Bench Bleed Dual Master Cylinder.....

[silverton_ford]

First of all, sorry to bore anyone with such a simple question here.  I couldn't find a "tech" section in the forum, so this is in the lounge.  I have searched youtube, other forums, searched shop manuals, called friends and I am not getting anywhere, so I figured I would ask here as well..... I know many here will have more experience than I do.

I am working on my 1978 Ford F250.  I am changing the dual master cylinder on the pickup.   I figured the basics of this system is the same as far back as when dual master cylinders came out in a Ford...so should be the same as any 1968-1970 Shelby.

I am struggling here....   I have bench bleed the master cylinder using the solid plastic plugs provided to plug the brake line ports, the plugs with the hoses attached and routed back into the reservoir and now I just made some solid lines with fittings and routed them back into the reservoir.   Still getting the same result.  Each method I have zero fluid leaks.  I can get all the air out of the front reservoir, but the large rear reservoir still spits an air bubble out consistently with each push on the piston.  I would really like to have zero air bubbles so I can be assured it is bleed and ready to install.  I am on my second master cylinder this weekend with the same issue.  I have never had this much trouble with this procedure. 

This is such a basic procedure, I don't understand why I am having this issue.  What is the problem here?  What am I doing wrong?  Are these master cylinders being produced today with cheap materials and it is a common problem(I would hope that is not the problem, but I'm uncertain. First one was a Dorman and the second is a Centric.)?   Does anyone have any pointers?  Do I need to remove the piston and try to service it (never have had to do that before)?

Again, sorry for the mundane question.   Thank you for your input.

[429rb]

I've recently bled a few old master cylinders, one NOS, the other rebuilt with brass sleeve.  Both were a pain.  The second one I wound up buying a big hypodermic turkey baster\injector from local hardware store and injected brake fluid in a reverse manner through the brake lines.  Method worked great.  Air pushed out of piston chamber up into fluid chambers and gone.  One that process was complete I set things up like normal brake bleeding to confirm all worked when piston was pushed in.

429rb