Yes and no. They are not made to hold pressurized air. But they do hold pressure in the form of liquid pressing against the walls. Also, CO2 pressure will not build in the carboy if there is an outlet - and their is - the racking cane. In this case, the pressure in the carboy headspace will equal the weight of the beer from the beer surface to the bend of the racking cane (because that is the weakest source of resistance). It's only a few ounces of beer providing resistance, so pressure in the carboy would never even get close to 1/2 psi.Are you going from a carboy to a keg?
If so, I have used the following system very successfully:
1. push a racking cane through one hole in one of those orange carboy caps with two nipples.
2. connect a hose to the output side of the racking cane.
3. connect a black keg connector (for beverage out) to the other end of the above hose.
4. connect that connector to the beverage out post on a purged and depressurized keg
5. connect your CO2 tank to the other carboy cap nipple and pressurize to 2-3 psi.
This will pressurize the whole carboy, forcing your beer out the racking cane and into the beverage out, filling your keg quiescently from the bottom, all under a blanket of CO2.
I was under the impression that glass carboys are rated for 0 PSI and this is a very bad idea. is that incorrect?