My voltage multiplier (cockroft-walton) will not charge my capacitor bank past 150v. How can I fix this?
Since your high voltage source apparently outputs 500 V open circuit, but only charges your capacitors to 150 V, it's pretty clear your capacitors are leaking.  This may be the reason they were available for free.

Try just charging up one of the capacitors and see how high it gets, then add the other capacitors accross it incrementally.  Perhaps only one of them is leaking excessively.

Another issue is measuring such high voltage.  What is the impedance of your voltmeter?  If it's 10 M&Omega;, for example, then just the voltmeter alone will draw 50 &micro;A at 500 V.  How much can your high voltage source put out and still maintain 450 V?

If the voltmeter is the culprit, then allow the caps to charge without the meter connected.  Wait a while to make sure the voltage has leveled off, then connect the meter and quickly look at the reading.  In the short term it will show you what the caps got charged to, even if that voltage goes down as you keep the meter across the caps.

Another thing to do is to carefully clean all the high voltage caps.  Dirt can cause significant leakage, especially at high voltage.  I don't know what your caps are made of, but probably mildly soapy water won't hurt them.  Get everything really clean, then rinse with distilled water and let it completely dry before powering up again.  Let everything sit for 12 hours or more in a warm and dry place after all noticable moisture has already been removed.