Does having the tail lights on or off change the brake / signal lamp problem any? You have switched the bulbs right to left? Checked the lenses for clean and hold the lenses up to the light
Very first thing I'd do is check grounding. If you can't do anything else, scrape / sand the shell of the socket and (at least temporarily) clamp a wire around the shell with a hose clamp, or solder if you know how, and run a dedicated ground wire, such as over to the trunk lid latch mount.
If that does not do it, make sure the socket contacts are clean and the sockets are clean internally, and that the springs are nice and tight. If that is no help, move up front to the left kick panel, and dig out the connector, where the rear harness disconnects from the instrument panel.
Work that connector in / out several times. Get your voltmeter. Hit the brakes, If alone, figure a way to jack the pedal with a scrap of wood, etc, from the dash lip, and check the brightness is still unequal.
If so, check the voltage at the left and right signal wires at that kick panel connector. If they are different, move up to the harness connector at the turn sigal switch, work it in/ out several times and recheck voltage. Check voltage CAREFULLY on both the left and the right sides, and on both sides of the connectors. If this is the trouble, they should be different.
If they are different coming "out" of the turn signal switch, the trouble is in the switch itself.
