1992 318, automatic 4 x 4. Brake light and ABS light came on, the I got a low brake pedal. Appeared low on fluid, I topped it off and found no leaks. Replaced master cyl., no change, still low pedal and lights. Hear a vacuum noise by brake pedal when applying brakes. Any thought appreciated.

 

Well I'd start by identifying the noise your hear. Vac leak or something? You should be able to figure that out by having a friend press the pedal whille your under there listening\ looking. You're not losing fluid though?

 

Don't forget to bleed all 4 good.

 

your vacuum booster is bad. the diaphram inside is bad and leaks when you apply the brakes

 

I'd have to agree.