sounds like the master cylinder seals have perished, the srvo costs 120 by the way,ive just got one,if it was that your car would air leak,when running,the fluid is getting through,then theres no blockage,the rear ones run through the load valves,theyll only stop working(ie.stop doing the pulse pressure thing they do under heavy load when emergency breaking) if there is debris jamming the arm actuators,(stopping them from closing,ie.like a small stone jammed in between),or if they are leaking(visable),and the rear calipers were leaking too ,so ,the fronts would also leak if it were them also,(calipers=visable),
which is why me thinks its the master cylinder seals on the push rod .(inside the master cylinder,the arm is pushed ,shoving pressurised fluid through the pipes,if its split ,the fluid bypasses the arm through the seal=no pressure.)

hope this helps, nuts Rus