If a rim is not a round circle it will appear to move up and down as it passes through a rim brake or goes by some other index point. You can true a rim by adjusting the spoke tension.
Remove The Tyre And Tube
You can’t true a rim wich is non-circular rim without removing the tyre and tube.
Spread The Load
Tighten in the middle more than those at the end of the misalignment. Make small adjustments don’t give any spoke more than a half turn before checking and re-diagnosing.
If the rim has a dip loosen all the spokes under that point.
Make The Test Harder
As the rim improves move the index point to find a new position where it drags on the rim. Take your time and have patience, there is an art to being able to true a rim.
Stress The Spokes
Between each round of adjustment and re-diagnosis stress the spokes using the procedure checking spoke tension and squeezing hard. This releases any twisting forces trapped in the spokes. If you don’t do this the forces will be released the first time the bike is ridden and the wheel may go out of true straight away. This stressing process is the main difference between a hand-built wheel and one built by a machine.
If you can’t get the rim straight without very uneven spoke tension the rim may be damaged beyond repair and need replacing- this is an example of when you cannot true a rim.