Use DOT 4. DOT is DOT 3 with added drying agents using something with Boron in it. I forget, yet should be compatible with your (what year?) PV 544 and the seals therein.

Or if DOT 3, remember to flush new DOT 3 every two years or so.

Use a Motive or like power pressure method to push the fluid. Observe the brake bleed order. Maybe solicit advice on what PSI of pressure to bleed at.

I have a 65 544 and use DOT5 (silicone) as I completely rebuilt the stock brakes with new cylinders brake lines etc. If I have a leak or spill it doesn't hurt any painted surface. It works fine and the car stops straight and quick enough.
I am sure the cars as they were built used DOT3 or a lesser grade as nothing better was readily available. Unless you are racing or live at the top of a mountain, DOT3 will work fine. Tom