Replaced brakes, is that new pads only or new discs as well.

If new pads only you won't get a proper pedal till the pads have bedded in to the existing pattern established on worn and grooved discs, this can take a few hundred miles.

Presumably the calipers pistons are moving freely in their bores, rears usualy suspect for seizing.

I have changed hundreds of pads and never had a soft pedal ,its not caused by grooved discs but by air in the system.

I beg to differ. If the discs are well-worn a nice, new and perfectly flat pad will only engage with the "high spots" at initial contact and additional pedal movement will force some deformation to the surface, giving a spongy feel.

Once the pads have bedded in to match the grooving / dishing of the disc, a firm pedal returns.

Less common these days as most places'll recommend a disc swap if there's any visible wear, while we used to happily run 'em while they were like waffle irons and with a monstrous lip at the outer edge....