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General Air Brake Requirements
FMVSS-121 identifies minimum requirements for air brake systems on
commercial vehicles built in the U.S. (Requirements in Canada are covered
under CMVSS-121 and are virtually identical.) These regulations cover
requirements for new construction. Once put into operation, proper use and
maintenance is covered under standards such as:
• FMCSR 393 – Covers required equipment
• FMCSR 396 – Covers inspection and repair.
The requirements for spring brake performance and operation, reservoir size
and air timing have not been changed with the introduction of ABS. Because
the addition of ABS components can have an affect on air brake timing, it is
important to verify acceptable performance on new ABS installations. The
maximum application and release times permitted under FMVSS-121 are
shown in Figure 4.
Measured at
Initial Condition 0 PSIG
Towing Trailer
Converter Dolly 550
Single Trailer
Figure 4, Air Timing Requirements
Apply Time (mSec)
From pedal
From pedal
movement to
movement to
reach 60 PSIG
reach 60 PSIG
Brake Chamber
50 CI Reservoir
0 PSIG
500
500
550
600
N/A
MBS-1P Installation
Release Time (mSec)
From pedal
From pedal
movement to
movement to
reach 5 PSIG
reach 5 PSIG
Brake Chamber
50 CI Reservoir
95 PSIG
5 PSIG
1000
1000
1100
1000
1200
N/A
322UX032
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