[Ducati] ST4 rear brakes

[Andren Appelquest]

Rear brakes on my '01 ST4 are weaker than most Ducs, getting worse. 
Rotor, pads, fluid OK. The pistons seem to have insufficient pressure. 
Front brakes oustanding. 5500 light miles on bike. Any suggestions as to 
where I should start looking for something not right? The books I get a 
chance to look at do not address the problem at the wrenching level.
Andy in Seabrook, TX