So you can tilt back, but not forwards? How slowly does it tilt forwards when you push your toe down? or is there just no power from it? Even if it lets only a small amount of fluid through, it should still be able to lift the front of the machine without any effort, just slowly.
The only thing i could think of is your bolts that hold the pedal to the frame may have moved causing your spool adjustment to be wrong. When you push the pedal forward it may be bottoming out? this would explain your problem. Its at least worth a look at the simple things first. If you can feel neutral and you feel it moving forwards and backwards it would have to be a problem with the spool.
*maybee* even a stuck load check on your control block, but i think these ONLY hold high pressure, and will not allow it to by-pass.