Brake Servo

Hi, Originally my 1600 Hi Hi My Mk1 Hatch was originally  fitted with a 7" Servo No 172 612 107B. Brakes had a long pedal and were spongy. This servo had the nonreturn valve inserted in the neck. I fitted an 8" bought second hand and now no vacuum what so ever. A local mechanic fitted it for me. I assume that I bought a lemon  I even tried placing an inline non-return valve. I'm thinking that the layout for a GTi is different as it has the non-return valve inline and I've made an error in the fitting.
If I were to fit a Mk2 complete with master cylinder would it be a straight fit? I wouldn't have to worry about clevis pins and shortening rods etc. Most local mechanics don't want to tackle this job Can anyone decipher this mess and provide a solution