Craig Dickson , Sep 30, 2010; 10:24 a.m.
Please be more specific about exactly how you are doing this.
When your Pentax lens is mounted on your 5D Mark II, does the camera display an aperture of "0.0" in the viewfinder, or some other number? If it says "0.0", then the camera is in stop-down metering mode, but otherwise, it isn't. If it isn't, then you need to set the aperture on the camera as wide as it will go, and leave it there. The trick here is that with a non-EF lens, you are really always doing stop-down metering because the camera has no way to control the lens' aperture, and for accurate metering you need the camera to think the lens will be shot wide open regardless of the actual aperture you're using.
I've used Nikon and Pentax lenses on my 5D Mark II, but not in Live View mode. Using Canon's Eg-S focusing screen, I find that I can get sharp, accurately exposed images using the viewfinder by focusing with the lens wide open, then stopping down to meter and shoot.