valve, unexpected functionality

[Thornton, Keith]

Hi,
The following pipeline
gst-launch-1.0.exe -v videotestsrc do-timestamp=true pattern=snow is-live=true num-buffers=900 ! video/x-raw,width=1920,height=1080,framerate=30000/1001,format=YV12 ! tee name=t1 ! queue name=displ
-queue max-size-bytes=0 leaky=2 ! valve name=display-valve drop=false ! glimagesink sync=false t1. ! queue name=encode-queue max-size-bytes=124416000 ! valve name=encode-valve drop=false ! x264enc
trate=32768 ! h264parse ! qtmux ! filesink location="f:/temp/out.mp4"

produces snow on my display (somewhat delayed which is not important) and a mp4 file.
This is running the current git master version on a Windows 7 machine.
When I change the first valve drop to true I neither get snow on the display (expected and wanted) nor do I get a file (perhaps expected but not wanted sid-effect).
Is this due to the fact that the pipeline as a whole cannot preroll?
Would I be better using a pad probe and letting the pipeline preroll before blocking the relevant pad.
If anyone can provide an explanation I would be grateful.
regards
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