Buy a new metal radiator.
Don't buy a Volvo radiator. That Da!*
Plastic tank radiator has two common failure modes. Your problem and the
engine killing split all the way down the side. I have had resonable luck with
after market Radiators. No leaks so far an lifetime guarantee.
79 245 D,
81 240 D,
86 240 320,000mi and running fine when traded
94 944 Green 257,503 mi CRUNCH Parted out
94 944 White 149,000 The New one
94 944 Maroon 135,000 mi