Originally posted by GalacticPhoto Auto-ISO should still function in manual mode. I could use this lens on my A7r in manual with Auto-ISO, this seems more to be an issue with Pentax not doing things the right way.
Actually it's because people want Manual to be Manual. Pentax has given auto-iso to the K-30/K-50 and I think the K-01, but people complained that wasn't really Manual. So they don't give it on the K5, K5ii, K3, etc. You can use other modes with auto-ISO which is what may be the issue - the OP may have had auto-ISO with a K-50 or K-30 and want to use that method again. Essentially as I understand it - he would like to set Aperture on the lens, then press green button and set Shutter speed (make adjustments if needed) and then allow AUTO ISO to float to keep the exposure accurate to what it was originally set to.
If I recall TAv is what approximates M with auto-ISO. Not sure if it works with green button.