New Master Cylinder Wont Bench Bleed

I am trying to bench bleed my new dual master cylinder, and I cannot get the air out. I first used the kit that Master Power Brakes supplied, which rely's on the seal of 2 cone shaped plastic tubes to seal the rubber bleeder hoses too the master cylinder outlets. I kept getting more and more air, so I though that maybe those wernt sealing properly, and they were actuall drawing air in. I now formed 2 brake lines with fittings so they submerge into each chamber, and I keep pumping, and pumping, more pumping, slow even strokes, and I am waiting for the piston to get firm. It wont. I keep seeing small streams of bubbles re-enter the master. The master cylinder IS level in my vise. Any ideas?

 

You're not using silicone brake fluid are you?? It will never bleed...

 

No, this is DOT 4 fluid that MP Brakes supplied.

 

It should never get firm. You should see a steady stream of fluid with no air bubbles. When the air bubbles stop coming out of the end of the submerged lines you are done and it's ready for mounting. You have removed the air from the M/Cyl.

 

Ok, it didnt make sense to me that it would get firm, but that just something I read online. But it seems that the tiny stream of air bubbles never stops coming. What could cause that? I have probably made about 35+ slow even pumps.

 

With a little more work, I can get push the piston in slowly, withdraw slowly, and get no air bubbles, but the 2 strokes later, I get bubbles. Also, sometimes, the piston doesnt return all the way to its fully open positiion, without using a screwdriver to lightly pull it out. Othertimes, I slowly release, and it fully returns to its resting position. Still stumped.

 

put the thing on the car and bleed the brakes, it should work ok.

If you use plugs in the ports to bench bleed it, then it will get firm. If you use the tubes, just bench bleed it enough to get most of the air out, then you're done.

 

Thanks, I figured a small amount wouldnt be terrible, especialy with my Motive pressure bleeder, which does a really good job. I'll bleed it all tomorrow and report back.

 

I have bench bled master cyls for years with just my fingers on the ports. Never had a problem.

 

I bled the brakes tonight with my Motive pressure bleeder, and it took a bit, but I got a nice firm pedal. Thanks for the help guys.