Is the tank designed to release pressure when coolant overflows from the radiator into the catch tank? I'm asking because the aftermarket resevoirs look like they're sealed, but the stock plastic tank looks like pressure can leak through the top.

 

yep it leaks at the top. Just flip the reservior upside down and you will see. The pressure is regulated at the radiator cap i believe. If the reservior was sealed the displacement would have nowhere to go.

 

Well, even if it was sealed, the pressure built up in the cooling system would still cause the coolant to leak into the catch can until the pressure was equal. Is there any reason Not to use an aftermarket can?

 

Are you sure the aftermarket can isn't vented somehow? It should be, because heating & cooling of the air in the can will change the pressure too.

Some cars have pressurized reservoirs (typically European cars). But then the radiator doesn't have any kind of cap, and the reservoir cap has the pressure relief valve & vacuum break. Maybe the aftermarket can you're looking at was supposed to be used on that kind of system?

 

i have a summit tank havent gotton my car to work yet though seems fine to me its twice the size but it fits in the spot under were the ab used to sit im all about as little as possible in the engine bay its also black so oout of site