You can disassemble the brass prop valve and try to clean out the passageways and install new O-rings (make sure to use O-rings made of a proper material like Viton or Buna-N or Neoprene, not just generic "rubber" off the shelf. Any O-ring you could possibly need can be had from
Small Parts Inc. Is your '75 the iron type or the brass type? See also
this thread.
