Which chemicals could be used to dissolve Polybutylene Terephthalate (PBT), but not dissolve brass?
PBT is a thermoplastic and its melting point is only 223 °C. Depending on its composition, brass doesn't melt until 900 °C or so. High enough heating may change the structural properties of the brass, but perhaps there is a temperature to which you can soften the plastic enough to detach the pins without damaging them. 

Unfortunately, brass is easily attacked by many chemicals and PBT is resistant to many solvents, but something like hot DMSO or N-methyl-2-pyrrolidone may swell the polymer enough to separate the parts.