Help with power steering

[tein]

My power steering cap/dipstick is leaking where can I get a GOOD replacement cap? Or is there a reason why it's doing this? i.e. too much pressure in the system. Another wierd thing is I can not seem to completely get the air out of the lines. I turn the steering wheel back and forth while watching the bubbles shoot out of the reservoir

Update: Anybody know why my reservoir is becoming pressurized? I placed a plastic bag over it, along with the cap and I hear a low squeak. I remove the plastic, the squeak goes away. Your help would be GREATLY appreciated.

[DocWalt]

You have a small leak somewhere. It's easier for air to get into the system than it is for fluid to get out.