I had the same problem many years ago on my PV544. It would pull to the left, and then to the right on braking. Slow stops weren't a problem, but when you needed the brakes, you were all over the road, first left and then right.

I turned out the front brake shoes were mismatched. It had one kind of lining on the left side, and another kind of lining on the right side. The coefficients of friction and fade qualities were different between the two sets of shoes. On a 544 the leading and trailing brake shoes are the same, so I made one set of shoes the leading shoe on each side, and the other set the trailing shoes. That completely fixed the pulling problem.

Drum brakes are more sensitive to shoe mis-match and contamination since there is a leading and trailing shoe. The leading shoe sort of "boosts", or adds to the force on the trailing shoe.

I would inspect the shoes very carefully for any kind of mismatch and contamination. As planetman wrote, it is a single cylinder brake system and the hydraulic pressure will be the same throughout the system
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john