bleeding rear brakes

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xs650dude63

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Hey everyone, im trying to bleed my rear brakes, I bought a new master cylinder from Mikesxs, I used the syringe method to makes sure very little air was left in the system, ok so I got the master cylinder back on, went by the directions in the work shop manual and its not working, I keep pumping the brake pedal and nothing is coming out,I got the bleeder valve open all the way open, resivour is full with fluid, when I pump the pedal a tiny bit of fluid comes out and thats it. I did adjust the push rod so it would go into the MC farther and that didn't help, I got the other end of the rubber tube going into a container submerged in fluid, but the brake pedal goes full travel up and down and nothing is coming out. Is it possible the Master cylinder I bought from MikeXS is bad???
 
You're supposed to pump the pedal several times with the bleeder closed to build up pressure, then open the bleeder while holding the pedal down. After 10 to 15 seconds, or after fluid stops coming out, close the bleeder and pump the pedal again several times. Repeat, repeat, repeat.

I have a Mityvac vacuum pump so I pull fluid through the system with that first to fill it, then I switch to the normal pump-the-pedal/brake lever method to finish up. The vacuum pump makes this a pretty simple task, getting most of the air out right at the beginning, and quickly filling the system with fluid.
 
Thats what I am doing right now pump pump pump open bleeder a tiny bit comes out close the bleeder then I repeat, i will pump it about 30 times open up the bleeder keeping the pedal down so no air is sucked back in, and a teaspoon or so comes out, close the bleeder then repeat, it does feel like the pedal is a little harder to push down, but no where near putting on the back brakes, do I need to do this for an hour or should it go faster then this?
 
That rod adjustment can influence the MC function. If it's adjusted in too tight and partially depressing the MC piston, the seal on the piston may be blocking or partially blocking the fluid feed port from the reservoir. The bore in the MC won't properly re-fill with fluid after each pump.
 
Does anyone know how to set the push rod to the proper length? The original one I had the length was changed so I don't know the original setting.
 
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I've bled brakes by myself on a car using this method. Seems like when I need help, I have no friends.. Haha! Crack the bleeder valve, put the tubing over it and submerg the other end in brake fluid. Let's the air out thru the bottle but no air back in. Drill a hole in the bottle cap to hold the tubing. Make sense? It has worked for me. Also, I had an issue when I aggressively pumped the system. Seemed to create more air. Slow and steady pumps. I've had a few things recently come up on XS650 brakes. 1: I had a really pitted, shitty piston that wouldn't seal. (You bought all new) Old brake lines had shit that kept breaking loose even tho I thought I had them cleaned. I bought new lines and problem solved. Hope this helps. One more thing, I got rid of that junction on the front forks and ran a straight line. Got a zip tie holding it to the forks and away from the tire. Looks like shit but it's rideable for now.
 
The rear brakes have air pockets when installed, remove the caliper and suspend it so all air can go completely up hill to the master cylinder or caliper then bleed. Another trick is to insert a thin bar where the rotor would be between the pads. Pump the pads out to touch the bar then force the piston all the way back into the caliper, this sends fluid AND air back and out through the reservoir. Did you rebuild the caliper? If you don't rebuild it the gunk behind the rubber seal forces the seal to stick to the piston, the seal stretches when you pump up then simply returns the piston to where it was before you started pumping, result you never gain.
 
I'm going to try the mighty vac first, if that doesn't work I will remove the caliper and try to get the air bubbles to flow towards the caliper. Since the caliper (not rebuilt) doesn't work I can't do the "bar" trick. Thanks, any other suggestions would be appreciated!
 
So, let me understand this correctly if ever the system is opened up an air gets into it you need to rebuild the calipers, and using a device like the mighty vac, will do no good until the calipers are rebuilt?
 
No you just always need to rebuild ANY 20+ year old brake system, stem to stern. Has the caliper ever been R&R'd?
 
I don't know, I know the bike was a drum brake bike then converted to disc. I've only owned it for a year and a half, and the rear master cylinder croaked on me.
 
When you remove the caliper to rebuild it, put the hose end in a container, it should completely drain all the fluid from the reservoir within say 15 minutes by gravity, if it doesn't replace the hose. Well ANY 30 year old rubber hose needs to be replaced.
 
I'm wondering if it was converted to disc if the components are newer, and need not to be replaced. I can take a picture of the hose and get your opinion on the condition.
 
You beat me to my next and last comment on this. Rubber hoses degrade from the inside out they can look BRAND NEW on the outside and be cracked and shedding rubber inside, creating blockages. You can keep on hoping the "PO" was a conscientious guy and rebuilt everything he touched, good luck with that thought. OR you can be the guy that does it right and strip rebuild the caliper. It often can be done without buying parts, depending on your tolerance for buffing out rust pits on the old caliper piston. When you've done it a few times a caliper overhaul takes less than an hour, with most of the time spent oh so carefully digging the crud out of the seal groove with some bent heavy COPPER wire, the caliper body is aluminum, scratching it with steel tools; not a good idea.
 
There are no "newer" components on these if they're stock. The parts added to yours to do the conversion were from another late '70s-early '80s Yamaha. All those parts are 30+ years old. That doesn't mean they're no good anymore. They just require a complete disassembly and thorough cleaning.
 
I blow them out with compressed air directed into the brake hose hole. I use the tapered rubber tip on my blowgun so it can be pushed tight into the hole and seal around the edges. If the piston is stuck in there, you may have to break it free with a big C-clamp. You may need to "work" it. Press it back into the caliper with the C-clamp, blow it out as far as it will come, press it back in, etc. It will come out more and more, little by little, as you "work" it, eventually popping out completely. Keep your fingers out of the way and lay a thin piece of plywood in the caliper in front of the piston because when it pops out, it can shoot across the room, lol. Be prepared for a pretty loud "bang" too, enough to startle you if you're not expecting it.
 

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