I set the compressor at 10-15psi (1 bar). Air will leak out easier than oil so this is plenty of pressure. I just held it in the rubber plug for about 10secs and then started squirting suspect areas with soap. The bubbles show up right a way if there is a leak. You can see the bubble grow from the exact leak point. Here is a picture of the process showing a leak at the flywheel seal.
After I put in new seals I did the test again and also used the reverse of this process to verify that it was leak free. Using a Mighty Vac vacuum pump, the case held a steady vacuum for about 5 minutes.
