Hi 

How did you bleed/flush the system ???   I take it you have no leaks .....  And I also take it , when you lost rear brake and firmness , (after bleed/flush),  the fluid in the  reservoir was as the same level ???  

If you are confident you have flushed all the air out of system , and there are no leaks ,, it points to a mechanical problem ....   Items 15 and 17 are kits you can buy,,, or I suppose you can buy individual parts .... The prices are US $ , that will give you an idea.....

I replaced the equivalent of no. 17 on my old Sporty, that component is the plunger mechanism that has the rubber o-rings.

at a guess i'd say your primary piston cup [ seal] is passing fluid back behind it so no level loss and usually no visible leaks,but loss of pedal after a few days [after bleeding], new kit will fix it[ or complete cylinder ]. hope this helps,cheers.