Baffling break problem on minivan

Maintenance/Repairs
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1

Anyone want to venture a guess? My mechanic is stumped on this one…



I’ve got a 2002 Chrysler Town and Country minivan. About 6-8 weeks ago the brakes felt soft so we brought it in. Mechanics couldn’t find the problem, so they guessed and bled the brakes. That solved the problem.



A few weeks later, the problem returned. Since they couldn’t find a leak, this time they guessed that it was the master cylinder. Once again, when they gave it back to us, the brakes were firm.



Then, you guessed it: a few days ago, the brakes went soft again. This time, the mechanic, and even the local Chrysler dealership, have no clue what could be causing the problem.



Anyone?

2

sounds like you have a failing brake booster, or a vacuum leak to the booster.

maybe some other wizened mechanics could help decipher this too?

3

I got clarification from my wife, who’s the primary driver. Here’s what the brakes are like when the problem exists:

  • When she first presses the pedal, it goes nearly to the floor before any significant resistance is encountered.
  • If her second pressing of the brake pedal is rapid, she makes no progress on brake pressure.
  • If she lets the pedal come all the way back up, she’s back to square one on the brake pressure.

So her technique is this, each time she needs to brake:

  1. Press the pedal to the floor.
  2. Let the pedal up most of the way, but not all the way.
  3. Slowly depress the pedal a second time. This usually does the job. Sometimes she needs to do steps 2-3 twice before she gets real braking power.

But as mentioned above, after each incident in which she needs to use the brakes, she loses pressure pretty much immediately. She says that even if it’s just a few seconds between braking events, she’s back to square one on the brake pressure.

4

Are they finding air in the brake system each time it’s bled? If so, what might be happening is air is being pulled into the hydraulic system each time the brakes are operated. How does this happen?

Each time the brakes are applied, hydraulic pressure is forced on the caliper pistons to push them out onto the brake pads. This hydraulic pressure is also applied to the caliper piston seals which causes them to expand slighty. When the brakes are released, the hydraulic pressure is released which allows the caliper pistons to retract slightly back into their bores. At the same time, the hydraulic pressure is released from the caliper piston seals which causes them to shrink back to their normal size. If the seals or the bores in the calipers are even worn slightly, this is when just the slightest amount of air can be pulled past the seals and into the hydraulic system. This air then accumulates each time the brakes are used, until you end up with a soft brake pedal.

Now I’ve seen this problem on older vehicles with high mileage, so I wouldn’t think this would happen on a vehicle as new as yours. But hey. That’s all I got.

Tester

5

when this happens does the parking brake still function normally? Maybe there is a problem with the rear brake self adjuster thats allowing the brake shoe to retract fully.

6

Thanks for the tip. As far as I know, they’re not seeing anything unhealthy when bleeding the brakes. At least, they report that there’s no fluid missing. Does that usually imply that there’s no extra air, either?

How do mechanics normally determine whether or not the problem you describe (air slipping past the pistons) is happening? Do they just guess, and replace the rear calliper assemblies?

7

We didn’t think to check. Fortunately, we never needed to employ the emergency brake.

But would we expect the problem you’re describing to be fixed each time the brake lines are bled?

8

Possibly. Using the parking brake is what keeps the brakes adjusted.

9

yes that’s what is going on,air is entering the system.

10

There’s no way to test to see if this is what’s happening. You look at if you’ve exhausted all other possibilties, and replace the calipers.

Tester

11

does this NOT sound like a brake booster/ MC issue?

12

plus 66

13

I’m not sure. I’m not a car guy. I was just trying to give more info that I thought might be relevant.

14

Can you imagine it being anything else?

15

I thought hitting the brakes to arrest backward motion was what adjusted the brakes?

16

Omit. I answered before I read up on the booster.

The air could have entered during the brake bleeding process. (a helper pumping the brake instead of holding it steady)

17

Hello,why do you think the Booster is the problem?

where is Air entering the sys,from that point?

18

  1. In these vans (with rear drum brakes only), the rear brakes are adjusted when the parking brake is used, not when backing up and braking. Rear drum brakes way out of adjustment will cause a soft pedal.

  2. A bad power booster results in a hard pedal, not a soft one.

19

Supposing the rear drum brakes are way out of adjustment. Would it make sense that the problem temporarily goes away the two times we had work done (first bleeding, then replacing the master cylinder)?

20

Not likely, but I suppose it’s possible. The rear brakes might have been just on the fringe of being too far out of adjustment.