Why can you "pump up" the brakes with engine off? - JeepForum.com
Search  
Sign Up   Today's Posts
User: Pass: Remember?
Advertise Here
Jeep Home Jeep Forum Jeep Classifieds Jeep Registry JeepSpace Jeep Reviews Jeep Gallery Jeep Clubs Jeep Groups Jeep Videos Jeep Events Jeep Articles
Go Back JeepForum.com > Models > Jeep CJ Forum > Why can you "pump up" the brakes with engine off?

The Original 3/8" Ruffstuff Diff Cover!Don't forget the Jeep Forum discount, 10% off!Ruffstuff's Monster Shock Brackets!

Reply
Unread 04-13-2012, 12:16 PM   #1
McFly0815
Registered User
1986 CJ7 
 
Join Date: Jul 2007
Location: Germany
Posts: 19
Why can you "pump up" the brakes with engine off?

Hi there,

still chasing the gremlins inside my brakes... I need some lesson from the experts...

If you have power brakes, you normally can pump up the brakes until you get a stiff pedal (with the engine shut off).

Why can you do that? What is happening physically inside the booster? Or is it not related to the booster?

The reason I am asking is: I can not pump up the brakes in my CJ - altough the booster seems to work (there is definitely brake assistance if the engine is running).

So what's happening inside the booster while "pumping up"?



__________________
'86 CJ7 4.2L - 3" suspension lift - 1" body lift - 31x10.5x15 MT
'89 XJ 4.0L - 3 door - 5 speed - out of order...
'99 XJ 4.0L Limited - stock
McFly0815 is offline   Reply With Quote
Unread 04-13-2012, 12:21 PM   #2
CSP
Registered User
 
Join Date: Aug 2002
Location: Colorado
Posts: 12,073
You're not really pumping up the brakes and getting a stiff pedal. You're bleeding off the vacuum assist from the booster. The stiff pedal isn't really a stiff pedal like you're thinking. It's just reverting to manual brakes which requires more leg effort.

It's the same thing as having power steering and the steering requires more effort if the engine isn't running. The power assist goes away.
CSP is online now   Reply With Quote
Unread 04-13-2012, 12:29 PM   #3
firewalker909
Ok, one more.
 
firewalker909's Avatar
1973 CJ5 
 
Join Date: Jan 2011
Location: Yucaipa, California
Posts: 378
The power brake booster is only there to assist with braking action when the engine is running and you are getting vacuum to the booster. When the engine is not running there is no assist from the booster, the brake master cylinder will work the same as a vehicle with non-power brakes.

The brake master cylinder creates the pressure for the brake system. The power booster only assists in applying pressure to the brake master cylinder.

If you are having to pump the brake pedal to create pressure ("a stiff pedal") you have other problems with your brakes ("gremlins").

You need to make sure your brakes are bled properly at the wheel cylinders and the master cylinder. If after bleeding you still have soft pedal problems, you may have internal problems with the brake master cylinder and it may need rebuilding or replacing.

If you haven't done so, you should also pull your wheels and check the brake components. Ensure you wheel cylinders/calipers are not leaking, the brake shoes/pads are in good condition, and the springs, adjusters, and other components are in good shape.
firewalker909 is offline   Reply With Quote
Unread 04-13-2012, 02:20 PM   #4
CSP
Registered User
 
Join Date: Aug 2002
Location: Colorado
Posts: 12,073
Quote:
Originally Posted by firewalker909 View Post
When the engine is not running there is no assist from the booster, the brake master cylinder will work the same as a vehicle with non-power brakes.
Yes and no. There is vacuum stored in the booster for a reserve in case of an emergency and the engine dies. This is about enough to two to three applications of the brakes, and then it reverts to manual.
CSP is online now   Reply With Quote
Unread 04-13-2012, 02:29 PM   #5
LT1CJ7
Registered User
1983 CJ7 
 
Join Date: Aug 2004
Location: [duhm-as]
Posts: 4,512
Actually a CJ power booster with no vacuum requires more effort than a manual system.
The power booster bracket linkage is offset so it requires more effort than a direct rod like the manual MC uses.
booster-brackets.jpg  
LT1CJ7 is offline   Reply With Quote
Unread 05-21-2012, 09:14 AM   #6
HokieCJ7
Registered User
1983 CJ7 
 
Join Date: Sep 2011
Location: Lansdowne, Va
Posts: 26
I too have added a power booster on my 83 CJ7. When the engine is off, I can pump the peddle and it gets stiff, but when the engine is on, I can never pump the peddle to get it "hard". Is this working as it is supposed to?
HokieCJ7 is offline   Reply With Quote
Unread 05-21-2012, 09:26 AM   #7
Matt1981CJ7
Web Wheeler
 
Matt1981CJ7's Avatar
1981 CJ7 
 
Join Date: May 2011
Location: Elbert, CO
Posts: 10,015
Quote:
Originally Posted by HokieCJ7 View Post
I too have added a power booster on my 83 CJ7. When the engine is off, I can pump the peddle and it gets stiff, but when the engine is on, I can never pump the peddle to get it "hard". Is this working as it is supposed to?
If you read and comprehended CSP's earlier post, your answer has already been stated.

