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Unread 06-29-2008, 09:15 PM   #1
Tarukai
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1998 XJ Cherokee 
 
Join Date: May 2008
Location: Delmar, NY
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Brake pedal has return pressure, but very little when actually braking

If the title is too confusing, let me explain it here:

I finally got my exhaust back on in full thanks to my uncle, and I go to start it and take it off the ramps so I can finally get it inspected. I start it, and I go to shift, but before I even press the shifter button, I push in the brake pedal and it simply floors. Now, My first thought was "Oh crap, my brakes are out!", but when I lifted my foot, the pedal returned to its standard at-rest place. Needless to say, I rolled it off the ramps and the brakes worked enough to stop me so I could shift to park. I turned the Jeep off and got out, telling my dad about it. He told me to drive around the block to see how it felt -- not too bad an idea since we have a bunch of stop signs around our neighborhood.

I get in the Jeep and start it up, starts fine of course. I pump the pedal a bit and the pressure builds back up to how it was the last time I drove it. My dad speculates it might have been the fact that it was on ramps for a few months. So, I shift into reverse with my foot on the now firm brake, and I feel the pressure sink under my foot. Mind you, it held the brakes, but as I would find out in a moment, not too much. As I backed out, I did it slowly, and I pushed the brakes, and stopped. I shift into drive and go to the first stop sign.

As I'm driving, I'm pushing the brakes a bit seeing if the pressure is changing or building up at all. I can tell the brakes are still working, since I'm slowing down when I push the pedal. So as I come to the stop sign, I decide to start slowing down earlier than usual. I pushed the brakes to the floor and came to a stop, albeit a mite slow. I drove it around the block and it stayed the same the whole time.

I checked the fluid levels and it seemed fine, since it's full to the brim on a slant (this was on the ramps), but just plain full otherwise.

I'm assuming broken line or air or something in the lines. I'm going to bleed the brakes within the next few days with a friend, and hope that I don't snap my bleeders off (since it's so rusty in so many spots).

Please help me with this, as I would like to have the brakes working right before inspecting, so I don't crash into the garage XD

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Unread 06-30-2008, 07:00 AM   #2
CharBroiled
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bleeding them is the 1st step, cant hurt and fresh fluid is always a good idea.

if the pedal falls away with steady pressure it usually means a leak or the seals in the master cylinder are bad and allowing the fluid to seep around the seals.

if it is the master cylinder seals you wont loose fluid while if its a leak you obviously will.

air in the system usually just gives a spongy pedal, not a sinking pedal.
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Unread 06-30-2008, 09:37 AM   #3
Saudade
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Sounds like your brake booster is shot.

To test the booster,
Start the engine and let run for a few minutes to build vacuum. Then shut off.

With the engine off. Pump the brake pedal a few times and it should "pump up" (pedal gets higher with every pump).

Keep pumping until it "tops out", then, on the last pump, press and hold the pedal. Start the engine, the pedal should go down.

With the engine running press and hold the brake pedal, then shut the engine off while still pressing the brake. The pedal should not change height.

If it does these, then the booster is OK.

Last edited by Saudade; 06-30-2008 at 11:39 AM.. Reason: Addedd Text
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Unread 06-30-2008, 10:39 AM   #4
Tarukai
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Quote:
Originally Posted by Saudade View Post
Sounds like your brake booster is shot.

Start the engine and let run for a few minutes to build vacuum. Then shut off.

With the engine off. Pump the brake pedal a few times and it should "pump up" (pedal gets higher with every pump). Keep pumping until it "tops out", then, on the last pump, press and hold the pedal.

Start the engine, the pedal should go down.

With the engine running press and hold the brake pedal, then shut the engine off while still pressing the brake. The pedal should not change height.
this is EXACTLY what happened! Ah great, now I get to replace my brake booster. Suggestions on where to get a good one (junkyard or remanned?) would be very helpful, and I'll search for a write-up on doing it myself.

and I am still definitely bleeding my system, because the fluid is black. I really need to replace all of that.
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Unread 06-30-2008, 11:37 AM   #5
Saudade
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Just to be sure, if it does the above, the booster is OK. For example, if you pump up the pedal, then turn the engine on and the pedal DOES NOT drop down, then the booster is bad.

I should have made that more clear above. I'll edit it to correct.
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Unread 06-30-2008, 01:32 PM   #6
Bushytails2
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A bad booster will cause an overly firm pedal, _not_ a soft pedal that sinks to the floor. If you're having to use two feet to come to a quick stop, that would point at the brake booster...

