Have a look at my reply to the question in this thread:
http://www.cnczone.com/forums/machin...s_warping.html (303 stainless warping)
All you need to do is substitute 'brass' for 'stainless'.
When you are machining the outer layer off any material that has been cold worked, that is squeezed between rollers to the final dimension, you are almost always going to get warping. This applies to stainless steels, alloy steels, brass, copper, aluminum alloys, etc. The deformation of the material during the rolling process creates different stresses in the surface of the material compared to the inside. These stresses are balanced but when you remove material from one side they become unbalanced so the material warps.
Without annealing it is difficult, sometimes impossible, to avoid or correct this warping but sometimes annealing is undesirable. In the case of brass it reduces the yield strength markedly so there are techniques you can try:
Start with material 1/16", or so, larger than you need.
Face off a skim cut of about 0.020" from one side. When you remove the material from the vises it will probably warp.
Put it in the vises with the machined side down and hold it down 'perfectly' flat as you tighten the vises.
Face off the skim cut, same amount from this side. When you remove it from the vises if you are lucky, the warping will be very small.
Turn it over, hold it flat and take off the remaning 0.010" from the first side.
Do this on the second side and you will have it about as flat as you will get it.
The surface stresses are normally not very deep and removing 0.03" from each side may remove enough stress that the material will now stay acceptably flat when the final machining is done. However if you now start machining pockets you may find some warping occurring again. And the direction of the warp can depend on how deep the pocket is, shallow will go one way and deep the opposite.
When you have had a bit of experience machining cold worked bar stock sometimes you can plan your overall machining sequence to balance the stress removal on both (all) sides and finish up with an unwarped part. Sometimes you have to resort to straightening it after machining. The important thing is to always make sure it is clamped 'perfectly' flat so the dimensions come out correct.