But I will repeat it. When you pump your power assisted brakes with the engine off, you are merely bleeding the vacuum from the booster. Without the vacuum, your brakes are basically manual. That's why the pedal gets stiffer feeling.

Matt
Matt1981CJ7 is offline   Reply With Quote
Unread 05-21-2012, 09:31 AM   #8
John Strenk
Moderator
 
John Strenk's Avatar
1976 CJ5 
 
Join Date: Feb 2005
Location: Shalersville Ohio
Posts: 13,007
Quote:
Originally Posted by HokieCJ7 View Post
I too have added a power booster on my 83 CJ7. When the engine is off, I can pump the peddle and it gets stiff, but when the engine is on, I can never pump the peddle to get it "hard". Is this working as it is supposed to?
My 76 I converted to Power brakes gets stiff right quick with or without the engine running.

But I also have braided lines that help. because old rubber lines can expand a little.

also check the Initial adjustment on the rear drum brakes if you have them. These are self adjusting eventually but if you are backed off to far, it will take forever, if even possible, to get adjusted correctly. You should manually adjust the rear shoes before you start bleeding the system.

A lot of people forget about the initial adjustment on drums to get them to work right.
John Strenk is offline   Reply With Quote
Unread 05-21-2012, 09:36 AM   #9
PROJECT 23
Registered User
1985 CJ7 
 
Join Date: May 2011
Location: North Ridgeville, Ohio
Posts: 2,348
Quote:
Originally Posted by John Strenk

My 76 I converted to Power brakes gets stiff right quick with or without the engine running.

But I also have braided lines that help. because old rubber lines can expand a little.

also check the Initial adjustment on the rear drum brakes if you have them. These are self adjusting eventually but if you are backed off to far, it will take forever, if even possible, to get adjusted correctly. You should manually adjust the rear shoes before you start bleeding the system.

A lot of people forget about the initial adjustment on drums to get them to work right.
He said "gets stiff right quick". Hee hee : )
__________________
[B][COLOR="DarkRed"]Gone Fishin'[/COLOR][/B]
PROJECT 23 is offline   Reply With Quote
Unread 05-21-2012, 09:59 AM   #10
yayaisland
Registered User
1976 CJ5 
 
Join Date: Feb 2011
Location: BAKERSFIELD, CA
Posts: 14
I to have a cj5 and it has power/booster assistance. I have a question on the topic. Every once in awhile I'll be pressing the break to stop and the peddle gets so stiff I can't press it any farther and I'm still rolling. I have to let off the peddle and press it again to get the breaks to fully stop the jeep. I have a disk break changeover on it. All four are disk. Yes the break pads are good. No there's no air in the line. Any ideas?
yayaisland is offline   Reply With Quote
Unread 05-21-2012, 10:04 AM   #11
CSP
Registered User
 
Join Date: Aug 2002
Location: Colorado
Posts: 12,073
yayaisland it sounds like you have an internal leak in the master cylinder.
CSP is online now   Reply With Quote
Unread 05-21-2012, 10:09 AM   #12
John Strenk
Moderator
 
John Strenk's Avatar
1976 CJ5 
 
Join Date: Feb 2005
Location: Shalersville Ohio
Posts: 13,007
Do you have your brake warning light disconnected?
It sounds as if the safety valve inside the combination valve has moved and is blocked off one side of your brake system.
The only way to tell is if your warning light is hooked up.
John Strenk is offline   Reply With Quote
Unread 05-21-2012, 10:45 AM   #13
HokieCJ7
Registered User
1983 CJ7 
 
Join Date: Sep 2011
Location: Lansdowne, Va
Posts: 26
Hello Matt '81. Thanks for your clever answer, which really didn't answer my question at all. Perhaps you should re-read and comprehend before answering.

Others- Can someone please tell me if I should be able to pump my brake pedal with the engine ON, and build up a firm pedal, or does this only occur with power brakes when the engine is off?

Thank you for those which choose to answer others, and not condescend.
HokieCJ7 is offline   Reply With Quote
Unread 05-21-2012, 10:48 AM   #14
PROJECT 23
Registered User
1985 CJ7 
 
Join Date: May 2011
Location: North Ridgeville, Ohio
Posts: 2,348
He said pump and firm......
__________________
[B][COLOR="DarkRed"]Gone Fishin'[/COLOR][/B]
PROJECT 23 is offline   Reply With Quote
Unread 05-21-2012, 10:49 AM   #15
HokieCJ7
Registered User
1983 CJ7 
 
Join Date: Sep 2011
Location: Lansdowne, Va
Posts: 26
Hi John,
You mentioned having that the rubber lines can expand a little under pressure. Perhaps this is what I have going on. Is this expansion actually noticeable by sight, or can you feel it bulge under pressure, as I had my wife press the pedal, while I felt the lines, and did not notice any change in wither their appearance or feel. Also, would a slight expansion be enough to cause your pedal not to get firm when pumping with the engine on?

Thanks
HokieCJ7 is offline   Reply With Quote
Reply
Thread Tools


Suggested Threads





Jeep, Wrangler, Cherokee, Grand Cherokee, and other models are copyrighted and trademarked to Jeep/Chrysler Corporation. JeepForum.com is not in any way associated with Jeep or the Chrysler Corp.