You can test the plastic to determine what it's made of. If it's styrene then nail polish remover (acetone) will soften it. Place a drop on an unimportant portion of the plastic and let it sit for a couple seconds and then wipe it off with a paper towel. If the plastic now has a rough or hazy apperance then super glue will work well to bond it back together.

If the plastic remains unchanged then the only thing I can recommend is to use an epoxy and figure out a way to reinforce the bond, such as splicing some pieces of material on the sides of the crack. Epoxy doesn't bond very well to plastic so sand it first to give it something to bite in to. I would use some thin pieces of hobby-shop grade plywood for the reinforcement, it comes in sheets as thin as 3/64"

I think the coolant tank is polyethylene. Nothing likes to stick this material as it is very inert. I'm not familiar with the moroso epoxy mentioned above but I would think the only thing with any hope of lasting more than a few hours would be a soft silicone, particularly the clear type. I don't think RTV silicone would work since it sets up too hard. Polyethylene must really be melted in order for the repair to have any staying power. I can't imagine any repair on coolant tank lasting since the plastic becomes distressed and brittle.

-Robert

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