Mu brake caliper barrel will not retract

hi Ed.
It's nearly time to install the new brake pads. But there's one more step to complete before you can do that. You have to manually retract the caliper piston.
If you look at the inside of the caliper you'll see a cylindrical piston coming out -- this part pushes on the inboard side of the brake pad. You'll see that it has adjusted itself to match your worn-out pads, so you'll need to reset the piston to its original position before the caliper, along with your new (and thicker) brake pads, will fit over the rotor.

Before you reset the caliper piston, it's a good idea to remove the cap that covers the brake fluid reservoir. If you don't, you'll be fighting against a significant amount of brake fluid pressure.
Now that the reservoir cap is removed, this is the part where you'll need that C-clamp. Place the end with the screw on it against the piston. You can use a small piece of wood to protect the surface of the piston, if you choose. As you turn the screw, the clamp will increase the pressure on the piston. Keep tightening it until you're able to slip the new pads into the caliper and fit the caliper and the new pads over the brake rotor.
If you find that the piston isn't properly retracting, don't force it. You may have a caliper that's been designed with a piston that slowly turns as it extends. If that's the case, then you'll need a special tool to thread the piston back into the caliper. A brake caliper tool -- a tool specifically designed to retract this type of brake caliper piston -- is available for purchase at most auto parts stores. If you can't find one there, then you can order the tool online -- or if you're really lucky, maybe your neighbor will have one that you can borrow for the afternoon.
Once the brake caliper piston has been fully retracted, you can re-cap the brake fluid reservoir. It's not a good idea to leave the cap off of your brake fluid reservoir for any longer than is absolutely necessary. Debris or even water may find its way into your brake's hydraulic system. Brake fluid is hygroscopic, which means that it absorbs and retains water. Water in the brake lines leads to serious safety concerns and more complex repairs than the relatively simple pad replacement that you're finishing up with right now.
Installing the new brake pads themselves is just a simple matter of slipping the new pads into the slots where you found the old ones. If they don't easily fit in with your hands, you may want to tap them in gently with a hammer or rubber mallet. Then, replace the pins or bolts that held them in place and you're nearly done.
Next, move the caliper back into position on the brake rotor. Make sure it fits snugly, and tighten the bolts that hold the caliper in place. Make sure everything is back in place and then give the brake pedal a few pumps from inside your car just to make sure the pedal feels right to you. It may take several pumps of the pedal to get the brakes to properly seat themselves into their new position.
The rest of the brake repair is easy. Put the wheel back on, tighten the lug nuts, remove the jack stands and lower the car to the ground using your jack. It's just like changing a tire. Remember to fully tighten (and properly torque) the lug nuts once you have the vehicle back on the ground.
Also, don't forget to test drive the car to make sure that your brake work was successful. Fixing brakes is one thing; making sure they work properly is another.

if you need more info go to this link.
there is youtube videos showing you how to do it.

how do you open brake caliper so new pads fit Google Search

Remove the caliper mounting bolts and remove the caliper. Replace brake pads as necessary. Special tool is required to retract caliper piston to fit over new pads. (see picture below)
Retract piston in caliper using special tool to rotate clockwise and make sure the notch in the piston is aligned properly so the tab on the inner pad will fit in properly.
Reinstall caliper assembly and caliper mounting bolts.

open the bleeder on the brake caliper, if you can then retract the piston then replace the brake hose that aitches to the caliper if after you open the bleeder you still can not retract the caliper piston then you will need to replace the caliper.....GOOD LUCK

With the wheel removed, look behind the caliper there are 2 bolts that hold the caliper in place. Completely remove the lower bolt and loosen the top. The caliper will then swing upwards exposing the old pads. Remove cover off master cylider reservior. Leave both old pads in place for now. slide the far side pad over to the near side pad and using a channel-lock wrench, squeeze slowly and steadily. this will cause your brake cylider to retract, pushing the fluid backwards through the line and into the reservior. Once the cylinder is retracted, remove old pads and place new pads on either side. Align caliper and swing it down. Replace lower bolt and tighten upper.

Have you tried opening the bleeder screw? Is it on the car?
It's unlikely that the caliper is bad if it is new. So, it might be a restriction in the hydraulic line leading to the new caliper.

Hello, to replace the front pads first open the hood and remove the brake fluid cover (will make retracting the piston easier). Next losen the lug nuts till you can turn them by hand than jack up the vehicle and use a jack support stand. Once the vehicle is supported remove the lug nuts and wheel, remove the lower caliper screw and rotate the caliper upward. Remove the old pads, use one of the old pads and place it so it centers the caliper, using a large C-clamp position it with the solid end on the back side of the caliper and rotating handle contact the pad in the center or close as possible. Slowly rotate until the piston is fully retracted into the caliper, your brake fluid may run over but its ok. Install the new pads return the caliper to the normal position over the pads reinstall the lower caliper screw. Put the wheel back on tighten the lug nuts till the wheel is evely and firmly in its normal position. Remove the jack stand lower the vehicle, put the brake fluid cover back on and pump the brakes until they are firm and check the fluid, add some if needed. Tighten the lug nuts, check for leakes. After starting the vehicle pump the brakes a few times, now the fun part the test drive :) allow you self more than normal distance to stop. Tips, I always set the parking brake when working on the front brakes, some people use a 2x4 instead of a brake pad to retract the piston into the caliper, double check your work, i always do and take your time.

