I can't collaspe the rear brake caliper to remove the brake Pads

Front and back rotors are different. For front rotors, after you remove the wheel you need to remove the bolts holding the caliper to the mounting spindle. The bolt can be a standard 6 face head, a recessed hex, or even a torx or star head recessed bolt.

With bolts removed, the caliper can be worked away from the anti-rattle clips the caliper slides on. These clips are called "hardware" and should be replaced.

The caliper pistons need to be pressed back into position because the new pads are thicker than what you removed. The calipers should be tied and suspended and not allowed to hang by their brake lines while you work with the rotors. Use a "c" clamp and an old brake pad to press the pistons back in. Sometimes it is good to remove the cap on the Master cylinder to allow for brake fluid to travel back to the Master.

Either replace or turn the old rotors and then put the brake hardware in place. Lube the caliper slides. then as a unit position the pads into the caliper and hang the caliper over the rotor. The caliper will hang and pivot into place. Replace and tighten the mounting bolts.

The rear brakes are more of a problem due to a second set of Emergency brakes in rear assemblies. So you have 2 sets of rear brakes, the Emergencies requiring clearance to remove the rear rotors. Some car models require readjustment of the Emergency cables before reassembly.

In all, I would suggest that you be an observer for your first brake job. At least get a Video of the process at a number of autoparts stores. You can wind up with a car you can not drive or reassemble without help.

Depending on vehicle. Find level ground and park there. Put chocks in front and behind wheels not being lifted. Loosen lugs, jack up vehicle, set jack stands, lower vehicle on jack stands, remove lugs and wheel. Open hood remove brake fluid cap. Remove two bolts holding caliper, pull caliper off rotor disc and bracket, remove pads, place a used pad in front of caliper piston, use a c clamp to push caliper piston in caliper, remove c clamp and old pad, hang caliper up, remove 2 bolts holding caliper bracket, remove bracket, remove rotor, install new rotor, install caliper bracket, install new pads, install caliper, repeat this on other side, put cap back on brake fluid tank, remove bleeder fittings and keep pressing brake pedal until an even flow of brake fluid sprays out. Install bleeder fitting. Make sure brake fluid doesnt get on paint and is contained and disposed of correctly, top off fluid install wheels and raise car up remove jack stands, lower car and break in new pads

Here ya go! This is for the rear brake pads removal:

Removal & Installation

1. Remove the brake master cylinder filler cap. Check brake fluid level in brake master cylinder reservoir. Remove fluid until brake master cylinder reservoir is half full.
   
2. Raise and support the vehicle.
   
3. Remove the wheel and tire assembly.
   
4. Inspect the brake pads for wear or contamination. If worn, damaged or past specification, install new components.
   
5. Remove the caliper pin bolts.
   
6. Remove the rear disc brake caliper.
   
7. Remove the brake pads and rail clips.
   
8. Inspect the disc brake caliper for leaks. If leaks are found, disassembly is required.
   
9. Installation is the reverse of removal. Observe the following torques:
   
   • Caliper bolts: 27 ft. lbs. (36 Nm)
     
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You'll want to remove the two bolts that hold on the caliper. Remove the caliper and brake pads. You'll need to collapse the caliper piston back into the caliper to get the caliper on with the new brake pads. You can use a C-clamp, or a caliper piston tool to push it back in. I would remove the reservoir cap when you collapse the piston into the caliper.

Park the polo on level ground, turn the engine off and apply the parking brake. Pull the hood release handle and move to the front of the car.

Lift the hood and remove the master cylinder. Remove one-half of the brake fluid inside the reservoir with a syringe.

Loosen the lug nuts on the rear wheels with a tire iron. Place the jack beneath the frame at the back of the Volkswagen and lift the back tires off the ground. Rest the rear axles of the Volkswagen on two jack stands. Remove the lug nuts and take the rear wheels off.

Reach behind the brake caliper to locate the caliper mounting pins. Remove the pins with a 12 mm socket and ratchet. Lift the caliper from the brake rotor and suspend it above the wheel assembly with a bungee cord or wire coat hanger.

Remove the rear brake pads from the slots in the caliper bridge. Inspect the caliper bridge and rotor for damage and significant wear. Replace the rotor if it is warped or badly scored. Scoring of the rotors can lead to break failure and will prematurely wear the new brake pads. Place new brake pads into the slots of the caliper bridge.

Attach the piston depression tool to the caliper. The tool will fit over the back side of the caliper and against the piston inside. Twist the tool's handle to force the piston into the side of the rear caliper. Remove the tool once the caliper piston is totally depressed within the caliper.

