I have a Troy Bilt pressure washer with a pin-hole leak where the manifold attachs to the pump unit. Can I just back-off the fasteners, scrape the old gasket off and replace with liquid automotive gasket maker, rather than replace the entire pump unit?

Most likely the piston seals are leaking This is a good thing as since it leaks out it is keeping the water from entering the drive mechanism. If you can find the PUMP brand and model you can get parts fromm a place like www.pressure-washer-parts.com . The parts are inexpensive for just the seals before you ruin the complete pump.When replacing thee seals, you might as well replace the pump packing too since they wear. If you cannot find the pump numbers, then get the Troy Bilt model number and check their web site for parts. http://www.troybilt-gen-pw.com/pressure_washers/ They have parts listed there but i think they only list the complete pump not pump repair parts.