soft brakes and headaches

Heres a thought; isolate the fronts to see if you have a hard pedal.That is to say, disable the rears. There are several ways to do this. You can 1) pinch the rubber hose at the back( not recommended), or 2)plug the m/c output to the rear, or 3)remove the brake shoes and clamp the pistons into the wheelcylinders, or 4) disconnect the rear flexhose and cap the steel line. I have used all four methods. Method 1) is the easiest but it is possible to damage the line. Methods 2) and 4) will require re-bleeding. And method 3) will not prove accurate if there is air in the line somewhere.
- I would start with method 4) because; a)it does not req. re-bleeding and b) if the pedal gets hard you instantly know that the problem is in the rear mechanics.
-But if method 4) doesnt get you a hard pedal, then I would first observe the rear flexhose to see if it is expanding due to internal failure. Then if it is ok I would be tempted to use method 1)
-then if still no hard pedal, I would go to method 2)
-At this point you have totally eliminated the rear brakes and since you have already roadtested the car, the fronts are working, and the pedal should be hard.
-But if its not hard, the problem is in the front, or in the m/c itself.
-To prove the m/c just plug the front brake port as well. If still soft, I would bench bleed it one more time, with it being level to expel the air which can get trapped in the nose. This would be a good time to also prove the compensating port is working. So now if the pedal is hard you have a good m/c. But if still soft shes defective.
-Lets assume the m/c is good. Reconnect the fronts and bleed the line to the p-valve. Now the pedal should be hard. But if its not; theres either air in the front system or there is a mechanical problem. So again we observe the flexhoses.If theyre ok, the next most likely place is the interface between the calipers and the discs. So we have a helper pump the brakes moderately at regular intervals while you are observing the action. If the pads were correctly installed and the calipers are free to self-center, then there will be very little to see. You may be able to see the pistons moving back and forth a few thousands but thats all. If however there is more action, then thats the problem. To isolate the hydraulics I remove the calipers from their mounts and clamp the pistons into the bores with a big C-clamp. Now the pedal has got to be hard, unless there is air in the lines. If soft, re-bleed.If hard the problem is in either the pad to disc interface or the pad to piston interface. The face of the pad friction material has got to be parallel to the steel backing material. If the steel backer is bent it will act like a spring forcing the piston back into the caliper after every application giving a soft pedal. So will any silicon material pasted on there. If the pad is worn at an angle,or the pad is sitting on a worn rotor,the caliper will tend to rock. So fix the interface.Thats it we are done with the fronts, and the pedal is hard. So,reconnect the rears and continue trouble shooting; ie go back to the top of the post.