Removal & Installation

1. Before servicing the vehicle, refer to the Precautions section.
   
2. Remove or disconnect the following:
   
   • Negative battery cable
     
   • Front wheels
     
3. Place a C-clamp over the caliper as illustrated, place a screw drive head against the outboard pad and hook against the rear of the caliper. Slowly tighten the screw drive and retract the caliper pistons into their bores and breaking the outboard brake pad from the caliper fingers.
   
4. Once the pad is free, slide the caliper in on the guides to provide clearance between the rotor and inboard pad.
   
5. Place an appropriate prytool through the center opening in the top of the caliper behind the inboard pad between the pistons using care not to contact the piston boots. Pry the pad to free it from the pistons.
   
6. Remove the two caliper guide pin bolts and position the caliper aside. Remove the brake pads.
   

Place a C-clamp over the caliper as illustrated


View of the caliper guide pins and boots (1), anti-rattle clips (2), brake rotor (3), outboard pad (4), caliper adapter (5) and inboard pad (6)

To install:

1. Completely compress the caliper pistons
   
2. Install or connect the following:
   
   • Brake pads with anti-rattle clips onto the adapter
     
   • Caliper and align the guide pin bolt holes with the guide pins. Install the pin bolts and tighten to 32 ft. lbs. (43 Nm).
     
   • Banjo bolt with new washers on each side of the hose fitting and tighten to 35 ft. lbs. (47 Nm)
     
   • Wheels
     
   • Negative battery cable
     
3. Check and adjust the brake system fluid level and road test the vehicle.
   

I always use a c-clamp to push the piston in with. you won't be able to replace the pads unless you push the piston in.

Installation

1. CAUTION: If the caliper piston is retracted into the housing, brake fluid will be displaced from the master cylinder.
   Fully retract the caliper piston.

1. CAUTION: Make sure the brake disc faces are clean before installation.
   Install the inner brake pad.

1. CAUTION: Make sure the brake disc faces are clean before installation.
   

2. Fully retract the caliper piston

3. Install the outer brake pad.

1. Push the outer retaining clip over the spring retaining lugs.

1. Install the caliper.
   1. Install the bolts.
   1. Install the bolt covers.

1. Install the brake hose onto the support bracket.

1. Locate the outer pad retaining clip.

1. Install the wheels.

1. Check the fluid level in the brake fluid reservoir.

If you can't see diagrams(sometimes happens) let me know and I will get them to you

1. Raise and support the vehicle. Remove the rear wheels.
2. If equipped, disconnect the brake pad wear sensor from the harness.
3. Counterhold the lower sliding pin using a thin open-end wrench and remove the caliper-to-sliding pin hex bolt.
4. Pivot the caliper upward or if necessary remove the upper caliper-to-sliding pin hex bolt.
5. Using a brake caliper retraction tool, select the appropriate sized piston press arbor and install the tool onto the brake caliper. Following the tool manufacturer's recommendations, use the tool to rotate the caliper piston clockwise as it is pressed into the brake caliper housing, until it is fully retracted.

Use a thin open-end wrench to . . . . . . support the caliper sliding pin when removing the caliper mounting bolt Use a disc brake caliper piston retracting tool that will rotate the caliper piston as it is pressed into the caliper The rear caliper piston is slotted to allow a retracting tool to rotate the piston when replacing the brake padsTo install:

1. Clean the brake caliper and all sliding surfaces. With the caliper piston pressed fully back into the caliper housing, check for leaking fluid and damaged dust boots.
2. If equipped, install a new brake pad wear sensor in the pad if the plastic part has been worn through on the old sensor. New sensors are not required unless the wire inside the plastic part has been exposed.
3. Lubricate the slider bolts with approved brake grease.
4. Install the brake pads onto the bracket assembly, and then carefully lower the caliper over the caliper mount. Check and make sure that the pad springs are correctly seated.
5. Replace all removed caliper-to-sliding pin bolts and torque to:
6. 1990-97 Passat models: 25 ft. lbs. (35 Nm)
7. 1998-00 Passat models: 22 ft. lbs. (30 Nm)
8. If equipped, connect the brake wear sensor wire to the harness. Check for a good connection as most problems with the sensor circuit are caused by faulty connections. Check that the wire is held by the loop in the dust cover and the connector is held at the clips.
9. Install the wheels and hand tighten the lug bolts. Lower the vehicle carefully until the tires begin to contact the surface and torque the lug bolts to specification in a crisscross pattern to:
10. M12 x 1.5 lug bolts: 81 ft. lbs. (110 Nm)
11. M14 x 1.5 lug bolts: 89 ft. lbs. (120 Nm)

Feel free to contact if you have any questions