IndigoBlueWagon
TDIClub Enthusiast, Principal IDParts, Vendor , w/
I doubt you need either. Boosters are such a rare failure item they're not sold in the aftermarket, AFAIK. and I'd bet the sinking pedal is either a bad brake booster hose or a leaking brake hose. I chased this problem in my Golf including replacing the master only finally finding a small drip of brake fluid under the car after a track session. Pumping up the brakes at 100 MPH at the end of a straight isn't fun.