huh? where did you take the pressure reading from? A low side pressure will be somewhere in the range of 20-35 psi and high side something in 180-220 psi range. If it's zero, there is obviously no refrigerant in the system.
Oh and Wedley,
something I just noticed this weekend,the lousy heater valve on the accord doesn't close all the way (yeah I know some are designed to stay slightly open), but first turn your knob all the way to max cold and then manually close that water heater valve (located just underneath the intake plenum on the 5th Gen accords). You'll notice it moves just a notch more in the direction towards the front bumper, that's the complete closed position. Try not to turn your heater knob until winter hits and you need heat. Apparenlty for me, I tried adjust the valve, but the way it is designed it just won't allow to close completely, unless you manually shut it all the way. Dude I tell you, big difference, the damn a/c almost froze me out over the weekend, quite impressed indeed, especially for the R-134 system and in a black car
What's probably going on is when the system is off and the pressures "equal" the low side looks like it has pressure. Then when he turns on the compressor it zero's out the low side. Either way if the low zero's out it's probably leaking.
And yes I have seen tons of heater valves out of adjustment on Honda's that would not cold for anything. Fixing this fixed alot of a/c warm problems. Also on almost all accords between 90-96? the hot/cold knob on the dash always breaks internally and why you may turn the knob and it feels fine, it may not be turning the heater valve at all. I seen this every summer for the last 8 years and about 50% on accords with an A/c problem needed a new knob.