There's no check valve in that line, shouldn't be.
The HX sends the heated hot raw water to the injection nipple on the exhaust riser to the muffler. This sounds like what you're talking about.
There is a vented loop on that line, before the nipple.
On our boat it is a Marelon loop with a small top fitting with the tiny little rubber flap. Made by Forespar, IIRC.
Ours is a 5/8" hose and vented loop.
Go to the Forespar website, or Google marelon vented loops, or use WM and you should be able to buy just the rubber ducky part to replace yours.
This is pretty simple. Good work on your part to notice it.
You may be interested in our Tech wiki which includes a lot of M25 engine info. www.c34ia.org
, click on tech wiki, scroll down to engine.