Is the reservoir full?
The above may sound like a stupid question, but I had this problem the first time I changed the fluid in my 87 300TD. The reservoir has a divider in it for safety and has a very strange shape. The rear cylinder is also very slow to fill. The most forward compartment is the most visible and the rear is hardly noticeable. I have changed and bled brakes and master cylinders hundreds of times and I was completely stumped when I was just pumping air to the rear wheels. be549 (David) steered me in the right direction. Take another good look at your reservoir, fill it up and then try using the standard two man bleeding method.