If the pedal slowly sinks under constant pressure, and the fluid level is ok, a bad master cylinder would be a good place to check. If the pedal slowly sinks under constant pressure, and the fluid level gets increasingly low, leaking wheel cylinders or calipers are likely.

Bleeding the brakes is a good first step, as any air in the lines will make finding other problems a lot harder.

--Bushytails
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Unread 06-30-2008, 11:48 PM   #7
Tarukai
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okay, so I'm glad that my brake booster is fine, then.

Should I have a friend check the fluid level while the top for the cylinder is off? If so, I'll be doing that. I have a feeling since I'll be replacing the system's fluid either way, it couldn't be too bad.

mind you, the fluid is at the level it was when I bought it, so I don't think it's leaking (or at least not much). Hopefully, at least.
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Unread 07-01-2008, 01:33 AM   #8
Bushytails2
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A leak large enough to cause stopping problems like you describe would cause the fluid level to drop to empty rather quickly...

--Bushytails
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Unread 07-01-2008, 10:46 AM   #9
Tarukai
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okay, so now I'm working under the assumption of not a leak, since it's nowhere near empty.

I'll have a friend check the fluid today.
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Unread 07-01-2008, 12:36 PM   #10
440_Magnum
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Quote:
Originally Posted by Saudade View Post
Sounds like your brake booster is shot.
.


I disagree! If the booster is bad, the pedal WILL NOT sink all the way to the floor. It will be very hard to push, and the brakes will feel very ineffective because of the huge pedal effort required... but it will not sink to the floor. A pedal that sinks to the floor is always a sign of either air in the system (bleed the brakes!) or a leaking master cylinder, wheel cylinder, or caliper.

The symptom you are describing sounds a little more like air in the lines, because you can pump up some braking pressure. Brake systems are designed to work with NO AIR in the lines or cylinders since air is compressible. Follow the recommended procedures for bleeding the brakes (in a nutshell have a helper apply pressure to the pedal while you open the bleeder on each wheel cylinder, starting with the one furthest from the master cylinder (right rear) and ending with the one closest (left front, assuming left-hand drive). Close the bleeder valve BEFORE you have your helper release the pedal, and top off the brake fluid after each wheel.
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Unread 07-01-2008, 01:48 PM   #11
lhp21
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More than likely a part problem. If it was the fluid, you would have serious problems and it would be down (obviously). Have you tried bleeding yet? (Only read OP)
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Unread 07-01-2008, 04:43 PM   #12
Tarukai
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Quote:
Originally Posted by 440_Magnum View Post
I disagree! If the booster is bad, the pedal WILL NOT sink all the way to the floor. It will be very hard to push, and the brakes will feel very ineffective because of the huge pedal effort required... but it will not sink to the floor. A pedal that sinks to the floor is always a sign of either air in the system (bleed the brakes!) or a leaking master cylinder, wheel cylinder, or caliper.
Not to be rude, but you need to read down further. He corrected himself.

Quote:
The symptom you are describing sounds a little more like air in the lines, because you can pump up some braking pressure. Brake systems are designed to work with NO AIR in the lines or cylinders since air is compressible. Follow the recommended procedures for bleeding the brakes (in a nutshell have a helper apply pressure to the pedal while you open the bleeder on each wheel cylinder, starting with the one furthest from the master cylinder (right rear) and ending with the one closest (left front, assuming left-hand drive). Close the bleeder valve BEFORE you have your helper release the pedal, and top off the brake fluid after each wheel.
I figured as much, and my grandmother told me this happened to one of her cars a few years ago (one of my uncles is a mechanic), so she had him bleed the brakes for her. And I know the steps of brake bleeding, I first learned it when I got an issue of PopMech with the steps in it (and I got a DIY supplement with my last issue which has a bunch of vehicle DIY stuff, which is nice).

I'm actually going to see if I can run it to 55 MPH, and if I can and the brake still work right, I'm going to take it to a place around here called Warren Tire and have them bleed the brakes and inspect it finally.

and before anyone gets up in arms about my not bleeding it myself: I've had enough with this, I don't want to break off a bleeder valve and not be able to replace it for a few days, and this way the brakes should be better and it will be inspected to boot!

also, I have work Thursday, so right now I'd rather not risk this kind of thing. Next time I have to do it, I guarantee I'll be doing it with a friend.

EDIT: oh, and lhp21, I'll be having it bled tonight, so yeah. Hopefully it's not a part problem, else I might have them replace it at Warren Tire. Unless it's too expensive, in which case I might not.
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