... if you want the 300D to work better than the 240 that has 134 in it, put 12 back in.
if not 12, NAPA stocks freeze 12, which is a blend of 134 and something else to give close to 12 performance.
did you totally flush the lines, AND the condenser, AND the evaporator? with a/c flush kit?
www.dieselgiant.com has a great pictorial on this.
I would start there. oh, and a 12 year old filter/drier would be junk anyway... they don't keep long.
you should replace the expansion valve while you have it open as well. it's all explained in the diy on DG's page.
ebay and craigslist are great sources for R$12, I got my last 30LB jug for free!!! (well, I serviced the guy's Home A/C in trade for it. not a bad deal, 1.5 hours "inspecting" his system, and a pound or two of R22 and he was thrilled! and I walked away with a 34Lb jug of 12!