The National Highway Traffic Safety Administration is concerned that some defective Bendix air dryers could cause braking problems, but Bendix says it's been working on correcting the problem for more than a year and there's no risk of drivers losing their brakes.
If drivers find air reservoirs are not pumping back up to their full pressure following brake applications, there may be a problem caused by a faulty air dryer cartridge or canister.

The problem is extremely limited as involves two Bendix air dryer models, the ADSP and the ADIS. Both are used predominately as standard equipment on Kenworths and Peterbilts, but are also optional on other makes of air-braked trucks and tractors.
According to spokesman Jeff Malarik, Bendix identified some defective air dryer canisters manufactured in 1999 for the two air dryer models noted above. It is believed almost all of these canisters have been replaced, starting in late 1999 when Bendix issued a technical bulletin explaining the problem. The subject surfaced again recently as the result of a National Highway Traffic Safety Administration engineering notice.
If your truck or tractor has a faulty air dryer cartridge, here is what may happen. Due to a manufacturing defect, some of the dessicant pellets (the little pellets inside the canister that absorb moisture in the compressed air before it passes into the first air tank) may find their way into the orifice of the air dryer's purge valve and block it open or partially open. Should this happen, when the compressor cuts in to recharge the air reservoirs, the air is pumped out through the purge port instead of into the reservoirs.
Here's what you'd probably notice. Following one or more brake applications, which draws down reservoir pressure, air fails to pump back up. (Normal compressor cut-in pressure is about 105 psi.) Pressure will continue to drop as brakes are used to the point that your low-pressure buzzer alarm sounds and/or you start to sense dragging brakes (spring brakes partially applying) from the dropping air pressure.
At no time is there a risk of no brakes. If you continue to apply brakes, the dropping pressure will apply the spring parking/emergency brakes. If this should happen, your rig will stop where it is and won't be moveable until either the spring brakes are manually released or the air pressure is restored.
Bendix points out that this problem first surfaced in the fall of 1999 and the company promptly notified the truck manufacturers and Bendix distributors to look only for the models ADSP and ADIS air dryer canisters and to replace them at once. Bendix' Malarik thinks that both truck dealer and brake distributor supplies were replaced before many faulty units got on to trucks. Nonetheless, NHTSA is concerned enough that there are still defective air canisters in service that it is further investigating the issue.
Bendix' official position is that "there is no undue risk to safety. There is no loss of braking pressure, no 'surprise' brake applications and no increase in stopping distances."
In the unlikely event that you experience the situations outlined above, try to drive your rig to a safe spot before the pressure drops to the point where a spring/emergency brake application would prevent further movement.