I would try to get a variety that flowers at about the same time. A lot of catalogs (One Green World, Raintree, Burnt Ridge) list their fruit trees in order of ripening. Some might list in terms of pollinating. Internet search for pollination order might work better. My two cents.
John S
PDX OR

Greg, funny – I just searched “Valor plum” on Google and first up was your post here on the forum! …Either we’re real popular, or it's a very rare variety… …Actually, it’s new!

Found this: “(H4) 1995 ~ blue, medium-sized plum. Good quality. Moderately good, regular crops. Not self-fertile. Season: late Aug.”

And this: “There are a group of plums called the prune plums. They have a very high sugar content. This is the type of plum that is dried to make prunes. But these plums also taste delicious right off the tree… …The Valor prune plum is dark purple with a greenish pulp that does very well in zones 5-7. This prune plum ripens mid-season.

Quick Facts

In general European Plums are a tree that can spread up to 20 feet wide and 20 feet tall. This plum tree prefers zones 4-9 and loves full sun with fertile well-drained soil. The white flowers bloom in the spring and plum fruit harvest time begins in late summer and runs through the fall.”

Here we go: Plums and Prunes
“There are two types of plums, Japanese and European. Most Japanese plums are used for fresh fruit only. With a few exceptions the Japanese plum is usually larger, a pleasant blend of acid and sugar and somewhat juicier. European plums are actually prunes. They have a higher sugar content which enables them to be used for drying without fermenting at the pit. As a fresh fruit they are sweeter then a Japanese plum.”

[Something we all need reminded of:]

“Japanese and European varieties of plums do not cross-pollinate. European plums have been known to be self-fertile. Two different Japanese plum varieties will be needed to produce fruit.”

So “Valor” is a European plum; and your Santa Rosa is an Asian plum. I see the branch of “Imperial Epineuse” (needs pollinator) you grafted to the Valor is a European prune/plum, but I didn’t search pollination charts… Did you buy it from Raintree? Is the Imp. Epineuse too young to have flowers?

I’ve had Satsuma, Ozark Premiers and now Shiro’s blooming much of this week; with my European ‘prunes’ just beginning to open. So your Valor does sound early for a European…

I’m dreading these killer freezes along with every other fruit grower I was watching some Hood River pear growers interviewed this evening on the news … considering millions of dollars in losses if it’s as cold as predicted. I guess the good deal for us amateur-armchair fruit growers is we’ve such a menagerie that if it’s a bad year for one thing – it's usually bountiful for another... Now Let it snow - let it snow - let it snow