Vince Dhimos answered a question at Quora.
Why does Russia support China, in South China Sea issue ?
Vince Dhimos Master's in Russian from Kutztown State University
Your question has a sound basis in fact. Russia does support China in this issue but not very strongly. In fact, Russia declares it wants to stay out of the issue, as reported here:
Putin: Russia is staying out of South China Sea dispute
The US is now starting to feel the effects of its irrational position toward Russia. If the US and its puppets in Europe etc had taken a more reasonable stance toward Russia, that country might be more amenable to nudging China away from what the US is calling an expansionist policy. But after all, US-controlled NATO and the US Pentagon have officially declared Russia as an “adversary” of the US. That’s unfair on so many levels. I have shown, for example, that Russia is not the aggressor it is alleged to be in world affairs: https://www.quora.com/How-does-a-country-like-Russia-which-GDP-based-on-totally-natural-reserves-low-technology-screw-up-population-meddle-in-the-affair-of-other-countries-Moldova-Azerbaijan-Georgia-Syria-or-say-other-Shouldnt-Western/answer/Vince-Dhimos
Further, Russia is supplying cheap gas to Europe, and since Europe is supposedly an ally of the US, then the US ought to view that positively. But no, Congress has decided to slap unfair sanctions on the gas pipeline project Nord Stream 2 in a bid to prevent completion of the line, thereby causing great economic harm to both Russia and Europe. These stupid policies have greatly damaged Russia-US relations, perhaps irreparably, precluding any possibility of persuading Russia to pressure China to soften its expansionist policies in the South China Sea.