Foxx Fuzz & Wa & Volume weirdness

[dualaud]

hiya,

a friend of mine recently gave me an old foxx fuzz & wa & volume pedal as a gift. 
now i wonder whether it is normal, that the pedal has a significant loss of volume just being plugged in. i would say, playing with the pedal plugged in, but with the wah and fuzz switched off, i have about "half" the volume compared to playing without the pedal in my fx chain.
is this a flaw in the design, or is something wrong with the pedal?
if its a flaw, is there anything i could do to correct it?

i found this schematic:
http://www.harpamps.com/schematics/foxfzwah.pdf

unfortunately its without the volume pedal option.
in case the pedal is damaged, what could i do to troubleshoot?

any ideas are greatly appreciated,
dualaud