If it won't go down you have air bypass someplace. Either a seal is broken (likely on the piston inside) or in some cases you may have your guage on the wrong side of your regulator (Done that on my bob long torpedo before).
Its not a tank problem, 600 is about what they dump depending on your propellant of choice and tank reg used.
I would remove your reg, disassemble it, and look for any bad seals and replace as necessary- All your necessary o-rings should be able to be found at home depot. While your there get yourself a tube of dow33 and re-lube before assembling. Dow33 is the same as sl33k and alot of the other non-oil lubes and will not have a negative effect on your marker.
'07 Angel G7 Joy Division
'05 Dye DMC /w X94 bolt
'05 WGP Black Magic /w MQ2
'03 Dye Matrix /w X93 bolt
'01 Shocker Sport Turbo /w old school Maxflo 4500