yet another problem with my wonderful stang this time its the brakes

[lmarinukjr]

ok i have a 1989 5.0l coupe.dropped it off at his shop for safety inspection.i could have done it at my shop but i got a traffic ticket for no safety sticker so i figured when i go to court it would look really shady if my license number and name was on the paper work.so heres my list of things wrong. front brakes,front rotors,rear shoes,spring kit,drums and wheel cylinders.yeah i know it seems like a lot but i already did the front brakes and im putting a complete rear in my car tomorrow from my totaled hatch all the brakes are new. ok so heres my question i suspect that my master cylinder is bad here's why if i have the engine off and i pump the pedal it comes up really high and as soon as i start the engine the pedal drops pretty far down.what do u think?

 

[87'GTstang]

You are building line pressure when you pump the pedal. When you start the engine, you are then getting vacuum-assist. With the engine off, a good system shoukd give you a very nice and firm pedal feel when you stomp on it. If your M/C is going bad you would have to replace fluid periodically, not to mention you would start getting air in the system and that would create a very "spongy" pedal feel and it would get progressively lower to the floor with time. Also when the M/C starts going, you can notice a rusted/pitted/discolored/paint missing on the booster indicating the seal is worn. Usually if you see it at this point, replace the M/C asap, or you will be replacing both.

 

[lmarinukjr]

ok so u think its just the rear wheel cylinders