I ordered. Even if I had it reversed it would not prevent being able to bleed with the vac would it? You are right about the MC not being sealed (I could not quite wrap my mind around that one but was willing to try anything). I do get slight movement on the calipers but when released they move back never pumping down near the discs. My vac will not produce pressure so I tried to siphon (with the vac on one end) from the front caliper to the rear caliper. Then I reversed it and after a couple hours I still get air bubbles. Did notice the bubbles increase as the vacuum increases (told me that I must have a leak?). I have the MC capped off while siphoning. With the MC capped off I believe that virtually eliminates it being the problem. There appears to be no leakage from any connections. OBTW, I did try to pump it thru with air pressure and made a "duh" mess. Only thing that I see is I keep getting air. After all efforts the brake pedal remains the same and I still cannot understand the backflow up the tube???

[jmnjrpa]

Thanks Dan

Soon as I (with help) solve my brake issue I will put her back together and post pics.

[draketrumpet]

When we were trying to use the mity vac on mine, I know we had a lot of trouble getting it to seal and we got air bubbles entering at the mity vac hose connection to the bleeder valve. You probably thought of that, but just in case.

[randy]

John,

I just bled my brakes last weekend and when I did the back, i had a similar issue with it. i found that my M/C piston was not fully relaxing out of the bore, covering the return hole. I had the actuating rod turned up too tight. When i released the rod, it all went like **** through a goose. i know the midnight system is different than mine, but it's worth a look-see at the M/C piston.

[jmnjrpa]

Thanks guys

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.

[FANNING507]

'81 mns

I cleaned and bled mine last year. Left front caliper must be bled after any changes, as well as the rear caliper. I found it easier to work from the right side of the bike, remove the left front caliper and position it on the right side front for access to the bleeding valve. Otherwise its a two person job. Might try it. Kim