Contact Hasselblad USA in New Jersey to see if they still offer the speck to check camera service. Let Hasselblad figure out which backs need to be repaired. Your 500c body if it has never been serviced might require a complete overhaul, rather than just CLA. Send them your lenses as well. It will save you time and effort in the long run to get everything serviced at one time.
I sent them my Hasselblad kit which consisted of a 1962 500c, and C12 back from 1960, and a 80 mm C Lens from 1958. Four weeks and $500 later I received back a wonderful operating camera kit. The camera was covered for 6 months if I had a problem with it.
If you are wondering why I didn't use David Odess instead, Mr. Odess doesn't really like working on Hasselblad equipment that was made earlier than 1960. I had a good conversation with him about the camera, but since the lens and back were so old I just sent it off to New Jersey instead. Let me add that after talking to Mr. Odess on the phone for 20 minutes he did tell me to send in my camera to him, and he would have repaired it. David seems to be a good guy.
Get your own stuff fix, don't buy more stuff from KEH that will have to be fixed too.