Hy Coyote, there are many valves that could have been swapped into your truck since it was built, if you could give us some pictures of what is leaking, that would help identify the part. Once the part is identified you can probably purchase a replacement part or repair kit at any place that supplies heavy duty truck parts. Hotrod Lincoln said there are two basic types of air brakes, that is true as to how the linnings are made to touch the brake drums, those two types are "S" cam brakes [not cam and lever, that was a "Ross" steering box] and wedge brakes. There are also single system and dual system and anti-lock so it can be as simple or complicated as system as you require.
