Might just be the gasket between the booster and master cylinder..... that's where I'd start, even with caulking/sealant to see if that's where it leaks. Think about finding an air leak in the power brake system.

Diesels are deceptively tough on brakes because they continue to combust and propel the car forward even when you take your foot off the accelerator. Practice gliding up to red lights and stop signs, you will see this is true.

And buffing old paint as you have done is better restoration technique than repainting the entire car, imho.