PS pump help!

[Texasbigblock]

Hey everyone,
I'm in need of some help with my power steering. My power steering pump on my truck whines non stop. I've bled it by turning it lock to lock and just can't get the whine out. Also, the pump builds up extreme pressure to the point where it'll blow the cap off. The fluid always is foamy so I figured I must be getting air in somewhere but the pump doesn't leak a drop when the truck is off and sitting so It seems like there aren't any leaks in hoses or anything. Anybody have any ideas? The ps pump and hoses are only a couple years old. I've taken out the valve and checked it and everything is working fine there.