You're absolutely right about your answer, that the warning light is supposed to be based on a difference in pressure in the two triangular circuits, because the car in question was reported to be a 1990.

I just want to clarify, only for the sake of those readers who have a '92 or '93 car, that theirs is the exception -- i.e., your answer applies to those readers only with pre-ABS 240s (up to '91).

If you've got a ABS 240 ('92 and '93), a pressure comparison between two circuits is not possible because there are no longer two independent circuits -- ABS precludes that. Instead, for these cars Volvo had to resort to the more mundane or conventional warning based on mere fluid level -- like most other cars, the brake fluid reservoir has a fluid level switch, and this is what sets off the warning light.