Do you still have the in line pressure switch? There are two terminals...12v in and the other is out to the brake lights. The fault could be internal to the switch.
I'd first test that the 12v to the switch is Ok......using a test bulb. If the bulb lights....fine. If that blows the fuse, then the fault is upstream, but I doubt it will.
Then put the test bulb on the out terminal, with the brake lamp wire out of the way. If the bulb lights as a brake light, the fault is downstream towards the lights. If the test bulb blows the fuse, it's the switch.
If that checks out, I'd remove both brake light bulbs, and test the circuit. If it still blows then it could be either side and it gets harder to pin down unless you can trace where the brake lamp wires split to each side.
