Can't get rear brakes bled. Help!

I just replaced the master cylinder, rear brake shoes and rear wheel cylinders on my '99 Suburban.  I can't seem to get the rear brakes bled.  The first series of the "pump and hold" technique goes well with air being bled out along with some fluid, but each subsequent time I loosen the bleeder there is hardly any air, fluid or pressure.  It also isn't using very much brake fluid at all.  The level hardly drops at all.
 
I went ahead and bled the fronts just to see if there was a difference and the fronts bled just fine right off the bat.
 
If I remove the master cylinder lid and replace it (even without adding fluid) the pressure in the rear lines increases for that first "pump and hold" series, but quickly decreases to a dribble again...
 
Am I doing something wrong?  Any help is appreciated.  Thanks!
 

give this a shot!
 
With out any pressure on the brake pedal open the bleeder on the right rear wheel then have someone push the brake pedal to the floor and hold it there while you close the bleeder. repeat as necessary, this sometimes works on stubborn brakes!

Thanks!
 
I'll give that a shot when I get home from work this evening. 

on that yr u have to use the GM Tech2 to bleed the ABS system also.

Thanks twizzle12.  That helped.  I discovered that in my fear of stripping the threads on the fittings going into the new master cylinder I had not gotten the rear brake line quite snug enough.  It was barely leaking, but enough to cause the rear brakes not to bleed properly.  As soon as I snugged it down the rear brakes bled just fine.  My son and I took it for a test drive with the new brakes and fuel pump and all worked great.
 
Thanks all for the help!