SC400 coolant reservoir replacement
 

Is there any reason I couldn't use a $20 generic overflow tank? It's such an easy replacement I figured that if that was all we needed there wouldn't be such a market for 20 year old brittle plastic overflow tanks on ebay.

Plumbing is easy of course. The only part that makes me pause is that the stocker is obviously pressurized, thus the radiator cap, while a regular overflow tank is just a big jug of coolant that holds no pressure.

What am I missing?

I was gonna do the same thing until I found a used oem tank w/o the sensor on top since that's where it tends to leak, but I have the same question you do.

u can use a soda bottle if u please, whatever u use must have the hose under water

The sc400 tank has pressure in it as opposed to the 300 that does not.

That's what I was worried about - that it was more than just an overflow bottle. It's actually an integral part of the radiator.

Subarus (at least I know the 2.5 turbo does) use a separate reservoir that is made of heavy plastic like our OEM piece. Even if they aren't any cheaper on ebay (haven't looked) they are 2 decades newer so they won't fall apart again.

The easy permanent fix is a new radiator from a newer model that didn't use the tank. I may do that since it's time for new hoses anyway.

I have a 2001 Tahoe and its under pressure. I know it's to big but maybe a smaller GM engine will have a suitable reservoir

There are 2 options

1 use a supra radiator and a home made overflow (lots of fab work)
2 find a good used one which can be a bit pricey and hard to find

Don't the later SC400s use a radiator without the extra piece? I thought they had a "regular" radiator / flimsy overflow tank setup, maybe 98+ cars? I'm not too proud to have a generic overflow bottle under the hood if that's all I'd need after a new radiator.