Dump valve stem: 1.5 x 7mm (The dump valve uses two of these, one to secure the inlet to the valve, and one between the plastic guide bushings where the valve stem slides against the valves body to open and close the dump part of the valve. These fit pretty damn tight. I usually have to cut them off when changing them. You might want to order some 1.0 x 11.5mm rings to try and see if they offer a bit looser fit. Otherwise at $0.06 a piece it isn't the end of the world if you have to destroy them every time you break the valve down to clean.
All of these orings are EPDM, 70 Durometer. They can be found here:
Metic: https://www.theoringstore.com/store/index.php?main_page=index&cPath=368_2278
AS568: FDA Buna-N O-Rings - We make getting o-rings easy!!
So I got these about two weeks ago, and all of them seem to be fitting pretty well. I pulled the dump valve apart completely (I never have before, and it was nasty in the shaft). The original oring between the two bushings was broken when I pulled the shaft out. I got to this part of the oring bag and the 1.0 x 11.5mm orings are way too thin, they leave a lot of play in that union and leak. I now see you said 1.5 x 7mm at first, so this must have been a typo. The 7mm ID ones were out of stock, so I'll look and see if they are back now. I appreciate the time to list all of these. They are much better than the stock silicone ones for sure!