Hi David,
There can be a number of causes of pin-holes like this but I'd say that 99% of the time it can be prevented by allowing the laminate to be a little more resin-rich. When a pin-hole free finish is a priority (as opposed to ultimate lightness being the #1 priority) then follow this sequence for pin-hole free perfection:
1. Allow a good few seconds of 'flushing' resin through the laminate when the laminate is fully infused and resin is making its way down the resin line (unless you have a peel-ply resin break in place, in which case let the resin crawl through that for a minute or so).
2. Clamp off the vacuum line.
3. Wait a little while (vague I know but it depends on the size of what you're infusing: small parts, 10-20 secs, large parts, up to a minute or two).
4. Clamp off the resin line.
This sequence goes against logic slightly for an infusion but what it does is allow a little extra resin into the laminate, probably decreasing the fibre fraction from 60:40 to maybe 58:42. This subtle amount of surplus resin does wonders for the pin holes you're experiencing and is these steps are pretty-much standard for the professional infusion guys we supply.
I hope this helps; let us all know if it does!
Matt