Yesterday i found out that my Pentax 6x7 got some problems with slow shutter speeds from 1/125 and over, the shutter and the mirror stucks open forever.
Like this:
So i found a not so well explained solution online and this is my guide:
1- Gently remove the leather from the left side (the side with the 6x7 print)
2- Uscrew the 2 circled screws DO NOT UNSCREW THE THIRD.
3- Unscrew the two circled screws inside the focusing screen zone
4-Gently remove the piece by pushing it up and forward
5- Unscrew the two screws
6- Remove the piece
7- Cock the shutter, remove the battery and shot to open the contact of the magnet and clean it easier.
8- Clean the contacts with a small tissue paper wet with some contact cleaning detergent sticked to a thin screwdriver to reach every zone.
9- Test
10- Close everything
11- Stick back the leather
This would solve your problem, hope that helps!