You're testing this with the bottom cover securely fastened, right? Any light leaks into the box will turn the LDRs on and reduce sustain.
Here are some easy tests you can run:
Set TONE to noon and BALANCE to max for all of these tests. The position of the toggle switch does not matter when BALANCE is maxed.
Play softly with SUSTAIN all the way down and then again with SUSTAIN is turned all the way up. Do you hear a difference?
Turn SUSTAIN all the way up. Strike a chord and listen to the decay while switching the pedal in and out. Do you hear a difference?
In a very dark room, with the bottom cover removed, turn SUSTAIN all the way up, play some chords thru the pedal and see how brightly the LEDs light up.