Brake Master Cylinder

I am trying to rebuild the master cylinder on my '77 MGB. I have the master cylinder off of the car. I have dis-assembled everything except the Pressure Differential Piston Assembly. Applying air pressure to the primary outlet as the manual directs, nothing happens. Very little air passes thru the PDRA chamber and the assembly does not move. Any advice is greatly welcome.

I hope that even though there were no responses to my question posted, someone read it. There is not a great deal published on the subject. I received a couple of e-mails that I tried,(tapping the end with a soft piece of wood,ect.) I did it with air, not with air, very violently againast the wood bench (not recommended), but unsuccessfully. The piston is stuck solid. The only possibility is that one or both of the O-rings have become lodged, or the end of the piston is in contact with the housing and has fused itself together. No incidental movement to encourage additional movement with brake fluid, grease or penetrating oil. Ordered a new one, one that will not be so neglected, and while I am there, rebuild the servo. Thanks!

I read it, even picked up my Chilton's to look at a drawing of the master cylinder. Yours is different than mine (73 B), so I didn't have any worthwhile advice. Except, of course, that rebuilding mine was a pain in the neck, too. Same deal, taking it apart. Putting it together was easy, though.

Thanks for the encouragement. I am still going to rebuild the servo . I ordered a new master cylinder. It should be here tomorrow! Did you also rebuild your servo while you were in there?
They make it look very complicated without the special tools. I am going to fabricate them if necessary. Since it will be a one time (likely) use, perhaps I try wood first. Tad bit easier than metal!

No, mine doesn't have a servo so I was spared that adventure.
Rebuilt the master cylinder twice, once around '88 and again around '95. First time around, getting it apart was a two week exercise. This big teflon washer would not come free. Finally pulverized it and shook out the pieces. Second time wasn't too bad. Third time? I'll just replace it. I think it is worn out.
Good luck. Sounds like you're having fun.

Pulled the servo last night and what a bear. What concerns me is re-assembly. The two lower nuts are removable from the footwell but installing will be a challenge. The clevis and cotter pin for the brake pedal will be just as tough. But you are right, I am having fun. Hope to get it on the road soon!