Pressure washer pressure gauge readings

Hi, I have two pressure washers and wanted to check what pressure they were really producing so I bought a gauge. One is a 2,650psi washer. I bought some quick disconnects and attached the gauge between the unloader valve and the wand hose. When I fire up the pressure washer the gauge reads 0 but when I pull the trigger it reads 1,300psi. On the other pressure washer, more of an industrial model and older, I installed the pressure gauge in the same location between the unloader and the wand hose. When I fire this one up the gauge reads 3,000psi but when I pull the trigger (its harder to pull then the other machine) the gauge reads 0. Why am I getting pressure readings backwards from one machine to the other. On the one machine the reading is 0 then 1,300psi. On the other its 3,000psi then 0?
The smaller machine I bought new and have been using around the house for about 10 years.  The larger machine I bought used and have no history on it.