Pagerank works on backlinks no matter if it's Parked or not.
But PR doesn't affect google search results position.
So-called Trustrank is what determines Your position for keywords.
But no matter how high Your trustrank is, google will penalize Your domains when You park them. Penalize means if it was NR 35 result in google for "furniture", after parking it will be something like NR 400 bringing You almost none traffic.
PR doesn't decrease with Parking, You are only losing reciprocal links (if Parking expired sites).
Most of the Pagerank loss is done by moving backlinks from mainpages (like You have links in comments of BLOGS, and after a while, with new posts comming, You'd go to page 2 as main page contains for example only 5 latest POSTs) and You're moving to subpages which has usually lower pagerank.
That's all. If I'm mistaken, please tell me, I like to learn :D