Replace the caliper onto the caliper bridge and screw in the caliber mounting pens by hand. Replace the rear wheel onto the wheel bolt and screw on the lug nuts.

Repeat the process for the other side of the rear of the polo. Lift the rear end of the vehicle with the jack and remove the jack stands. Lower the rear tires to the ground and tighten the lug nuts with a lug wrench.

Return to the engine compartment. Remove the master cylinder cap and check the level of brake fluid. Place a funnel into the container and fill the reservoir with brake fluid as needed.

you have to loosen the emergency brake adjustment on the cable give it some slack push the shoes together by hand and rotor will fit . please dont forget to rate this answer . hope this helps

Rear To Remove:

1. Remove the brake master cylinder filler cap and remove fluid until the reservoir is half full.
2. Raise and safely support the vehicle.
3. Remove the rear wheel and tire assembly. CAUTION
   Never allow the brake caliper to hang from the brake hose.
   
4. Remove the two disc brake caliper pin bolts.
5. Lift the disc brake caliper from the anchor plate and position the caliper out of the way. Do not disconnect the fluid line.
6. Inspect the disc brake caliper for leaks and replace the caliper if necessary.
7. Remove the pads and retraction clips.

To Install:
CAUTION
Do not allow grease, oil, brake fluid or other contaminants to contact the pad lining material.

1. Install retraction clips.
2. Install the brake pads.
3. Slowly compress the caliper pistons with a large C-clamp. CAUTION
   Use care not to damage the bleeder screw or front disc brake rotor shield.
   
4. Install the disc brake caliper.
5. Install the disc brake caliper pin bolts. Tighten the pin bolts to 26 ft. lbs. (35 Nm) (F-250, F-350 SRW). Tighten the pin bolts to 56 ft. lbs. (76 Nm) (F-350 DRW). Tighten the pin bolts to 46 ft. lbs. (62 Nm) (F-450 F-550).
6. Install the wheel and tire assembly.
7. Lower the vehicle.
8. Fill the brake master cylinder reservoir with DOT 3 motor vehicle brake fluid.
9. Inspect for correct brake operation.

Renault trafic brake calipers on the front can be pushed in with a G-clamp, but rear calipers need to be wound in, if you look at the piston it has notches for a winding tool!

Rear disc brake pads offer better performance and are not as affected by moisture like conventional brake shoe style brakes are. Rear disc brakes are similar to front disc brakes. The main difference is that rear disc brake systems must incorporate the emergency brake system. There are two methods widely used for the emergency brake with rear disc systems. The first system is a brake shoe inside the brake disc that is actuated by the emergency brake lever. The second is a screw style actuator inside the brake caliper. When activated the brake pads are forced into the brake disc and held tightly by the emergency brake lever.
READ COMPLETELY BEFORE STARTING
Step 1 - Identify Rear Disc Brake Components
Rear disc brake assembly includes; rear brake disc, rear brake pads, brake caliper mount and a caliper mounting screw. (Note: Some vehicles do not have the rotor mounting screw.)
Step 2 - Removing the Rear Brake Caliper Mount Bolts
To replace rear brake pads and rotors the rear brake caliper needs to be removed. First loosen the rear brake caliper mount bolts and remove them. Turn counter clockwise.
Step 3 - Lift Rear Brake Caliper from The Caliper Mount
After the caliper mount bolts have been removed, gently lift the brake caliper from the caliper mount. Inspect the caliper slides; they should move freely in the caliper mount. Remove rear brake pads and hardware.

Step 4 - Removing Caliper Mount Bolts
With a socket wrench or other appropriate removal tool, loosen the rear brake caliper mounting bolts. Remove bolts and lift the caliper mount and remove it from the vehicle. Remove the retaining screw from the disc mounting hole. Tap the rotor gently to release any rust that has accumulated between the rotor and bearing hub. Lift brake rotor from wheel hub holding on tightly, using both hands. You do not want to drop the rotor.

Step 5 - Removing Rear Brake Rotor
Remove the retaining screw from the disc mounting hole, tap the rotor gently to release any rust that has accumulated between the rotor and bearing hub. Lift brake rotor from wheel hub, hold on using both hands and do not drop.

