4w disk normal driving brakes ok.

You have air trapped in the braking system somewhere. Have you bled the system thoroughly at all four wheels?
Have you changed the front calipers recently. Many years ago, I helped a friend mount new calipers on his vehicle, and we put the new calipers on the wrong side -- right one on the left side and vice versa. In this configuration, the bleeder screw wound up on the lower portion of the caliper instead of at the top, where it needs to be to allow trapped air to come out. Just a thought.

Michael:

You must start bleeding the brakes at the wheel farthest from the master cylinder (usually the right rear), then the next farthest from the master cylinder, then the next, then the closest. If your master cylinder is at the left front of the car, start with the right rear, then the left rear, then the right front, then the left front. If you don't bleed the brakes in the correct order, you are just shifting the air in the lines from one line to another. Make sure that you close the bleeder before letting the brake pedal up, and the engine should not be running when you bleed the brakes... Make sure that the emergency brake is off. Make sure that the master cylinder does not run out of brake fluid at any time that you are bleeding the brakes.

Does it pull to one side or the other when rolling? If so, then you may have a stuck caliper. If not, then you likely have a bad master cylinder and need to replace. If it is a stuck caliper, it would be wisest to replace BOTH calipers. If the brake pedal feels hard when you push, that would be another indicator the master cylinder is bad - but you may not have the hardness in the pedal. If pulling up on the pedal relieves the problem, I would suspect the master cylinder first. Driving on it could damage many components, and replacing them all may be expensive - I recommend verifying the master cylinder is bad and replacement before driving!!

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

The extra brake travel can be;
1. air still in the brake lines and need bleeding
2. if there are brake drums on the rear they may be out of adjustment
3. if there are warped brake disc or caliper problems that will add to the brake travel

If the car has drum brakes in the rear, Jack the rear up and have someone push the brake pedal hard and you try to turn the wheel. If you can turn it you need to adjust the rear shoes. They may be traveling out so far that the fluid level drops causing the pedal to go to the floor. You must adjust them with the drum on till you hear and feel the shoes rubbing against the drum. Hope this helps.

This may sound dum but its been done a few times and is a simple mistake , the bleeder screw is up top, with the caliper on the bleeder screw should be up top, if not then calipers are on the wrong side, it sounds like air in the system, also you pumped the pedal up to push out the piston so pads hit rotor after bleeding,i dont see anything else, if the pedal is good with pinched lines at caliper then it got to be calipers are wrong or on the wrong side and the bleeder is not atop caliper,also you bleed the rear also? these trucks and cars use left caliper and rt rear on one system of the master and rt front and left rear on one side, let me know about the caliper, heres a picture to show you what i mean. hope this helps

renew the master cylinder immediatly ,the seal between the front and rear chambers has failed. This is a dangerous vehicle and shouldnt be used in this condition it WILL FAIL SOON

You may have a leak in the booster or master cylinder. You can put a pressure tester on the system to test for leaks. It does sound like you have an air leak.
You can try to bleed the master cylinder, then slave cylinder, then brake lines.

Either you did not bleed the brakes properly or they need adjusted. (rear only) Front most likely are disks. If you put pads on the front and spread the caliper pistons, you should pump the brake pedal 10 or 15 times to move the pistons out. Some ABS systems are notoriously hard to bleed.