Extruded polystyrene (insulation sheets) does not have enough compressive strength. This density styrofoam will crush at 20 psi.

http://www.dow.com/PublishedLiterature/dh_0053/0901b80380053c3d.pdf?filepath=styrofoam/pdfs/noreg/179-08040.pdf&fromPage=GetDoc

For the piston you need to use a much ...

Has anyone found a USA supplier of pressure-rated ABS? The only stuff I can find is from UK or Austrailia. I don't want to pay ridiculous shipping and VAT taxes if I can help it.

Also, in your opinion what is the maximum reasonably safe pressure for 1.5" sch 40 PVC rated for 330 psi, with a 150 psi ...