Anyone happen to know the brake line size or sizes on a 75 cutlass? Would like to replace all mine.

 

The line that runs from the M/C to the back of the combo valve to the rear brake hose is 1/4". All the rest (including those on top of the rear axle) are 3/16. This is pretty much standard for all Oldsmobiles since 1967. Note that the flare nuts at the M/C and combo valve will not be the same thread as the ones that come on lines from the auto parts store. GM used unique threads to prevent improper connections on the assembly line.