The stream is very slow and does stop also in browser itself.If the stream is bad like this then you need to wait a while till rtmpdump starts but you also could add some timeout & buffer into your link to gettings started the stream quicker.
Code:
rtmpdump -v -r "rtmp://rtmp.zerocast.tv:1935/live" -a "live" -f "WIN 14,0,0,179" -W "http://cdn.zerocast.tv/player.swf" -p "http://zerocast.tv/embed3.php?a=1&id=&width=580&height=400&autostart=" --live -y "rGCi7bg0Qxvs8q7Sl13l" -T "8964316ec8772dfa" --buffer 4000 -m 2 | "player" -
greetz