If you drilled into the cap, there would have to be a clog below it for it to back flow. You also have protection from back flow due to the dosing pumps being a stop point when they are resting.1. Stop telling me it's not legal you are ruining my case for plausible deniability.
2. I like that drilling into the cap would not be permanent. Not that I would ever stop reefing, but if I sold the house one day this is something that could be replaced beforehand. Also if I screw it up I can replace the cap and try again.
My question is will the water from the house flow back through the cap and into the RO drain line?
I would still do a "loop" right at the cap to prevent the chance of gas getting beyond the loop.
If you wanted to get really creative, you could use some push connect fittings and make a "ring trap".
You may even be able to install a small check valve inline. One with a spring may end up rusting, but there are others that are just plastic balls. Those need to be installed, so the ball rests on the input side of the valve (from your sump side) and should be placed to where that is downwards in the run. This would mean that an up/down run for the line would need to be made, but could be done right at the cap and in an up down section of the loop you make. This would prevent gas or back flow of liquid also.








