Here's another thing to try in order to avoid the "thumb bump" that can make scales uneven. This is for RH going up or LH going down. One thing that causes the "bump" is that you move the thumb quickly to get it into position, passing it under the hand, and you use that same motion to strike the key. That tends to make it too loud. So try this. Do the scale slowly, very slowly, but when you are going to move the thumb under your hand, make that motion of the thumb quite quick and then stop the thumb over the note you will play without actually playing it, then strike the note with the appropriate volume. If your scale is slow enough, you can do this in perfect time. So everything is slow, except the motion of the thumb positioning itself over the note. Doing this will force you to separate the movement of the thumb into position from the actual striking of the note and give you better control of the volume you produce with the thumb. Of course, when you speed up the motions will blend together again, but you will hopefully have learned how to control the positioning of the thumb separately from the striking of the key.
All the things that others have said about weight, and controlling the sound with the arm are important, too.