Proportioning valve leaks

First you must determine WHAT fittings this device has. Is it US or metric? Pipe thread or other? Brake tubing for older US cars is ALWAYS "inverted flare." That is not just a nickname, it is a specific fitting.

If the valve has pipe thread, you must have inverted flare --to--pipe thread ADAPTERS screwed into the device, and THAT thread MUST have sealer

The inverted flare fittings on the ends of the brake tubing NEVER uses sealer of any sort. They seal directly with the flare seating

Next, did you flare this tube yourself? Did you perhaps use the wrong flare tool? Inverted flare and SAE flare are both 45 degree flare angle. JIC and AN fittings are 37 degree flare, and they are distinctly separate tools.