FMVSS 121 sets timing requirements for air brake actuation. The brake application (build) time at the chamber farthest from the treadle valve must generally be:

1. No greater than 0.45 second to reach 60 psi
2. No greater than 2 seconds to reach 60 psi
3. No greater than 5 seconds to reach full pressure
4. Unregulated for school buses