How many antilock brake systems for air-braked vehicles comply with the National Highway Traffic Safety Administration's "close-looped" requirement?

Only one...the MSQR-5000® pneumatic (non-electronic) ABS manufactured by Air Brake Systems, Inc. In Washington v. DOT, 95-9513 (10th Cir. 1996), the Department of Transportation states:

"As NHTSA explained in promulgating the final rule, the definition of ABS 'does not require electronics for the sensing of wheel rotation or controlling signals,' and that although 'it is likely that electronic systems will be used, given currently available technologies,' these 'functions could be performed using pneumatic, hydraulic, optic, or other mechanical means' 60 Fed. Reg. 13227"

MSQR-5000® manufactuer's certification