Hey Frannie I have never used a pendulum to cleanse cards ...
so would not know what to say..
If my cards did need cleansing I normally would blow through them, visualising a white, purefying, positive air flow coming from my mouth and penetrating through , in and oaround my cards , dispelling any left over residual energy of whomever or whatever, and ridding negative energy.. I do this with my crystals too!
So maybe the pendulum would work in a similar way, instead of your breath, allow the swing of the pendulum dispell the energy you seek to rid.. through also the visualisation of it all going as well..
Just a thought