You may have bad flex hoses, plugged brake bleeders or bleeder ports or stuck/plugged proportioning valves.

If your flex hoses appear to be original, you should change them.

You can sometimes clear the bleeders by removing them and cleaning. Sometimes the port behind the bleeder is plugged and sometimes can be cleared with a stiff piece of wire.

If the proportioning valves are stuck, you may have to source replacements. I have never had to service that style.

IIRC, 1973 was the 1st year Volvo started using sealed non-rebuildable proportioning valves. 1972 and earlier have proportioning valves that can be easily disassembled and cleaned. I can still get reseal kits for the earlier style.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com