mirror slap

i have a Pentax K-r i have various lenes both auto focus and manual, the problem that i am having is the dreaded 'mirror slap' but only when i am using a 500mm with a t2 mount.Why? any ideas.
Cheers for any help Garry

What are you actually meaning by mirror slap?

the mirror starts to slap closed at the press of any button, dial or switch without letting me take a picture, and the only way that i can stop this is my putting on a 'standard' pentax lens

Thanks, I was wondering if you were talking about vibration from mirror slap, which is something else entirely.

Have you activated the aperture ring in the menus? "Using aperture ring" needs to be set to "permitted". Then the next thing is making sure all electronic contacts are clean, using a soft cloth. Obviously not on a T2 mount, but if it's a metal mount is it shorting out a contact perhaps?

Hope something along these lines might help.

i'm also going down the line of a short circuit but when i use a lens without the contacts i don't get the slap, its only when i use the t2 mount, so my first move is to try a new mount and the see what happens

Good idea, if that's the only difference in what you are doing it might somehow be the culprit.