I have had the same problem -- just two days ago. I stumbled upon the answer at this web page:
http://z31.com/brakepedal.shtml
In short, your brake pedal stoppers might be shot. They get hardened and brittle with age, crack and break. I found both mine in pieces on my floor. (I'm glad I don't vaccum very often).
Here is the recommeded part information from ZBum:Order these parts from you Nissan dealer. 2 of 46584-S0100 = $1.76 each 1 of 46512-H0101 = $0.91 each (manual transmission only)
Incidentally, my model is a 1986 300zx non-turbo.
sorry if this is the wrong thread -- thought I'd just post this info since others might be looking here for the answer.