with the compressor engaged,it should go into suction (below 0) as you add refridgerant the pressure should climb to about 20psi.don't let it go over 30psi esp if your putting it in as a liquid or you'll damage the compressor.
you can also do by superheat, hook your blue (low side) gauge to the suction line and clamp a thermometer next to the port, take the pressure reading and change to temperature reading using a pressure temperature chart, the key is to maintain 10 degrees of difference (superheat) between the gauges and the thermometer, it may sounds complicated but once you do it once it comes naturally.
Poppy! I like that. In answer to your question, NO, the hoses do not leak due to porosity (50 years ago maybe, but not now). The hoses leak at the connections, usually due to failed "O" rings, and sometimes due to corrosion of the metal coupler. The "Freon", which is a regestered trade mark of DuPont (like Kleenex), is what folks generically call A/C refrigerant. Refrigerant is a chemical compound and in its gasous state is very fine (from a molecular point of view) and it will leak out of any weak seal. I have a daughter in law that is a chemical engineer for a world class automobile manufacturer. She is one of a select few engineers responsible for ANYTHING chemical that goes into making one of their automobiles. We have great discussions about such things.
DIITTO the warning above. Add to that warning ; wear safety glasses and gloves. Liquid refrigerant is at -240+ degrees F, and if you get hit in the eye with a drop you can lose sight in that eye. If you are charging with a can then hold the can upright so that you charge with the refrigerant in the gasious state. DO NOT turn the can upside down and charge fluid into the system. This is dangerous and can be harmful to your system. Do not charge the system without knowing what the pressures are, and what they are supposed to be. The system may act like it needs a charge when it in facts needs a part like an oriface. You could overcharge the system and exaserbate the problem. If you discharge the system to make a repair then the system MUST be vacuumed in order to get all of the moisture out of the system. There are other ways to do this but they involve expensive professional equipment. If I open up the system then I replace every seal ("O" rings) in the system except the compressor shaft seal. The shaft seal is expensive and should be replaced if it shows signs of a leak. Check the metal hose couplings and replace the hoses that have serious corrosion problems at the couplings. Work safe, this can be hazardouos and should not be taken lightly.