CDL Air Brake PSI Numbers
Last updated: September 2026
What are the air brake PSI numbers for the CDL test?
The four pressure numbers to memorize are: cut-out about 125 PSI, cut-in about 100 PSI, low-air warning at 55 PSI, and spring brakes at about 20–45 PSI. Leak rates are a separate set of numbers.
Source: California Commercial Driver Handbook (DL-650)
The 4 Pressure Numbers
| Check | PSI | What it means |
|---|---|---|
| Governor cut-out | about 125 PSI | typically 120–140 PSI (manufacturer-specified) |
| Governor cut-in | about 100 PSI | Compressor starts pumping again |
| Low-air warning | 55 PSI | Legal range 55–75 PSI |
| Spring brakes | about 20–45 PSI | Parking/emergency brakes pop out |
Air Brake Leak Rates
Leak rates are tested with the engine off, in PSI per minute:
Static (foot off brake)
- Single vehicle2 PSI/min
- Combination vehicle3 PSI/min
- Triple combination (California)5 PSI/min
Applied (foot on brake)
- Single vehicle3 PSI/min
- Combination vehicle4 PSI/min
- Triple combination (California)6 PSI/min
Triple-combination leak rates are California DL-650 content — most other states only test single and combination.
What Each Number Means
Cut-out (about 125 PSI): the air compressor stops filling the tanks when the system is fully charged. Read the full explanation →
Cut-in (about 100 PSI): the compressor starts pumping again as pressure drops. Read the full explanation →
Low-air warning (55 PSI): the buzzer/light must activate before pressure falls below 55 PSI. Read why some guides say 60 →
Spring brakes (about 20–45 PSI): the parking/emergency brakes pop out and lock automatically.
Related Resources
This page is an independent study resource. It is not affiliated with the California DMV. Verify current requirements against official sources.