What is a abs modular on a Escalade Cadillac?

Jesse, The module will not stop fluid to rear brakes, a bad master cylinder will,leaking lines, air in brake system, faulty metering block if equipped, collapsed flex lines or pinched brake line faulty caliper or brake cylinders,etc. will cause the possible feeling of no rear brakes.
(ABS Module components) the ABS speed sensors and the ABS control module . When the brake is applied, the ABS control module reads the speed data from the speed sensor and sends the correct pressure to each wheel to prevent any wheel lock up, eg. sliding/skidding Links below have more info.
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Service Stability System AND abs Pump issue

ABS Control Module Replacement Service Cost YourMechanic

https://www.rockauto.com/en/catalog/cadillac,2002,escalade,5.3l+v8,1431695,brake/wheel+hub,abs+control+module,1844

I think your question is "what is an ABS module " if so an ABS or anti lock brake Module controls the power of the braking system sometimes only to the rear wheels but most times nowdays its to all wheels , this module takes info from the sensors around the car and determines if the "force" to the braking system needs to be reduced in order to keep vehicle under control in a stop , these systems are mostly "fail safe" in meaning if they fail or have a problem they just allow full power to brakes and don't "reduce" any longer till there repaired , but car will no longer have benefit of staying in control in a panic stop or slippery roads