old pentax lens on k10d metering issue

Hello
have just bought a pentax 300mm k fit lens not A or M as my other lenses.
I have altered the k10d to allow aperture ring however it works in aperture priority mode,but in manual it will not give me the meter reading in either screen.Is this normal? or is there a way to enable the meter to show.currently I am having a guess seeing the result and compensate manually
What are your experiences
Thanks

What make and model lens is it exactly?

Normally with Pentax K and M lenses (without an "A" on the aperture ring)...............

Av mode only works with the largest aperture.

With Manual mode, you need to select the aperture from the lens ring. Push the Depth of Field Preview lever to get a reading, or the Green button to set the shutter speed according to the meter. Unfortunately the stop-down metering doesn't work well and over-exposure is common, especially with smaller apertures. Manual compensation is then required.

If none of this works then perhaps there is something wrong or incompatible with your lens.

Its a 300mm f4 pentax smc .I think this was the lens before the M series
will try your suggestion
Thanks

It should work in the same way as M series lenses.

Good luck.

it will be interesting to see how the Pentax 300mm performs with the K10D. I have exposure problems with my shorter pre-A lenses, but both of my 300mm primes (both Tamron, one F5.6, one F2. meter more or less correctly.

Thanks
yes works when I press depth of field lever but overexposes by 2 or 3 stops.
Still that gives me a rough indication