Brake pedel hard,can't get fluid to back brakes,

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

In this case, you will need to leak the for wheels, never leak only one or two wheels when adjusting the breaks. If the leaking continues you need to check the conduit for potential damages. If pedal stay soft after checking everything else, check the break fluid unit. The unit has a piston if I'm not wrong. The piston has some parts that are of rubber material which can wear out or get damaged.

If you have a manual for your vehicle, check how to bleed the system. Certain vehicles need to be bled in a certain order of sequence. If you are using a one man bleeding kit, when you don't see air bubbles in the clear tube but see only fluid, then it is bled properly.

yes may need to bleed lines at master cylnder

The problem here is air in the ABS system, you must get the ABS hydraulic control unit bled, this can't be done with a special tool that holds open the poppet valves to each wheel so the air can be completely bled out. You must have the brakes bled at a shop OR the dealer that has this tool or its equivalent. There is no way around using the bleeding tool on this ABS system. I get this question over and over on FixYa.

make sure you are bleeding them by doing them as an abs system,do the right rear then the left front,then the left rear then the right front,the opposite wheels are how the system brakes.

Bleed right rear,left rear, right front, left front in that order to be sure of good bleed but the brake booster makes it easy to push pedal. Make sure of no leaks from master cylinder. Otherwise brake hoses being soft will also hurt baking quickly.

try bleeding the brakes as a system. start by making sure the master cylinder is full. then have a Friend step on the brakes and release 3-5 times. the last one they should hold as you break lose the bleeder on the passenger side rear, then do the driver side rear ,(check fluid) next passenger front and then the driver side front do each wheel until you don't see any or hear any air, thats the way i have always bleed brake systems. hope this helps

Let the fluid out of the master cylinder or from a line. If the pedel  then goes down w/o the fluid, then the booster is working ok. Rebuild or replace the master cylinder. 

1. You could have air in the lines yet, but more specifically, I would need to know if you have anti-skid system in your car. Also, you may have a proporation valve in the system that has to be activitated before the system can be bled.

In bleeding, you start at the rear wheels first and then work to the front, doing the master cyclinder last.
Finally, the brake master cyclinder is designed so that the front part of the cylinder pumps fluid to only one front wheel and only one rear wheel. The rear part of the master cylinder only pumps fluid to the other 1 front and 1 rear. This is so that if you loose your brakes, you will always have a rear and front working.
Bill