I have a really hard brake pedal,just rebuilt master cyl,old one was leaking into booster,new pads and I turned the rotors...my booster ..when you push pedal and clear the vacuum with eng off and hold pedal and start car..pedal drops a little..my rotors seem to be thinner than 1 in,my friends newer ones are 1 1/8 thick..my question is..could brake fluid ruin booster,my brakes were touchy a while ago and will to thin rotors make it hard to stop car..

so long as they aren't under spec, you should be okay for the thickness. Do you mean the pedal goes to the stop and bottoms out??Does it feel like you are stopping without power asist? Also, year of car can help. Might be a contaminated booster.

min thickness is .985 and thats where the rotors are now..70 chevelle ss 396..it feels like no power to me.I put semi metallic pads in the front,would the calipers not work with a min thickness rotor..proportion valve bad?

calipers will work until the piston blows out. Cnan you see if there is fluid in the booster?