TBH I don't think that Snap on Stud extractor tool will work, could be proven wrong, and happy to be so, but the end portion of the valve is hollow, and that tool works by gripping (hence squeezing) a stud to remove.
Unless you can get it flush with the pump body (ie gripping on the full length - which will be difficult)
You need to grip on the hexagonal (what is left of it) part. Hence the need for a Nut/Bolt extractor, and my advice to grip the most solid area (the Hex area)
As i say happy to be proven wrong.
Just my experience and thoughts.
Hope you get it out soon. I have a set of the Irwin Bolt/nut removers only after experiencing them at work, and have used them on various parts of a helicopter and on brake caliper bolts that had rounded on a MK3 Golf, and they do the job very well.