How to separate these shoes?
 

https://cimg4.ibsrv.net/gimg/bikefor...079a048e4c.jpg
Is it easy to separate these brake shoes?

1. Separate them from the rim for spacing.
2. Separate them from the rim for wheel removal.
3. Separate them from their holders for replacement.

Which?

1. Usually there is a barrel adjuster on the part of the caliper you didn't get in the picture. Screw it in some to open the gap. If no barrel adjuster on caliper or brake lever then undo the cable from the pinch bolt.

2. Some have a release on that part of the caliper you didn't get in the picture that you just flip and it opens much wider so as to let the wider tire pass. If no release, deflate the tire or turn the barrel adjuster all the way in.

3. That's for others to answer. I usually just replaced the entire pad and holder.

Centering next?

Depends. Sometimes you can just grab the rubber block with pliers and twist it out, or stick a flat screwdriver in the end and pry it. Then you have to tweak the holder to fit the new friction material - gently open the long sides until the block can be fitted, then close them up again.

1 & 2 never crossed my mind, I forget the varying experience levels here, and language differences.And some have a release on the lever.

I didn't realize that's what that thing was on the old style brake levers until just about the time I quit using them! <grin>

The Weinmann button is a neat design - I imagine you're not the only person who failed to notice its added function, in fact I have a vague recollection of someone complaining about its sloppy fit.

On some of these bikes there is no cable release lever, so you have to deflate the tyre to get the wheel out.

If you really like this bike, now is the time to upgrade to road calipers.

The best way probably begins with a more detailed question.

@juntjoo, tell us more about what you are wanting to do. Yes, some here will consider nearly anything you want to know a dumb question but others want to help. And there is a lot of knowledge here.

There are brakes more modern than yours that are much easier to work on and live with, plus modern levers that offer features seldom found in the older brakes. Also other parts and tricks that can address specific issues. Probably a few workarounds on what you have now. But to help you, we need to know what you want changed

#1 thanks. I'll look closer foe this "barrel adjuster" and report back...

I'd drop the wheel assembly out. While it's out, inspect & clean the mech parts.

Properly adjusted and lubricated, with good friction material and cables, shorter side-pulls like these are perfectly adequate. Some of us enjoy riding old bikes and prefer to leave them original, upgrading only the consumable parts.

I've found that removing one shoe from the caliper will allow wheel installation and removal without deflating the tire.

To get to the OP's apparent issue, to remove the pad itself from the holder, you can often pry the pad out with a screwdriver. If the holder is of the type that is stamped from sheet metal, with the edges folded up to retain the pad, you can bend one end of the holder down to create an opening through which you can drive out the pad and slide in a replacement. N.B. Most of these types of holder are aluminum, and can be weakened by bending the tab to create the opening; after doing so, position the pad with the weakened tab toward the rear, so that braking action does not bear against the weakened tab.

Another option is to replace the entire pad/holder assembly; these are usually quite generic. The Dia-Compe "G" caliper in your picture uses a 6mm x 1mm (M6) threaded stud to secure the holder to the caliper; this is a very common design, and any replacement with the M6 stud ought to work.

Oh right, because physically removing a shoe, then reinstalling and reseating it is much easier than reinflating a tyre.

How stupid of me.

In terms of physical effort required, yes (assuming you’re not using an air compressor).

https://cimg2.ibsrv.net/gimg/bikefor...bf398f6c2e.jpg
https://cimg3.ibsrv.net/gimg/bikefor...b1249287ae.jpg
Here's a closer look. Not sure how these work. My rim is a little off. I adjust often and can never get perfect, thus it rubs against the shoes a little. So I'm thinking pulling them apart might help

Those calipers were popular on almost every low-end bike from the 80s, it's centered by using two 10mm wrenches and you turn the entire assembly.

Rim width?
 

Just a thought...

Is the wheel original to the bike? If not, perhaps the replacement rim width is a bit too big for the caliper.

Still hard to ascertain exactly what issue you’re having from your description, but… if the brake caliper won’t open enough, with no cable tension, to clear your rim completely, you can pop each side of the spring out from behind the caliper and gently bend it out, essentially increasing the tension.

Your barrel adjuster is screwed right down, no further adjustment available, so you'll need to let the cable out a bit. Loosen the pinch bolt, gently pull the lever so a small amount of cable slips through and retighten the pinch bolt. If that's too slack take it up by screwing the adjuster out.

Ditto what @grumpus said. You don't have any adjustment to open them. I usually will undo the pinch bolt, then unscrew the barrel adjuster about half way and then squeeze the calipers together with my hand and snug the pinch bolt down with the other. Make sure to pull on the end of the cable before you tighten it to get all the slack out. As well, make sure the cable housing is fully in the barrel adjuster and any cable stops or ferrules all the way back to and including the brake lever.

It's easier to do than you might think from the description. But if the caliper springs are too strong for you, a quick clamp or even a old inner tube stretched and wrapped around the pads and wheel will hold them to the rim while you snug up the pinch bolt. After that you should have enough adjustment to screw the barrel adjuster in a few turns to open up the calipers. If your wheel is warped you might need more. If so, then just unscrew the barrel adjuster more before tightening the pinch bolt.

And it does look like you have the lever to open up the calipers for changing wheels quickly. It might be that someone set the pads with it open. And now that it's in the normal closed position the pads are too tight. You could just lift up on that lever with the black plastic on it and they'll open. But that's not the correct position intended for riding. That's just to open them wider for getting wide tires past when changing.

Actually, with the Dia-Compe G caliper, it's even easier than that. There are flats on the pivot bolt that can accept a single thin wrench to center the caliper over the rim:
https://cimg2.ibsrv.net/gimg/bikefor...05bd51b246.jpg

The caliper is dry and adjuster looks to need some wd or some penetrating oil. The barrel adjuster may be stiff action, or frozen