This may have been mentioned before, if so, sorry to waste bandwidth, but I am new here.

Recently, I needed to figure out if my headgasket was leaking  (this was an inline 4 motor transplant into my  van, but should be the same on a waterboxer) so, being a simple guy, I looked it over and came up with a simple  solution for a simple test.  I took out the sparkplugs and then put some compressed air into the pressure tank.  I disconnected the hose to the expansion tank and just put my air nozzel right onto that hose barb..There is a one way valve there to keep the pressure in, so I dialed up 20psi from my compressor and filled her up with air.   Started using my bicycle pump, but the compressor came to mind and made it even easier.

  Then, I went back to the cylinders and used a tube from my vacuum cleaner to listen for hissing, dripping, any sounds of leaking...Nothing there... but then I took a little dental mirror and worked around with a flashlight until I got some light into each plug hole..I saw some orange (Dextron coolant color) on one piston.  Reached in with a pipette (small tube) and drew out some coolant...

  Poorman's leak down test?  Anyhow, it worked for me..The bike pump almost fits right onto the hose barb, but the air nozzel from my compressor, I just pushed the ends together and got the air past the one way valve into the system..
  Again, sorry if someone else has written this up already (probably have)
  Don Hanson