Brakes were making a horrible grinding noise then all of a sudden the brake pedal went soft?

This is going to be unnecessarily expensive due to ignoring the symptoms for so long. By the time the grinding first started, the brakes were long overdue for an inspection. What has likely happened is the metal-on-metal grinding has worn the brake rotor down so far, the piston fell out of the caliper. Since that brake can no longer apply, you're not hearing the grinding. That also explains the leaking brake fluid. The right front and the left rear are the only brakes still working, but those are going to quit too once enough brake fluid is lost. At that point there will be no brakes except for the parking brake.

The next problem is you'll very likely need a replacement master cylinder. Over time, crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. When the brake pedal is pushed more than halfway to the floor, either due to a leaking flexible hose or steel line, improper bleeding procedures, or in this case, a caliper that fell apart, the pistons get run over that crud. That can rip the rubber lip seals. That causes a slowly-sinking brake pedal, and that often takes two or three days to show up. Once you mention the brake pedal went to the floor, many shops will automatically include a rebuilt master cylinder in their repair estimates. This doesn't apply to a master cylinder that's less than about a year old. That isn't enough time for that corrosion to develop yet.

Brake calipers can be rebuilt, in fact, we did that with almost every brake job in the 1980s, but today professionally rebuilt calipers are relatively inexpensive and are the better value. You'll need both front calipers so they're matched and provide balanced stopping power. Also plan on the normal stuff, meaning pads and rotors. If the van has anti-lock brakes and the master cylinder runs out of brake fluid, a scanner is usually needed to bleed the hydraulic controller. If the van doesn't have anti-lock brakes, or the reservoir isn't allowed to run empty, standard bleeding procedures will suffice.

If you want to pursue this yourself, start by looking at these articles:

https://www.2carpros.com/articles/guide-to-fixing-automotive-brake-grinding-causes-and-solutions

https://www.2carpros.com/articles/brake-caliper-replacement

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

https://www.2carpros.com/articles/how-to-replace-front-brake-pads-and-rotors-fwd

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

Let me know your level of expertise too. I have some tricks that will make the job a little easier. There's a lot of things you can do to end up with a better quality brake job, and a number of things to avoid to prevent other problems. I'll gladly share that wondrous information with you.

Check out this video too:

https://youtu.be/9S0im10Z2T0

They show replacing pads and rotors on a PT Cruiser. The calipers might mount differently, but the procedure will be very similar. Let me know how you'd like to proceed.

Unfortunately, no. The pistons have different diameters to design in the variables for a perfectly balanced braking system, front-to-rear. Drum brakes in your design are called "self-energizing". One shoe grabs the rotating drum and tries to rotate with it. That movement applies part of the second shoe. Hydraulic pressure also applies that second shoe too, so the forces are multiplied over the pressure you're putting on the brake pedal. Little pedal effort is needed to apply drum brakes.

Disc brake calipers don't have that self-energizing feature. More pedal pressure is needed which is why you'll almost always find a power braker booster is used with disc brakes.

To get the front calipers to apply equal braking forces to the rear system, a larger piston is needed. Besides that designed-in size, it is critical the pistons be identical on both wheels. There's two smaller piston sizes listed for use with rear disc brakes. If you are able to physically install either of them on just one side, it will result in a nasty brake pull to the side with the larger piston. I've run into that multiple times where someone was able to do that on a big, heavy pickup truck. Under even the slightest light brake pedal application, it would tear the steering wheel out of your hands. Obviously, very dangerous and tiring to drive.

If you want to check your new caliper, with rear drum brakes, the pistons must be three inches in diameter. The two listed for use with rear disc brakes are close to two and a half inches. That's a difference of roughly half an inch. On those trucks I mentioned, the pistons were just over 4" in diameter and the wrong ones were less than 1/8" different. That means the percentage of difference was much less than what you have, but it made those trucks impossible to drive.

The people at reputable parts stores would exchange a wrong caliper for the right one. I would question if you were told yours is not refundable after it had been installed. Some people don't know how to diagnose something, so they buy the part to try, then return it when it doesn't fix the problem. That I can understand, but you haven't installed it yet.