First question is...what makes you think it's the seal between the 2 halves? The most common leakage point is actually either using old PVC cement (or something other than PVC cement) between the ballast tank base and lower pressure hull or ballast tank or inattention to detail while assembling those parts.

Other areas to check:
Did you lubricate the pushrod seals?
Did you sand the hull before siliconing the seals in place?
Did you use silicone to secure the seals?
Is the ballast tank top completely sealed?
How about the antenna hole?
Or the screw holes in the pressure hull top?
Or the pump inlet/outlet hole?

Check all those carefully.

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