Uneven scales can result from problems other than lack of sheer strength (fine muscle development). The crossings with the thumb often can lead to "bumps" if you are twisting your wrist at these points and/or throwing your elbow away from your body. Keep totally relaxed and your arms should hang loosely and close to your body. This is the starting point. Obviously, your arms will move away from your body when necessary, i.e. the right hand playing in the high treble register while the left is playing in the extreme bass register. Otherwise, relaxed arms, elbows close to the body.
The movement in scale work is horizontal with no (or very, very litle) wrist-twisting to accomodate thumb-crossings. Try practicing each hand alone for starters and just do a five-note scale: CDEFG (in C Major, to start), ascending 12345, then, DEFGA (12345), DFGAB (12345), all white keys, and go up a few octaves, then descend the same way. Do the same exercise in G Major, then, D Major etc. and done slow enough that you can play them evenly. This will give you an idea of finger weakness without the complication of thumb-crossings. Do them slow enough to play them evenly, then pick of speed. Actually, when you do add thumb-crossings, you still want to experience the scale run as if it's just one group of five or four notes, followed by another, in a smooth movement up or down. No thumb or wrist bumps.
Do this slowly for the next 200 years or so, relaxed, and all will be well. Hopefully.