Left front brake stuck, revise or replace?
 

My car has 95000km on the clock, and recently my left front brake got stuck. My brother had the same problem with his 2000 Mazda 626 and the pistons in the calipers were completely rusted and done for.

Should I bother revising the caliper or just replace it with a new one? A new one would cost me rougly 80 euro and some work I have to do myself. I would prefer just replacing the entire caliper, they are breaks after all...

Just replace it. Faster, easier, probably cheaper, especially if you trade in your old one.

They are cheap and easy to rebuild yourself if the piston is not rusted on its outside surface. If it is rusted on the outside, you need to flush your brake fluid more often, as it is very contaminated with water.

Front rebuild kits cost as little as $3 in the US. Even if you need to source a piston, they are about $20 each, but they are a little bit difficult to find. So, with a little elbow grease, you can do it yourself for as little as $23.

Having said that, I just bought a pair of Wagner remans with steel pistons from Rock Auto for $100 total after returning a pair I got from a junkyard as cores. That gives me a set on the car and a set to rebuild and have on the shelf until needed.

Also be aware that rubber brake lines can collapse and act as a check valve.
Not common, but can happen.

I am aware of this.

However I believe it's really the caliper itself, as when I tried to stop with full braking power, the right wheel locks and ABS kicks in and the left wheel just doesn't do as much as it should. Tried it with 80mph and the car tried pretty hard to steer to the right.

It doesn't release, nor does it brake. If it would be a collapsed hose, it would indeed act so it can brake normally but would leave the brake sticking for a while or longer. Pushing the piston back isn't a problem either.

I remember at the technical checkup last year that the braking power during the brake test was lower on the left caliper which is giving problems right now. I have also had over the course of the last 6months that the left caliper would sometimes overheat and twisting the rotor from the heat, resulting in vibration.

So it seems that the caliper already has the problem for a year And got gradually more stuck, only it became clear just recently that it's completely finished.

Ok, just an FYI for others who may not know;
The way to determine if the caliper or the hose is bad;
Jack up the car to get the affected wheel off the ground.
(Jackstands please)
Remove the wheel.
Have a helper sit in the car.
Try to spin the wheel. If it does not release, unscrew the bleeder screw
and use a screwdriver to try to move it.
(Always open the bleeder screw to avoid forcing possibly contaminated fluid back through the system)
If the caliper is bad, it will be difficult.
If it moves easily, then have the helper press the brake.
This time if it seizes, crack the hose open at the caliper.
If it releases, it's probably the rubber line, but occasionally (rarely in my experience) the proportional valve goes bad.

Disclaimer:
This is from experiences in the 80s and 90s.
Newer technologies I am less familiar with.:)

I will crank up the car when I have time and quickly will check if it's a collapsed brake line :). It will still work like this.