Lost back brakes

2008 standard. Lost rear brakes. After bleeding they worked temporarily. Maybe a few days. Pushed the pads away from the disk then pumped the pedal. The pads pressed against the disk but no pedal pressure and the will not stop the bike.
Master cylinder piston or caliper problem ??
 
When Mufasa did this, I started investigating as is our way.

Found the seal inside the fluid reservoir had gotten all inverted and sucked down a bit, preventing a air tight seal.

Flushed, bled, no issues since.
 



Just take the thing off the bike and change the seals. Get a fast bleeder and flush the thingy. New pads on a very clean disc.



Clean and lube everything. Then, if it don't work, service the rear master cylinder. BTW, order a set of seals for the front calipers.

 

Would you provide a link so i can order thr correct kit? Thanks