Bleeder screws leaking

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.

Same story. If you're replacing bleeder screws, compare the old sealing ends to the new ones. I'm not aware of the ends being anything other than a common, standard sealing end, but replacing bleeder screws individually is not a typical job or one that I've done. I've cleaned blocked screws, installed repair kits that come with their matching seats, and replaced wheel cylinders. I do know you can buy individual bleeder screws, but I've never paid attention to whether there's different ones other than diameters.

I think a better suspect in this story is rust spots on the sealing surfaces in the wheel cylinders and calipers. Those were extremely common on the Ford-built Escorts and Tempos in the 1980s. Moisture naturally finds its way into the brake fluid, which is why all manufacturers have a time recommendation for replacing brake fluid, even though few of us actually do that. The water settles in the bottom of the bores, mainly in the wheel cylinders. Those rust spots resulted in leakage past the rubber lip seals, but it also showed up on the sealing surfaces for the bleeder screws. Those remained sealed until they were disturbed during the standard brake job. It used to be standard practice to rebuild wheel cylinders and calipers, (today it's much more cost effective to just replace those assemblies).

Remember, the threads are not where the sealing takes place. They only hold the bleeder screw tightly against the seat. I suspect it's those seats that are degraded. They do make repair kits, but those are mainly for rare vehicles when replacement parts aren't readily available. For most models, you can find wheel cylinders for less than ten dollars.

As far as air getting past the threads, that isn't likely to happen since the brake fluid is heavier and will flow down. What DOES cause concern for many people is when they use a vacuum-powered bleeder, it will draw air in past the threads, then out the through the bleeder hose. That hose is usually clear so you can see those annoying bubbles. The clue is if you're still bleeding air out of the lines, those will be big, long bubbles. Air that sneaks in past the threads will produce a steady stream of very tiny bubbles. Those can be ignored as they will no longer be there once the bleeder screw is closed.