the chinese (and the japanese) (artificially) lower the value of their currencies by buying immense quantities of US government bonds. their low value currency gives their products a competitive advantage over US products so they run a huge current account surplus with the US which they use to buy more US bonds.

So eventually the US runs out of buying power–which has happened now and the US prints up more money. Will this lower the value of the dollar against the Yuan?

The Chinese could keep the Yuan low but they would have to buy up still more US securities.

The Japanese have already gone into a current account deficit.