My K100D has a split prism screen, which aggravates the preset lens metering problem (perhaps because the screen's average central brightness increases strongly as f-stop increases, a geometric optical effect concentrating light in the center.)
Metering is improved if mulitzone metering is used (non-central parts of the screen are also used for metering.) Multizone metering is possible if the camera thinks the lens is an A type.
The camera can be fooled into thinking all lenses are A type by shorting out the A pin on the camera mount. This has the added advantage of making flash work properly with preset lenses.
I used a P&S camera to photograph the viewfinder' appearance for various f-stops with my K100D (brightness graphed with ImageJ):
(some of the vignetting is due to the P&S camera)
The center's relative brightness is much higher for higher f-stops; this may be the root cause of exposure problems with preset lenses. It demonstrates why using multizone metering helps.
Dave in Iowa
PS I'm not recommending you modify anything, just providing info about what might be going on.