Yeah, it can do that to valves for sure. One side of my property is all blackberry bushes and locust tree -thorn city (technically it's the neibors property so I can't do clean it out) and my tractor has slime in all 4 wheels as does my garden trailer. It's been the only way to keep the air in there.
The key is to put in the slime, add air and spin the tire for a few minutes to distribute the slime to the inner tread surface. Still no guarantee you won't get it on the valve but it does reduce it a lot. Oh, and don't put too much in!
Jill & Chris, Wills (14) Evie (11) & Toby our collie (5)
2011 Grey Wolf 28BH
2013 Chevy K1500 Crew w/ Reese StraitLine Dual Cam
Nights camped 2011: 11 2012: 18 2013: 12 2014: 12 2015: 13 2016: 3