I'm having trouble getting my rear disc brake set up on my DT build. (I've got the front working perfectly) I've worked very hard getting the caliper straight. When I adjust the inside and outside to where there is no rub, the brakes are very weak. The rub is intermittent, which makes me think the disc is not straight. I'm thinking about removing the disc and testing it to see if it perfectly flat. (or maybe I didn't mount it right)

Has anyone else had a disc that wasn't true?

you might get better and morer responses if you mention which brake you're using.
maybe say how you got the caliper straight....

two things you might check.

1. is the wheel correctly mounted in the dropouts?
2. are all the disc bolts tightened equally?

***if you've got a BB5, the business card trick works great to
get the caliper parallel to the rotor***

https://www.ecovelo.info/2011/04/15/a...5-disc-brakes/

The rotor could be out if true. Set it up so its rubbing, look down in the caliper to see what side it is rubbing on. You can use a small adjustable wrench to lightly flex it back in place.

The caliper is a little tricky to setup on the trucker because of its location. I find it easiest to loosen the caliper bolts just enough to let it move a little. Then hold the lever while you tighten the bolts. This will get it very close but you may have to fine tune it one bolt at a time. Loosen a bolt, eyeball where the caliper needs to go then tighten it, then move on the the other bolt.

Make sure that the arm on the caliper where the cable attaches is all the way out. I've seen many people have this too far in so there is just no braking power, the lever feels great but the arm is bottoming out on the caliper. For any other adjustment use the pad adjusters.

Hope this helps.

I would also point out that due to manufacturing tolerances and what not that many disc brake rotors aren't true and sometimes trying to true them can make it worse especially when minor. Other than that what "NoBrakeNate" and "saddlesores" are reasonably good suggestions (aside from the wrench bit as per what I said above)

A better question would be has anyone else had a disc rotor that was true. I've owned a lot of disc equipped bikes and never had a disc rotor that didn't need some adjustment. Rotors have to be much truer than bike wheels and they seldom are.

To true the rotor, leave the rotor on the wheel and keep it mounted on the bike. Sight down the gap between the pads in the caliper and see where the wobble is. You can use a rotor tuning fork or even a large cresent wrench to gently bend the rotor to trueness. Make only small corrections at a time and constantly check the wobble against the calipers.

Thanks all. I talked to the bike shop that is helping me with the build and was told they had a tool to true up the rotor. (probably a brick and a hammer) I plan to finish the build, then take it to the shop to show off and get the rotor trued.