I just swapped the rear tire on my sons bike (2000 KX-65) now he has very little rear brake. The disc does not appear bent as I took pre-cautions by setting the wheel on blocks so the disc would not hit the ground. I was also careful not to get any grease on the disc when I lubed the axle and bearings. I bled the brakes with new fluid but still you have to put maximum pressure on the brake lever to slow down. Any ideas?
Did you have to open the caliper , pry or anyting to get the disk back in? If so it may take some use to get the pads pushed back out to the disk, also make sure that you didn't loose a pad
It's not uncommon to force dirt past the square inner seal (square edged o-ring) when retracting the piston. The square seal works by twisting under pressure and untwisting when pressure is released so the piston can retract slightly off the disk. If dirt gets forced past the seal it can damage the square seal and significantly reduce the pressure applied to the piston. Most calipers have a dust seal that can be pulled back to check for leaks.
If everything looks good, take a close look at each end of each pad & makesure it is not hung up on a portion of the caliper bracket.
My RMX did that to me all the time. Everything looked right, but the outer pad would be hung up on the hanger.