Not sure of the Bosch variety/year of your van, but I have ATE calipers which I initially had a problem with the RSF caliper binding last year. This happened again recently. I thought that if it's happening to one side the other can't be far behind, so I'm just about to replace both of mine with a new pair, as I looked at the rebuild kits for them, but after factoring in the cost of those, my time taken to clean them up and the chances of a reoccurring fault, I've bought a new set to fit (cost difference of around £25 per side) and will keep /refurb the removed ones at a later date for a spare set. It'll give me some peace of mind as they have a two year warranty aswell.
Just a thought but if you do change them both for new, as long as you change the pair does it matter what type they are?
Most binding calipers or seized calipers are a result of negligent PO service.
Brake fluid is hygroscopic and will redily absorb water.
VW asked for it to be changed every two years.
Left too long or never changed it will absorb moisture.
This will cause some oxidation in the bore of the caliper and on the piston.
This crud collects behind the bore seal.
It forces the seal harder against the piston, until it can no longer release its grip.
Cleaning up the caliper and piston and, if there's no leaks, refitting the seals, it generally cures the issues
Ordered a caliper from Brickwerks today. Seal split some time ago and I was going to refurb but just want to swap over and be on my merry way. Looked to me that the seal might be tricky to get in so I'd be interested to see how you get on with it.