This LHD front caliper is really doing by head in. Its been dragging, resulting in the premature wearing out of the pads and leaving me with an under thickness rotor. Anyway I did as was suggested, bought a caliper kit and overhauled it. It was all pretty straight forward (Thanks Max for the advice when I got stuck) with no leaks.

While the caliper is not as bad as previously, it still drags excessively compared to the RHD side. After a drive its noticeable how much warmer the LHD wheel is compared to the RHD and there is still the slight smell of burning brakes.

So why wont the caliper piston retract like its supposed to?. I did the fault finding procedure to rule out the master cylinder and plumbing and found the diagonally opposite rear caliper doesn't drag. When the brakes are released shouldn't there be a slight negative pressure in the system to pull the piston back? The caliper bore looked pretty good and wasn't scoured when I inspected it.

I'm thinking of getting another caliper from a later model (not as old) magna from the wreckers and just throwing it in. If it fixes the problem great, if not I will throw a kit onto it and go from there.

Apologies for the long post.

Retracting the piston is actually done by the big rubber ring inside the caliper bore, believe it or not. It distorts when the piston moves out under pressure, and when you take the foot of the brake, it returns to its normal shape and pulls the piston back a bit.

Any number of reasons the caliper may not be retracting the piston:
Rust/wear/dirt in the caliper bore jamming the piston. (but you checked that)
Wear on that big rubber ring inside the caliper bore.
Distortion of the caliper bore/piston. (but you would see scuff marks on the bore.)
Guide pins lacking lubrication, dirty. Most likely. Results in excess wear on both pads. With the pads out, you should be able to slide the caliper back and forth by hand reasonably easily, so check that.
Disc pad nearest the piston too tight in the caliper.

Anyhow, just finding another caliper at the wreckers should solve the problem. Find one still filled with fluid and crack the bleed nipple to see if frees up ok before you pull it off a car. Take the mounting bracket and guide pins too. Take a plastic bag with you as well, don't want brake fluid on your boot carpet/metal. lol

Might have to grab myself a set at some stage for future use.

Thanks again Max. I didn't realize it was that internal rubber ring distorting that retracted the piston.

I also noticed that there wasn't any particular orientation for that internal ring. It was symmetrical in cross section so impossible to fit the wrong way around. Im used to the old school piston rubber rings from drum brake slave cylinders that have a cup shape, similar to the master cylinders . I was surprised that the internal rubber ring wasn't cup shaped.

That bit goes in either way, as you say. Never gave me cause for trouble, but sticky guide pins are another matter. lol

I solved a drag problem on two cars. It was the rubber brake hose clogged or the rubber bore perished. On one car it took me quite some time to detect a slight pulling to one side when the brakes were let off. The clogged hose was slower to let the fluid go back to the reservoir hence delaying the release. I'm just fixing another car and I found I couldn't blow through the front hoses. However, I could with the replacements.