Teves Caliper Query

My '81 1.3 has the Teves calipers. Pistons are free and mechanically everything moves as it should, new hoses fitted, new pads and retainer pins. When reassembled and pedal pumped up the wheel is very tight to turn. I have found that the force of the caliper spring is sufficient to cause the disc to bind. If I push back on the caliper against the spring force there is a small amount of movement that is sufficient to remove the force from the inner pad allowing the disc to rotate freely. As soon as I release the caliper the disc is once again tight. So it's a mechanical issue but I don't see how to rectify it as the spring will always cause the caliper to twist and put pressure on the inside pad. Any thoughts ?.
As an aside, what is the reason for the raised section on the piston nose ?. I have rotated this to 20 degrees as per the manual but don't see any reason for it as the pad doesn't locate on this in any way. Not unless there is an earlier design of pad that did have some form of anti-rotation profile that aligned with the piston ?.

From the photo, the spring exerts a force on the caliper tending to twist it within the cage and exert a force on the inside pad/disc. Pushing the caliper back against the spring relieves the pressure on the inside pad and the disc rotates freely.

My only thought is, have you cleaned up/sanded down and lubed all the moving contact surfaces of the calliper with brake lube, then lubed all pad contact surfaces, the piston face, parts of the calliper frame that contact the pad backing, sliding guides of the calliper frame, holes for the guide pins, whole length of the guide pins, all contact surfaces of the pad spreader springs? (the VW workshop manual for running gear takes care to point out all these lube points).

Regarding the cut-out at the top of piston face at a 20 degree angle, I guess that is intended to counteract uneven wear on the pads caused by the downward drag of the disc when braking

Everything has been disassembled, cleaned and greased. Before refitting the pads the cage can be slid fully both ways easily by hand to the full travel therefore there are no issues in this respect. It is purely down to the piston retainer spring loading the inside pad. I can put a bit of pressure on the caliper with one hand and it immediately frees off then binds again when I release it.

Did your pad kit include new spreader springs (823615269), or are these missing? I think these should counteract the force of the calliper locating spring. Unfortunately, it looks like this part is obsolete - VW Classis Parts have only one left in stock

New spreader springs fitted but these sit on top of the pad and push against the carrier therefore I don't see how these actually "spread" as per the description. I have always just thought of these as pad retainers not spreaders but the description does make me wonder how they are supposed to work. The pads and springs/pins were fitted new 25 years ago by my dad and the car was never back on the road so they are "new" but not today new. They are the correct items though  and are "as new" condition. Maybe the OEM spreaders are different though, I'll do some research.
Looking at the diagram I see how the backside of the spring should push the pads outwards, I don't think mine are like this they are just flat. It's worth trying the correct ones to see if this will help.

Yeah, there are a couple of images on Google against the part number, which look exactly like the springs in the parts diagram. I last worked with these early ATE/Teves callipers in the 1980s, perhaps someone with more recent experience can help.

Just a thought, are the pads too thick? According to the manual, there were two different thicknesses - 10 mm and 14 mm, depending on disc and calliper type. For the smaller callipers fitted in conjunction with 10 mm discs, which I think are correct for your 1.3, the pad thickness is stated to be 10 mm.

Pads fit fine therefore they must be correct, 14mm pads could never fit, with the piston fully recessed the pads are a nice sliding fit. I've ordered an ATE pad fitting kit to see if the spreader makes any difference, it's all I can think of as the spreader currently fitted doesn't have the spring part on the back so won't be pushing the pads away from the disc.

So I fitted the ATE retaining kit, the spreader looks identical to the current one but maybe a little thicker steel so may provide more spreading force. After a hard stamp on the pedal I can still rotate the wheels, previously it was almost impossible.  There is still a fair amount of drag but I think that will improve once everything is properly bedded in.