i will stand by my comment caliper seals are designed to de form under pressure and re form when pressure is released also not leak whilst in operation
if master cylinder ratio and adjustment is correct the fluid transfer required to apply brake pressure will return when released moving the piston an equal amount
if this did not happen no calipers would function correctly if pistons are too tight in bore brake will drag when released if piston returns too far
i would look at master cylinder adjustment and air leaks in the system air will compress under pressure and allow piston to travel further on release ( hence your extra movement )
on the return stroke brake fluid does not compress under normal loads air does and allows piston to travel further on return
As I said, I removed a perfectly functioning stock iron caliper, so the rest of the brake system was also perfectly functioning. But the Girling caliper worked as a described previously. So, in this case, the fault was definitely within the caliper.
As you say, the seals play an integral part in piston operation. And as others have said already, changing Girling seals for genuine AP seals solves the problem. So, on balance, given the available info, it’s seems fairly clear that something is wrong with the Girling seals.
As I mentioned earlier, the problem with Wassells model is that they have flooded the supply chain with these things, so even if they did listen and did improve the seals, an end user has no way of knowing what they’re buying from a retailer, and that’s likely to last a LONG time until the supply chain is purged. This is exactly the case with their good piston / bad rings offerings.
Your point about retailers and having a range of quality available is 100% bang on IMHO. This is why I’d alway recommend only a select few retailers to people who might ask. I’d also generally avoid unknown sellers on eBay etc. The point we’d differ on is when I am buying important parts from my short list of trusted sellers I always ask for the best option and always specify ‘not Wassell’ where possible!
We can debate that last point all day long, you’ll come up with examples of good parts and I’ll counter with examples of bad experience, etc. But I have been buying motorcyle parts for 40 years. And over time we all build up a mental data base of what works well, what causes us grief, etc. And my data base drives my choices. Had Wassell parts proved good over the years, I’d be an ardent fan. But they haven’t, so I’m not. Simple as that really.