You should have charged w/ two 6 oz cans. If you used 3 cans, that might have over-filled. I just cleaned and re-installed the R4 in my 1984 300D. There is a cir-clipped disk, that I assume has a spring-loaded relief valve (PRV) behind it, so sounds like your hypothesis is valid. In my other 300D, I put a Sanden w/ "GM Pad" head (to fit car's hoses) and added a screw-in PRV.

Don't go by the sight-glass. That is for R-12 and only a sanity check for at that. Judge by the pressures. I usually see 70 - 90 psig on the low-side w/ engine off and equilibrated, depending on outside air temp (say 80 - 105 F). With Duracool (HC), I usually see 120 - 190 psig high-side when compressor is running, again depends on outside temp.

Also, insure your electric radiator fan comes on when idling. If not, the condenser outlet gets almost too hot to touch and your AC won't cool well. The temp switch on the filter/drier often gets erratic. You can swap it without breaking into the system, and they are inexpensive here on Peach Parts. I tested 3 and all switched on @ ~83 F (found one was erratic). Alternative is do RollGuy's "relay mod. so always on w/ compressor".

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans