| Operate poorly when under load | |
| Vary with | |
| Show greater stopping power in bad weather | |
| Show greater stopping power when lightly loaded |
| Allow water and oil to be expelled from air tanks | |
| Allow excess air pressure to be released from air tanks | |
| Deliver air from the air tanks to the air brakes | |
| Deliver air from the air compressor to the air tanks |
| 100 | |
| 60 to 65 | |
| 60 | |
| 80 to 85 |