What is the material of snubber brakes?
This company provides brake components for the B737, 
the snubber brakes being among them:

 (Source)

They are made of what they call "AFT 200" material, which is described as:


  a phenolic treated, brass wire inserted cloth laminated under heat and pressure to a dense, strong composite. AFT 200 provides good fade and wear resistance and may be machined using standard, industry accepted practices. Its high strength makes it suitable for gear and lug driven applications.




I don't know if the snubber brakes in a KC-135 Stratotanker use this material as well, but the following picture shows where they would be installed:

 (Source)