Brake issue on MK7 GTI performance pack manual

[HK-gti]

Hello all! Hope you kind people can help me.

I've recently bought a 2014 MK7 GTI manual with performance pack.
Sometimes when driving slowly such as parking, the brake pedals bites lower down than normal. When I pump the brakes the bite returns back to the top. Yesterday I had my mechanic look over the car and he couldnt see anything wrong. I've had him do a brake fluid flush (activating abs). When I drove home the brake pedal sink was still there! I then looked behind the brake pedal to see the rubber boot slightly moist. Do I have a faulty brake master cylinder?

Receipts from the previous owner shows brake master cylinder, clutch cylinder, brake fluid was changed last year....

Except the brake master cylinder, what else could be causing the brake pedal to sink to the bottom?? Anyone has similar experience?
Does anyone know where I can find the correct part number for the brake master cylinder, online shows 2 options 25.4mm and 23.8mm? Which one is correct for my car with performance pack?

Picture of said car (I'm based in Hong Kong). I've had MK5/MK6 GTIs/MK5 R32 in the past and haven't come across a low brake pedal before.

[Redreels]

Could any moisture be as a result of the brake fluid change? I believe the master cylinder should be within the engine bay connected to the reservoir and servo. There shouldn’t really be anything leaking internally. Is the fluid level remaining constant? I.e no leaks

[joe6]

The symptoms you describe point to air still in the system. Either the bleed did not completely work or air is still getting in somehow. If a leak then the master cylinder level will slowly go down which presumably you are keeping a check of. The brake pedal does not directly connect to the master cylinder so not sure what the fluid is in the rubber boot. Vw manual says bleed sequence is passenger front, driver side front, passenger rear then driver rear.  Use dot4 low viscosity brake fluid. My first action would be to re bleed and then look at possible master cylinder issues. The lllparts website may be able to help identify the correct part number and give some idea of prices. Good luck.