Compare the sealing ends between the new and old bleeder screws. The sealing is not done with the threads. There's a formed tip on the screw that seats against a matching seat in the calipers and wheel cylinders. That's where any leaking is taking place.
Bleeder screws often get rusted tight, so many competent do-it-yourselfers put grease on the threads. That is a very bad idea. Grease will migrate around and find its way into the brake fluid where it will become contaminated. The slightest hint of any petroleum product in brake fluid will cause the rubber parts to swell. The only repair for that is to remove every part that contains a rubber seal or other rubber part that contacts the brake fluid, flush and dry the steel lines, then install all new rubber parts. That includes calipers, wheel cylinders, master cylinder and the bladder seal under its cap, rubber flex hoses, combination valve, and when used, a rear height-sensing proportioning valve and ABS hydraulic controller. Obviously that gets to be a very expensive repair. The better approach to rusty bleeder screws is to use a rubber cap on the end to keep water from getting inside the bleed hole. Water getting into the bleed hole is much more responsible for rusted threads than any water getting into the threads directly.
Tuesday, March 16th, 2021 AT 5:30 PM