Well , Ima thinking the fluid in the fronts is not moving. And the shoes in the back are adjusted OK. This gets you a hard pedal and rear brakes only. Since the rears are proportioned, you don't get a lotta stopping power, so you gotta press really really hard, and then, I'm guessing the booster finally wakes up and KaBlam! I have simulated this with front hoses clamped.
If the front chamber was drained, you would have a really long pedal travel before the rear brakes came on line, and since the booster is then engaged, you would again have normal modulation, just with super reduced braking power.
If both front caliper pistons were seized in the calipers, then the fluid would not be moving. But Both is highly unlikely
Same if the front piston in the M/C was seized, but again highly unlikely.
My guess is that the front hoses are internally collapsed or otherwise blocked, OR
the line from the master to the P-valve is plugged,
OR the C-valve is seized
All of these situations should have turned the brake-warning lite on in the dash.
