Ah, you have a servo. Right, well, I had a problem once with a BMW with brakes binding. Bleed them and it would be OK for a while then gradually get worse and when warm brakes would get hot, bleed them again and all OK for a while etc etc. It was a servo seal. Slightly different to your symptoms but could be something like this?

Simon
2002 4/4
1936 Austin 12 light 4 Ascot saloon
2023 AMG Mercedes C300e estate
2021 Audi A1 sport