Prime takes it's feed from the reserve intake. which is just a hole in the petcock casting.
The main feed is from a tube.
If it's not flowing in Prime but runs fine on Main then it sounds like yer reserve feed is blocked.
The petcock is easy to remove and take apart.
Remove the tank from the bike.
Empty most of the gas from the tank.
Put the tank on a towel on it's side.
Unscrew the 2 bolts and remove the petcock.
There should be a plastic and mesh shroud fitted over the tube.
Inspect the mesh for any tears or splits.
If it's OK, put a hose on the fuel fitting, set the petcock to Prime and blow in the tube.
Any crud in the system should get blown out into the shroud.
If there was crud, then I HIGHLY recommend buying a new petcock.
The rebuild kits are crap and usually fail after a few months.
A new Petcock will give years of problem free operation.
Murph sells 'em for $73
http://www.murphskits.com/catalog/product_info.php?cPath=1_94&products_id=140