Heater hose restrictor valve.
 

'95 Olds 88. Heater hose has a plastic restrictor valve in it. Replaced it once already with a dorman part. Now it has a small crack in it. Not expensive part, not hard to replace. Question is, is it absolutely necessary? Wouldn't want to question the wisdom of the Oldsmobile engineers and certainly don't want to crater my heater core if it controls pressure. Looks like a small piece of copper or PVC pipe would make a nifty restrictor eliminator.

I believe this restrictor is there to prevent stress on the heater core at wide open throttle or high RPMs.

I'm not a tech, but I've seen several replaced with unrestricted tubing of plastic or metal and no problems - yet. If you have a heavy foot, that might be a different story.