I can't respond technically to the OP's question, which, from the description, sounds like a more involved mechanical repair.

However, i wouldn't hesitate to recommend sending the camera/back in for a thorough CLA - especially if it's a camera you're very familiar with and enjoy using.

I've had CLA's done on a couple of my 35mm all-mechanical cameras and what an amazing difference it made. My RB67 Pro-S had been serviced before i bought it, but it's on my radar to send it in in the next couple years to tidy everything up - it's easily worth the cost to me to ensure my camera is 'running' smoothly.

When you buy used gear, you buy a ? and each "new" used camera is a "new" unknown. Considering the professional quality of a Mamiya RB67, a CLA should give years of flawless, perfect use.