Extremely low brake pedal with engine running

[gorp00]

In another thread I asked for help on an ABS pulsing just before coming to a complete stop. As I got into this problem I decided to replace both front sensors, front calipers, master cylinder and brake booster. Everything was old and beat up plus it wasn't worth trying to fix the sensors with the work involved only to find they needed to be replaced. Better to do it once.

Anyway, I got everything installed and the ABS problem is gone but... when the engine is started (creating vacuum in the booster) the pedal goes WAY down. The vehicle does stop and I can even get the ABS to kick in on a loose surface. I bench bled the MC and vacuum bled the lines. The pedal is fairly firm but does move down slowly with engine off.

I've gone through over a quart of fluid bleeding. I don't see any leaks but I do still see some bubbles in the fluid. Am I just not bleeding enough?

 

[MrQ]

Sounds to me like you still have air in there. I would not even do the vacuum bleeding this time and use the old method with the brake pedal, clear tube, and a jar of brake fluid. That method has always worked for me, the vacuum system seems to never get all the air out.

 

[gorp00]

Thanks Dan. I just re-bled. Pedal is rock hard with engine off but still goes low when I turn it on. Back to basics, as you say.

Rich

 

[rasouth]

I've got a '96 that the brake pedal goes way down when I start the engine. I think it has something to do with the brake booster creating that initial vacuum when you start up the engine. As soon as I release the pedal it goes back to normal and is working fine. I even tried my 2008 Honda Civic and it does the same thing except the pedal doesn't go down as far as in the Explorer. I think it's normal.

 

[RickOTR]

The only way to get all the air out of the system if you replaced the master cylinder is to have a dealer level scan tool to open the ABS module. This opens the valves in the ABS module and allows complete bleeding of the system.

 

[MrQ]

I disagree. I have changed master cylinders and not ever needed to cycle the abs. It remains closed until needed.

If you are sure you got all the air out, then maybe it's time to look at the booster.

 

[massacre]

The only way to get all the air out of the system if you replaced the master cylinder is to have a dealer level scan tool to open the ABS module. This opens the valves in the ABS module and allows complete bleeding of the system.

I was able to get air out by activating the abs on a gravel road.
Had to bleed after but it worked

 

[donalds]

I used forscan to activate cycle my abs pump and valves for bleeding

 

[gorp00]

I'm going to to bleed again using the the standard pedal pump method. I had not used the air driven vacuum before so maybe I'm doing something wrong. After the last bleed I ran around on a gravel parking lot to exercise the ABS. It does come on and the brakes do work but the pedal is still at the floor and, I noticed, no brake lights. That's probably the switch but I thought I'd mention it.

Lastly, I'm thinking it's not a problem with the booster because it's doing the opposite of what you would expect with a bad one, i.e., pedal effort gets easier. From researching, a non-functioning booster would give a harder pedal. So my guess is that whatever air is trapped in the system is being compressed by the enhanced pedal force. It seems a lot of air but it could be. We'll see after I bleed, again.

Thanks for the help; I'll keep you posted.

 

[410Fortune]

when you shut the engine off and pull the vacuum line for the booster does air rush in? A booster should hold vacuum

If the master cyl is depressed too far during initial bleeding process you can blow out the seals in the master and the pedal will go to the floor

Just FYI

 

[Mbrooks420]

It only takes a very small amount of air to ruin your pedal feel.

 

[Gary Crist]

Long ago I had that problem w/ 67 Cougar.
It turned out to be the rod length of the booster
output. The rod length determined the amount of
brake assist.

I do not know about yours, but mine made a lot of hissing noise too
I replaced the "new" booster. Fixed

 

[gorp00]

when you shut the engine off and pull the vacuum line for the booster does air rush in? A booster should hold vacuum

If the master cyl is depressed too far during initial bleeding process you can blow out the seals in the master and the pedal will go to the floor

Just FYI

410Fortune - If the seals were blown in this way would the pedal still feel solid with the engine off? I've got a good pedal until I start up. Just checking to see if I may have done this. Thanks

 

[gorp00]

I've bled and re-bled and still the pedal goes to the floor when I start the engine. The brake pedal is rock solid when the engine is off but as soon as the booster gets vacuum, no pedal. Only at the very bottom of the stroke do the brakes work.

I adjusted the push rod length on the booster but it was not that far off and it still doesn't work.

What else? hoses?

 

[MrQ]

Hmm, might be hoses. It may be rock hard without vacuum because you don't have the force multiplication of the booster, however with the booster you now have enough force to stretch the hoses.

 

[boominXplorer]

Calipers on the correct sides? Bleeder screw at the top?

 

[RickOTR]

I used forscan to activate cycle my abs pump and valves for bleeding

Forscan is what I use to bleed the system too

 

[410Fortune]

bad booster can cause a sinking pedal
Did you test engine vacuum?

 

[gorp00]

RickOTR, I just tried Forscan Lite and it told me that an ABS test could not be done, it was missing. Did you use it on your '97? If so, did you get the advanced module? I tried using a Foxwell before this and it told me the same thing. How did you do it?.

Thanks

 

[RickOTR]

Per FORscan... FORScan Lite for iOS and Android are simplified versions of FORScan optimized for mobile platforms. It has no configuration and programming functions,as well as some service functions.

If you're using Andriod, it may not have support for the ABS activation to open the ABS module valves. I use my laptop with FORscan and have no problem with ABS functions. You may have to ask FORscan if there is a difference between FORscan and FORscan lite.