dumb question, i dont have a return line for my catch can, would excess pressure build up in the rad if the can gets full of coolant? would draining the can for now and pouring it back into the rad be a problem?

 

If you mean the radiator overflow then no there wont be excess press build up in the radiator if the overflow is full. The overflow will just.... well overflow it there is too much coolant. It would be ok to reuse the coolant if it doesn't get contaminated with anything.

 

There is only one line to any coolant overflow. It works by suction - like when canning vegetables - the heat in the radiator cooling down creates a vacuum that sucks the coolant back out of the overflow - that's why there is a hot and cold mark on the overflow - when the system is hot it will be fuller, when it's cold it will be less. This is all controlled by the radiator cap - as pressure builds on the radiator cap it is spring loaded like a thermostat and it "opens" up, allowing coolant to flow to the overflow. When the system cools down, a suction is created and coolant returns to the radiator. That's why there are different pressure ratings available in radiator caps.