Are you using the manifold that connects to the high side and low side of the system? Or is it one of the small hose/gauge pieces for adding refrigerant? Also, was in the red low or high? Was the compressor running or off? If it’s low, it might just need to be topped off (but there could be a leak in the system). If it’s too high, then you likely have a stuck expansion valve or plugged orifice, both of which would require the system to be evacuated and the part replaced.