Put the car up on stands and use a brake pressure gauge on each caliper. Thread it into the bleed valve port.
Test it with the engine running and vacuum to the booster and with engine off.
Measure all four calipers.
You do want to know if the pressure to the left and right in front is equal.
The stock front brake proportioning valve was engineered in to reduce front braking with the original skinny tires.
Ford had a lot to do with that and I don't like the conclusion that Ford Engineering came to. How can it be bad to have too much braking?
Apparently it was known that the front and rear calipers are not an ideal match for each other and this was the factories fix to balance them better for the average driver who didn't expect super car braking.
Depending on how much engine vacuum you have, the power brake system should add around 150 to 250 psi to the gauge reading. Below 14 inches you will get almost no brake assist.
The stock power booster expects 18 inches of vacuum.
The fronts should read around 550 with no boost and engine off.
The rears in the 150 to 180 area.
My suspicions are that the brake proportioning valve is on it's way out and it is possible that you have lost power brake assist or that it is sporadic and needs to be replaced to keep the original function.
It is interesting to see how the philosophy of braking has changed. My Fusion has got the huge Mama brakes on it and is probably the best function of the car. The thing stops in anything...smoothly.
