I have a 89 CRX DX that had a soft brake pedal so we replaced the master cylinder. We did bench bleed it and we bled the brakes at each wheel afterwards until clear fluid came out. The problem is that the pedal feels firm when the car is off, but when I start it the pedal goes to the floor.

I've also noticed that when the car is off and I press the brake pedal, I can hear air rushing out through my cold air intake pipe. I took the vacuum line off at the booster and there is lots of vacuum with the car running. I blew on the check valve with it off the car, and I can blow air one way but not the other, so I assume its working.

Is the brake booster the only other thing it could be? I was thinking that the pedal would be super hard when the car is running, not go to the floor.

How hard is it to change the brake booster? I was reading in the manual and it says something about a special tool and measuring the pushrod or something?

 

did you put the check valve in backwards?

otherwise check the booster.

http://www.norcalcrx.org/tyson/pics/braketest.jpg

 

rebleed the brakes. And dont pump the crap out of the pedal before you crack open the bleeder. Makes air bubbles that wont come out. Open the lines with no pressure on the pedal, then push down and close the valve. And start furthest from the master cylinder and work your way closer.

Sounds like you are just hearing the booster discharging. Its normal.

You could also gravity bleed to make it easier

 

ok time to do a brake check, remove all bleed screws and reapply some new Teflon tape to prevent air bubbles from getting back into the system well you bleed and just get a cheap one man bleed set, just a bottle with a magnet or you can buy the vacuum hand pump which i use(very quick, but will make your master cylinder run dry quick so be careful of this tool)

also you never know who or how the car was handled in its lifetime, when it comes to the thing that stops the big *** metal beast, i buy brand new(and a new master and booster could run like 230 cash or 120 for an ebay knock off)

but none the less a good new master cylinder and new fresh pads and a brake system flush as brake fluid will break down overtime or sometimes old owners might mix different brands and that's a no no cheap brands don't go well with good ones and also check the bottom of you fluid tank for brake fluid gunk i had quite a bit in mine. that's what the filter basket is for!

also bleed in this order, PASSENGER REAR, DRIVER REAR, PASSENGER FRONT, DRIVER FRONT

IN THAT ORDER ONLY!!!!!!!!!!! will not stress this enough, also DON'T BEND/HIT/KINK YOUR METAL BRAKE LINES IN THE ENGINE BAY, THIS WILL CHANGE YOUR BRAKE PRESSURE ON THE LINE YOU BEND

 

Dont use teflon tape on any brake parts. Brake fluid will eat it up and contaminate the fluid. BAD IDEA!!!

 

test the brake booster. After the car has been running, listen carefully when you disconnect the booster hose. It should suck in air. If not it has a leak.

 

Sorry that's incorrect. According to Honda's service manual it says:

1. Passenger rear
2. Driver front
3. Driver rear
4. Passenger front

Our brakes are a dual system and work in conjunction diagonally.

 

Okay here's an update. We changed the booster out with a new one, reverse bled the system using the Phoenix bleeder, and did all the tests for the booster and check valve per the manual (it passed all the tests). After all that, I still have the same symptoms.

I did notice after we changed the booster that my brake lights stay on because the plunger on the brake switch is not making contact with the rubber stopper on the brake pedal. We measured the pushrod and the lock nut on the old booster and set up the new one as close as possible to that.

I'm not sure what else to check.