brake flush

I found several discussions on brake bleeding and flushing, but none included a reference to a question that came up today while trying to flush the system on my recently acquired '82 380SL.

I was using my bicycle pump/schraeder valve equipped master cylinder cap method and started on the rear. As the fluid in the front chamber of the master cylinder got down to the minimum level there was no equalizing of the level from the other (full) chamber. So now I have a full rear chamber and a nearly empty front one and don't see how the front chamber receives any fluid from the fill cap (located on the rear chamber) Who can enlighten me, please? Perhaps I have a blockage between chambers? Thanks.

Randy