Even if you could get a decent reading, the reading would not be correct due to the volume of the piping to the gauge. This increases the cylinder volume significantly. The only way to get a true reading is to have a check valve very close to the glow plug hole. Additionally, the inside diameter of the pipe should be small to cut down on the volume. You then must crank the engine through several cycles to compress the air in the pipes a little bit each cycle until the pressure in the pipes gets close enough to the pressure in the cylinder. After enough cycles, the gauge will stop increasing in value. That's fairly close to the actual cylinder compression.