It's probably just dirty. The pivots can get glommed up being down there.

Take the R wheel out, clean out/off the entire brake mechanism, and spray a little WD-40 onto the pivots (cover/protect the pads while doing this), move it thru its full range of motion several times, using your hands to pry it back open each time.

Make sure you wipe it down again, reinstall R wheel, and you should be good to go.

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float , hammer , and jog

I've had the same problem on my rear Magura RT 8 on my P5. I did what was indicated above. Took cover off, cleaned up the brake and then oiled the pivots. Worked fine after that without sticking.

Yea that worked thx.
Funny I tried cleaning and oiling before I posted. But I guess letting it sit for a couple of hours did the trick.

I have a similar issue with a brand new P5. First ride I noticed a rear brake rub. The right "arm" of the magura caliper isn't retracting all the way back. I can move it back by hand but it won't retract on it's own. I took the rear cover off and it started working. It also works when I loosen 1 or more screws that hold the cover in place. I then looked at the cover and noticed a rub spot where the tip of the brake caliper is touching the plastic cover. I've included the photo. Anyone else had this issue? I think the brake caliper is properly installed. Is there a chance that there were supposed to be some washers that needed to be installed to create some space for the caliper to move freely?

There are no washers between cover and frame on a P5, but I have experienced the brake arm rub on a few and so i do space the cover off when needed, you dont need much clearance, and yes, i've built 'several'

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'to give anything less than the best is to sacrifice the gift'...Pre

Do you have washers between the brake itself and the frame? if so, try and use a smaller one, to make sure you don't get contact between the brake cover and the rear brake.

Thanks for your reply Jensentri.

There appears to be a washer there. I don't build the bike so I'm not sure what thickness the washer is. I'm pretty sure the LBS used what was sent from Cervelo. I did notice that the brake caliper is pretty close to the frame (2-3 mm) so I'm pretty sure the washer is required and going down to a slimmer one might be an issue.

Thanks for your reply undies.

Was the rub at the top for the caliper of was it rubbing the arms at a different spot? Where did you get the washers? Did you use plastic, stainless steel or something else? What thickness?
Thanks

rub was on the 'top' forward protruding nubbin, if we use the front brake as a reference. I just used some washers from my drawer of 'useful stuff', about the same size as the rivnut you're bolting into, only about 1.0mm and i've probably only had to do this 3 or 4 times in over 60 P5/6 builds

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'to give anything less than the best is to sacrifice the gift'...Pre

Thank you for your response. I will try your recommendation. I also contacted Cervelo and they will send me a new cover.