Last night a buddy of mine brought his 93 Civic over to my shop to have the master cylinder replaced as it was leaking. He said that earlier yesterday he did a tune up and the idle of the car raised 300 rpms and was revving up and down. I instantly thought it was his IACV. Anyways we replace the master cylinder with a new autozone unit and get it all back together. When we bled the brakes the pedal was stiff until you started the car. As soon as you started the car, the brakes would go directly to the floor, if you pumped them while car was running they'd get stiff but still go to floor eventually. Also we noticed that when you applied the brakes the idling issues went away, the idle was smooth. As soon as you let go of the brakes the idle issues would come right back. I am about 90% sure its a bad booster, just wanna make sure before he goes and buys one and I have to tear this thing back apart.

 

While driving your coming to a stop at a traffic light or what ever. After you stop does the pedal go to the floor? Or at least act like it wants to go to the floor? If so, bad booster.

 

It goes to the floor as soon as you start the car and hit the brakes, does it while driving as well.

 

I just did my brothers master cylinder because his was showing the exact same symptoms as yours. Sinks to floor right after you start. Also hoping for an answer

 

Did you adjust the push rods?

 

Yes, the idle issue is what leads me to believe its a bad booster as well as the culprit for brakes. I will find out tonight as we're changing the booster

 

Is the booster vacuum line ok? I had these same symptoms in my Civic and it ended up being the vacuum hose even though it looked good upon inspection.

 

still have the same amt of fluid in it ....as in no leaks /air getting in the lines.......autozone parts suck

 

Yeah still has same fluid in it, I am pretty sure the MC isn't bad simply because of the idling issue which is usually common with a vacuum leak