Canon cameras (as most DSLR cameras these days) meter and AF wide-open. There is a limit on the maximum aperture wherein metering and AF can perform properly, and a cutoff where they are unlikely to work at all. In modern Canon cameras, the metering sensor is up in the viewfinder housing, just above the eyepiece you look through. At narrower apertures (lets say smaller than f/8), the amount of light lost between the primary mirror, the focusing screen and the pentamirror/pentaprism is probably so much that the metering sensor has too little light to work with (well less than 50% of the light that originally entered the lens).

Metering sensors are usually rated in a range of exposure values. I think all of the cameras you listed are rated to work between 1 - 20EV and a certain temperature (roughly room temperature, although the exact value sometimes changes from camera to camera.) The 5D II might actually work from 0-20EV. Top-end professional grade cameras and newer mid-grade professional cameras can operate from -1 or even -2 EV up through 20 or more EV, and therefor have up to three stops more low-light metering capability than your average entry-level.

If you stop down your Contax lenses enough in poor enough scene lighting, you might be reducing the EV at the metering sensor below the minimum of its operating range. Since the metering sensor is designed to operate wide open, before an electronic aperture is stopped down for exposure, if you are working in weak light with a stopped-down manual aperture, you'll make it even harder for the metering sensor to operate correctly. You might simply try half-pressing the shutter with the aperture wide-open, meter, use the AE-Lock function to lock in exposure, stop down, and fully press the shutter button to take the photo at the previously metered settings.