Brake Caliper Bleed Screws
The bleed screw on the left is an original one from Kirby Palm's 1983 XJ-S. The bottom of the port that this screws into is tapered, but it's not tapered as steeply as the point on this screw. So, when the screw is tightened down, that point is jammed into the hole in the center of the bottom of the port. Since this results in a small contact area, tightening the screw very effectively plugs the hole.
The screw on the right came from Gran Turismo Jaguar in an inboard rear brake rebuild kit. The threads are correct (3/8"-24), but the only way it's going to seal the port is if the edge of that flat bottom sits properly on the tapered bottom of the port. It might work, but it'll require either luck or overtightening.
The bleed screw works just fine when you toss a 3/16" ball bearing into the port before inserting the screw, though. The ball sits right down in the hole, and the flat bottom of the screw jams it down securely. It doesn't get stuck, either, which would be bad. It might work as a check valve when bleeding, which would be neat.
Note that these screws were used on the rear brakes on Palm's car, in which the bleed screws point upward -- so the ball would be expected to fall down into the tapered seat before the screw was even installed. The bleed screws on the front brakes are horizontal, though, so tossing a ball in might not work as well.
You can get 3/16" ball bearings at any bicycle shop. If you're in metric areas, a 4mm ball will probably work just as well.
By the way, the GTJ bleed screw is not made of stainless steel; it's plain steel. It has a "bright" finish that hopefully will resist rust for a while.
Walter Acker IV suggests the use of Speed Bleeders instead. They
are available in the correct shape, tapered tip and all. They have
a spring-loaded ball check valve inside, making it easier to bleed
the brakes. And they are made of 303 stainless steel. You can
look into them at http://www.speedbleeder.com.
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