Soon as you start the car the brake pedal goes to the floor

First, you should never change just one caliper. Brake repairs are always done in pairs.

It sounds like you still have air in the system. It may have to be electronically bled with a scan tool.

Hey Ryan,

It does sound like you have air in the ABS controller, you must take it in and have the valves manually opened to regain the pedal or the master cylinder has gone and and needs replacing, here is a guide that will help you.

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Please let me know what happens so it will help others.

Best, Ken

It sounds like you have not pumped the pedal up did you try cycling the brakes multiple times ? This video will help you see what to do with a guide that can help as well

https://youtu.be/9v9Xsx5Wjf8

and

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Please run down these guides and report back.

You will need to bleed the whole system first and then you need a compatible scan tool to bleed out the ABS unit. For now try the manual mode bleed process. If you still have problems then the ATF most likely damaged the system and master cylinder, calipers, and hoses need replacing.

Here is the process and hope it works.

Place a clean shop cloth beneath the brake master cylinder to prevent brake fluid spills.
With the ignition OFF and the brakes cool, apply the brakes three to five times, or until the brake pedal effort increases significantly, in order to deplete the brake booster power reserve.
If you have performed a brake master cylinder bench bleeding on this vehicle, or if you disconnected the brake pipes from the master cylinder, you must perform the following steps:
3.1. Ensure that the brake master cylinder reservoir is full to the maximum-fill level. If necessary, add Delco Supreme II, GM P/N 12377967 (Canadian P/N 992667), or equivalent DOT-3 brake fluid from a clean, sealed brake fluid container.
If removal of the reservoir cap and diaphragm is necessary, clean the outside of the reservoir on and around the cap prior to removal.
3.2. With the rear brake pipe installed securely to the master cylinder, loosen and separate the front brake pipe from the front port of the brake master cylinder.
3.3. Allow a small amount of brake fluid to gravity bleed from the open port of the master cylinder.
3.4. Reconnect the brake pipe to the master cylinder port and tighten securely.
3.5. Have an assistant slowly depress the brake pedal fully and maintain steady pressure on the pedal.
3.6. Loosen the same brake pipe to purge air from the open port of the master cylinder.
3.7. Tighten the brake pipe, then have the assistant slowly release the brake pedal.
3.8. Wait fifteen seconds, then repeat steps 3.3-3.7 until all air is purged from the same port of the master cylinder.
3.9. With the front brake pipe installed securely to the master cylinder - after all air has been purged from the front port of the master cylinder - loosen and separate the rear brake pipe from the master cylinder, then repeat steps 3.3-3.8.
3.10. After completing the final master cylinder port bleeding procedure, ensure that both of the brake pipe-to-master cylinder fittings are properly tightened.
Fill the brake master cylinder reservoir with Delco Supreme II, GM P/N 12377967 (Canadian P/N 992667), or equivalent DOT-3 brake fluid from a clean, sealed brake fluid container. Ensure that the brake master cylinder reservoir remains at least half-full during this bleeding procedure. Add fluid as needed to maintain the proper level. Clean the outside of the reservoir on and around the reservoir cap prior to removing the cap and diaphragm.
Install a proper box-end wrench onto the RIGHT REAR wheel hydraulic circuit bleeder valve.
Install a transparent hose over the end of the bleeder valve.
Submerge the open end of the transparent hose into a transparent container partially filled with Delco Supreme II, GM P/N 12377967 (Canadian P/N 992667), or equivalent DOT-3 brake fluid from a clean, sealed brake fluid container.
Have an assistant slowly depress the brake pedal fully and maintain steady pressure on the pedal.
Loosen the bleeder valve to purge air from the wheel hydraulic circuit.
Tighten the bleeder valve, then have the assistant slowly release the brake pedal.
Wait fifteen seconds, then repeat steps 8-10 until all air is purged from the same wheel hydraulic circuit.
With the right rear wheel hydraulic circuit bleeder valve tightened securely - after all air has been purged from the right rear hydraulic circuit- install a proper box-end wrench onto the LEFT FRONT wheel hydraulic circuit bleeder valve.
Install a transparent hose over the end of the bleeder valve, then repeat steps 7-11.
With the left front wheel hydraulic circuit bleeder valve tightened securely - after all air has been purged from the left front hydraulic circuit - install a proper box-end wrench onto the LEFT REAR wheel hydraulic circuit bleeder valve.
Install a transparent hose over the end of the bleeder valve, then repeat steps 7-11.
With the left rear wheel hydraulic circuit bleeder valve tightened securely - after all air has been purged from the left rear hydraulic circuit - install a proper box-end wrench onto the RIGHT FRONT wheel hydraulic circuit bleeder valve.
Install a transparent hose over the end of the bleeder valve, then repeat steps 7-11.
After completing the final wheel hydraulic circuit bleeding procedure, ensure that each of the 4 wheel hydraulic circuit bleeder valves are properly tightened.
Fill the brake master cylinder reservoir to the maximum-fill level with Delco Supreme II, GM P/N 12377967 (Canadian P/N 992667), or equivalent DOT-3 brake fluid from a clean, sealed brake fluid container.
Slowly depress and release the brake pedal. Observe the feel of the brake pedal.
If the brake pedal feels spongy, repeat the bleeding procedure again. If the brake pedal still feels spongy after repeating the bleeding procedure, perform the following steps:
21.1. Inspect the brake system for external leaks.

This is really odd. The system links the l/f and R/R and th L/R and the r/f wheels. Not front and rear.

Pull the drums and see if the piston moves on the wheel cylinders when the pedal is pressed. VERY EASY OR IT WILL BLOW OUT THE WHEEL CYLINDERS WHEN THE DRUM IS OFF. We just want to see if there is movement hydrolically. I am more inclined to think the shoes are not adjusted out far enough.

Good, at least we know that. Had to start somewhere. And your right about the adjustment.

I think What I would try next is to follow the lines back to the abs module, pick one of the rear lines, have someone apply pressure to the pedal, then start opening that line up at the module and see if fluid comes out. IF not, check each line going into the module in the same manner. IF it is going in and not coming out, it would appear to be a bad module.

Check the tire and wheel well for evidence of scraping.

You'll see rough worn areas on the tire. And you'll see shiny areas on the inside of the fender.

No. You'll just ruin another tire if the problem is what I think it is.

You are on the right track.

IT sounds like a fitting that is allowing air back into the system.

One wheel at a time, pinch off the brake hoses and begin bleeding. When you finally get a pedal, you located the hose, fitting or caliper most likley giving you a headache.

Was this a problem before the wheel cyls? Make sure those connections are tight.