Step 6 - Install New Brake Rotor
Check the new rotor against the old brake rotor to make sure they are the same size. Clean the mating surface on the wheel hub before the new brake rotor is installed. Reinstall rotor retainer screw.
Step 7 - Reset Rear Brake Caliper
Before new brake pads can be installed, the rear brake caliper must be reset. The reset tool winds the piston back into position so the new brake pads will fit. This style of brake caliper will not compress with a clamp tool; it can only be reset with the proper reset tool.
Step 8 - Reinstall Rear Caliper Mount and Install New Rear Brake Pads
After the caliper has been reset, reinstall caliper mounting bolts and make sure the bolts are tight. Then match up the old brake pads to the new brake pads. They should be exactly the same except, of course; the old ones will be worn out. Check the new brake pads for proper fit and install any brake hardware that is required.
Step 9 - Remount Rear Brake Caliper
Reinstall the brake caliper, align brake pad hardware and reinstall caliper mounting bolts. (Note: align the rear peg of the brake pad to the groove in the caliper piston.) Recheck and retighten all caliper and caliper mount bolts. Bleed brake system to relieve any air in the system. Before driving the vehicle, push the brake pedal down and let it up slowly. This operation forces the brake pads to travel to the brake rotors. DO NOT DRIVE VEHICLE until proper brake pedal operation resumes. When test driving vehicle listen for any unusual noises during the operation of the brakes.
WARNING! Always have the vehicle under inspection on level ground, in park with the emergency brake on. Always wear protective eyewear, gloves and necessary clothing before inspection or work begins. Never crank an engine over when anyone is near the battery or engine. Always have an operational fire extinguisher close by, obey all first aid instructions in the event of an injury. Never stand in front or behind a vehicle when cranked over or running. When engine is cranked over keep hands and clothing away from rotating components. Never move a car without proper brake pedal operation.

Are the rear calipers already fitted? I will assume not 1) open the hood and locate the brake master cylinder and brake reservoir. Open the filler cap. Put a piece of polythene Supermarket bag will across the opening and tighted the filler cap over it. This will provide a vacuum seal to the reservoir to prevent too much brake fluid loss whilst working on the brakes. 2) Jack up one of the rear wheels and remove it. Loosen by half a turn the flexible brake hose union in the old caliper. Put a piece of towel on the ground beneath the brake assembly to catch any brake fluid. About midway along the hose pinch it closed. It is very important that you use something rounded on both sides to do this to prevent damaging the hose. A self clamping wrench has jaws that are too sharp for the job. If you have a couple of drill bits of about 8mm to 10mm in diameter that you can put one either side of the tube and then set the self clamping wrench to squeeze these on to the hose with moderate force; enough to prevent fluid loss. Slacken now the brake fluid bleed screw in the caliper. Put a broad blade screw driver between the disc and the face of the brake pad and twist to lever the caliper pistons back into the body of the caliper. As you do so brake fluid will be forced out the slackened bleed screw. Remove the brake pads when suitably slackened. Undo the two very large bolts that secure the caliper body to the hub assembly of the car and lift it free. The following process avoids the need of disturbing the top brake pipe union: taking the weight of the caliper in the hand it should be possible to unscrew it from the previously loosened end of the brake hose union end as if it were a massive nut. 3) Fitting the new caliper is matter of again taking its weight in the hand and screwing it on to the brake hose union end. Do it only hand tight initially. Put a light smear of copper ease grease on the back of each pad...do not contaminate the disc or pad face. Copperease between the caliper piston faces and the pad backs prevents brake squeal. Some new pads are supplied with an anti squeal compound to be smeared on the pad backs in which case the Copperease can be omitted. Slip the brake pads into the caliper body and also fit the bleed screw finger tight. Fit the caliper with pads over the disc the disc and insert the mounting bolts. Tighten the large mounting bolts to the specified torque (very very tight). Tighten the hose union end with a spanner and take off the improvised hose clamp. Tighten the bleed screw moderately. 4) Under the hood remove the polythene temporary seal from under the reservoir filler cap. Pump the brake pedal slowly until some resistance is felt. Check the fluid reservoir level and if low top it back up to max. 5)With a 'helper' do the following: have them press on the brake pedal. Undo the caliper bleed screw about a quarter of turn to let the air out and then re- tighten it again. Your helper should only now allow the brake pedal up slowly. Have them press on the pedal again. Again loosen off the bleed screw fractionally to expel air bubbles. Keep repeating this process until brake fluid emerges for some time with absolutely no sign of air bubbles. Your helper should also report that the brake pedal feels hard. Make sure the caliper pitons have extended and that the pads have been pressed hard onto the disc. Tighten the bleed screw securely. 6) Check the reservoir under the hood, the level may have gone down, if so top it up to the max line with brake fluid. 7) repeat steps 1 to 6 on the other side of the car.
As a final check ensure that no polythene has been left beneath the reservoir cap and that the cap is refitted. Check that all clamps have been removed, that brake hose unions, bleed screws and caliper mounting bolts are all tight and that the brake pedal action reaches a point of hard resistance when stood on. Any signs of springy softness in the brakes means that there is still air left in the system that will need further bleeding. Ensure everything is clean and that nothing is leaking. Job done