Brake pedal losses pressure 1 to 2 pumps back to full pressure.

Brake lines could be leaking somewhere.

Hello Patricia,

Have you checked the master brake cylinder reservoir for brake fluid loss?

I don't suspect the master cylinder as you have replaced it... but am more suspect of a leaking brake line or fitting which should have
been discovered when you had the system bled.

The only time I ever had a rear brake problem with a 2500 series Chevrolet was due to seized rear brake caliper guide pins...as your calipers are also new that is not your problem.

If your brake pedal feels spongy and travels to the floor board you have air in the system, a brake fluid leak or a failure in the power steering system, such as a broken hose, broken power steering pump drive belt, or failed pump, would result in a loss of pressure to both the hydro-boost and steering.

Find more information here:

. Hydro Boost Power Assist Systems Operation Diagnosis and Repair

I have never bleed brakes more than one time per repair, in 49 years
of doing so & had no issues,so explain the bleeding process

You didn't bleed 4 times, as you have no pedal pressure.

Old fluid out-- new in ,pretty easy ,no pumping ,just a helper
& open the bleeder valve

Try checking with either propane or water mist on all hoses and surfaces where generated vacuum could escape.

Activator rod from Brake pedal to Master cylinder can be adjustable on some models.

THE MASTER CYLINDER NEED TO BE BLED FIRST.BEFORE PUTTING ON THE CAR.TO MANUAL BLEED BRAKE SYSTEM.YOU NEED 2 PEOPLE TO DO A BETTER JOB.YOU FIRST BLEED THE LEFT REAR RIGHT REAR LEFT FRONT AND RIGHT FRONT. WHEN YOU BLEED THE BRAKES.HAVE THE HELPER PUMP BRAKES 3 PUMPS.AND HOLD BRAKE PEDAL IN.YOU NEED A CLEAR BLEEDER HOSE IN A CLEAR BOTTLE HALF WITH CLEAN BRAKE FLUID. WITH BLEEDER HOSE ON BLEEDER SCREW AND OTHER END BLEEDER HOSE IN BOTTLE OF BRAKE FLUID.WHILE HELPER PUMP BRAKE PEDAL HOLD IT IN OPEN BLEEDER SCREW 1/4 TURN ALLOWING AIR BUBBLES LEAVE THE BRAKE SYSTEM.CLOSE THE BLEEDER SCREW BEFORE BRAKE PEDAL RELEASE.HAVE HELPER PUMP BRAKE PEDAL 3 TIMES. WITH PEDAL HELD DOWN OPEN BLEEDER SCREW GETTING AIR OUT OF THE SYSTEM.WHEN YOU SEE SOLID FLUID COMING OUT OF BLEEDER HOSE THE AIR IS OUT THE BRAKE SYSTEM.DONT DRIVE CAR ON HIGHWAY IF YOU STILL CANT GET SOLID BRAKES. HAVE CAR TOWED IN TO BE PRESSURE BLEED.

adjust your rear shoes up so that the rear drums just turn, with them ajusted to a low setting the brake pedal will be low .
also when you start your engine it is noral for the pedal to drop because of the boster ! [ but not to the floor ]
Bruce Pike

You are correct there is still air in the system. It is a two man job starting at the right rear and then left then front. Pump brake pedal then hold down while bleeder is opened then closed before pedal let off. Then repeat till no air. Be sure to check master cyl for fluid and keep it filled during process.

abs module shouldn't keep the vehicle from stopping but the light will come on if there is a leak in the system you could simply disconnect the module to find out though I would check the master cylinder and calipers/wheel cylinders though look for leaks on the floor board (under the brake pedal) around the wheel make sure when you bleed the brakes you only add new brake fluid dot 3 or 4 any water or contamination can cause failure+abs light a bad booster/and/or/vacuum line will cause excessive pressure on the pedal but should still stop the vehicle. if the vehicle was ever wrecked there could be kinked lines but not likely

how did you bled them back 2 front?

5 Inspect Brake Fluid Type Brake fluid loss or contamination 10 Inspect Master Cylinder (Brake System) Worn or leaking brake master cylinder. 11 Inspect Brake Power Booster Damaged booster vacuum line/check valve, or diaphram