is this the lens you have?
Pentax Lens Review Database - 135mm F2.5 Bayonet
if so then you can disregard Dave's post about m42 (most of us here associate Takumar with m42.)
Quote: It always meters 2 stops faster than it should.
older takumars (m42) usually have to have the exposure bumped up anywhere between 1.3-2.0+ so adjusting the exposure is no surprise with lots of lenses but I don't normally see too many people having to do so with a K mount MF lens. I don't have to adjust exposure on any of my M or A series (in non 'A' mode) lenses.
this is what I do when using a manual.
set the camera to M
I leave the aperture wide open so I can compose the shot.
I set the aperture to what I want to use
I point the camera at what in the scene I want to use to meter the light
(I use spot metering by default anyway)
I press 'AE-L' and the camera will stop down the lens, take the reading and set the shutter speed
(if the selected shutter speed is too slow the camera should give you a warning by blinking the shutter speed in the viewfinder)
I press the shutter
as far as I know you cant use the Av button next to the on/off switch to adjust exposure on the fly in M mode like you can in Av so you cant compensate like you can when using a takumar (m42) with adapter. if you are able to compensate are you trying to use Av or M? Av will only give you a correct exposure with the widest aperture. to be honest the lens should not be underexposing if its a K mount so im a bit stumped on that.