Intermittent soft brake pedal

[railer]

Wife's car - 2007 Mitsu eclipse:

She's complained a couple of times over the past month of the pedal going to the floor randomly while braking. The first time I checked into it, I romped around in the car for about 15 minutes, stopping in as many unique and different ways as I could think of, and of course had no problems. Fluid level was fine.

So today she mentioned that she had the same problem, except that it happened several times. I went out to check it out again, and upon first starting the car and stepping on the brakes before throwing it in reverse....the pedal did seem real soft, and almost went to the floor. I thought that was a good sign, and then proceeded to bang around in the car as I did the first time, but ultimately had no further issues with the brakes.

Why?

Should I just bleed the brakes and see what happens? Could this be a bad master cylinder? Anything else come to mind? Thanks in advance...

 

[C1]

You dont say if the brakes are power.

Yes, it could be a leaking master cylinder or if the brakes are vacuum powered, it could be a bad vacuum power assist unit or simple vacuum hose supply leak.

You will need to have the car inspected. Until then, remind your wife that in the event of a loss of brake emergency/condition, then also use the parking brake lever to help slow/stop the vehicle.

 

[nerp]

Better yet, do not let her drive the car until it's fixed. Do you want them to completely fail when she's on the road? Do you feel OK with her dying tomorrow?

 

[JCH13]

I saw a brake master cylinder fail where the seal got messed up in a really weird way. If you hit the brakes hard (like a sharp initial tap on the pedal) they would work fine, but if you eased onto them they went right to the floor.

This was, of course, an intermediate step towards brake failure, so I suggest you park the car until the braking system is fixed.

 

[monkeydelmagico]

Vacuum booster needs replaced. Not too hard to do. Laying on the floor under the dash is a PITA for that little car.

 

[JCH13]

Vacuum booster needs replaced. Not too hard to do. Laying on the floor under the dash is a PITA for that little car.

Please explain your logic behind both statements.

 

[railer]

Better yet, do not let her drive the car until it's fixed. Do you want them to completely fail when she's on the road? Do you feel OK with her dying tomorrow?

Thanks for contributing nothing to the thread.

I'm driving the car today. The brakes are fine, just like they always are when I drive the car. I'm trying to figure out what, if anything, is wrong. Thanks to those who have offered useful advice.

Someone asked if the brakes are power....I guess I just assumed that all modern cars have power brakes? I guess I'll have to verify that.

And I think I did probably tend to stab at the brakes yesterday. I think today I'll try some gradual stopping to see if I can make the problem pop up.

 

[boomerang]

When the problem occurred did either of you notice a light coming on in the dash? With an hydraulic failure (the symptoms of which would be the pedal going to the floor) the brake light should come on. As a test of the bulb, when starting the light should come on briefly and go out.

Where the MC attaches to the booster, are there any signs of fluid leakage?

Intermittent problems can be really tough to diagnose. I believe it's pretty much got to be the MC but I for one, hate throwing parts at a problem to try and fix it.

Also, you're thinking correctly, in that you couldn't buy a car without power brakes these days.

 

[monkeydelmagico]

Please explain your logic behind both statements.

Vacuum boosters are more prone to intermittent failure. You typically have disconnect one by laying in driver side footwell to unbolt it from brake pedal and firewall.

However, the vac booster is like 3rd in line of things to check and I should not have blindly recomeended replacing it. I apologize. There are several checks to do first to determine where the problem lies. check for master cylinder leaks, air bubbles by pumping brakes then opening brake fluid cap, caliper leaks, pinching vac booster line, etc.

 

[JCH13]

Vacuum boosters are more prone to intermittent failure. You typically have disconnect one by laying in driver side footwell to unbolt it from brake pedal and firewall.

However, the vac booster is like 3rd in line of things to check and I should not have blindly recomeended replacing it. I apologize. There are several checks to do first to determine where the problem lies. check for master cylinder leaks, air bubbles by pumping brakes then opening brake fluid cap, caliper leaks, pinching vac booster line, etc.

The reason why I do not think it would be a brake booster is that when they fail the pedal gets hard, not soft, because you now lack the power assist of the booster and have to depress the pedal entirely with your own strength. The pedal going right to the floor would be very unlikely with a failing, or blown, booster.

 

[Jimzz]

When the brake Booster fails the pedal gets hard, not soft.

 

[railer]

Thanks Monkey and Boomerang:

No lights on the dash at all. And oddly enough (to me anyway), the brake fluid is full to the brim in the reservoir. I don't remember ever adding any brake fluid, and I certainly haven't added any in the past couple of years. That would seem to rule out any leaks anywhere in the system, including at the caliper or MC.

My wife did mention hearing a whoosh at least one time when she thought the brakes were spongey, so maybe that would point me to the vac booster or a vac line.
Wouldn't a bad booster make the brake pedal harder to push down and not soft?
I'm thinking after I check the vac line to the booster, I'll probably go ahead and bleed the brakes only because it has never been done (approx 65k on the car).

 

[JulesMaximus]

You dont say if the brakes are power.

Yes, it could be a leaking master cylinder or if the brakes are vacuum powered, it could be a bad vacuum power assist unit or simple vacuum hose supply leak.

You will need to have the car inspected. Until then, remind your wife that in the event of a loss of brake emergency/condition, then also use the parking brake lever to help slow/stop the vehicle.

:thumbsdown: This is colossally bad advice.

If the brake pedal is intermittently going to the floor she should not drive the car at all until it is fixed.

Try bleeding the brakes first and see if that cures the problem. I would also inspect every line to see if there are any leaks anywhere.

 

[LTC8K6]

The brake fluid is full to the brim? And you have never added any?

Do you mean over filled?

Normally the fluid level goes down with pad wear.

 

[exar333]

Get the master cylinder checked out. This could be indicative of a future total failure, and thus loss of all/most braking ability. Don't mess around with issues like this, it could mostly fail at a horrible time.

 

[monkeydelmagico]

The reason why I do not think it would be a brake booster is that when they fail the pedal gets hard, not soft, because you now lack the power assist of the booster and have to depress the pedal entirely with your own strength. The pedal going right to the floor would be very unlikely with a failing, or blown, booster.

Agreed and you are probably right. However, I did have an Acura Integra where the booster was bad and it was allowing the pedal to go to the floor. I don't know how else to explain it other than it was sucking the pedal down. Pedal did NOT just firm up. I followed same logic and figured master cylinder. Replaced that first without curing problem. Turned out to be vac booster.

 

[nerp]

:thumbsdown: This is colossally bad advice.

If the brake pedal is intermittently going to the floor she should not drive the car at all until it is fixed.

Try bleeding the brakes first and see if that cures the problem. I would also inspect every line to see if there are any leaks anywhere.

I said the same thing and was told I had given useless advice. I guess some people like to risk brake failure on the road, especially when it's a loved one.

 

[boomerang]

Agreed and you are probably right. However, I did have an Acura Integra where the booster was bad and it was allowing the pedal to go to the floor. I don't know how else to explain it other than it was sucking the pedal down. Pedal did NOT just firm up. I followed same logic and figured master cylinder. Replaced that first without curing problem. Turned out to be vac booster.

I would think it would have had to have been a mechanical problem, not a vacuum problem with the booster. The pedal cannot physically go all the way down in a hydraulic system without a leak somewhere or air in the system.

I would have been very interested in taking that bad booster apart to see what the problem was.

http://www.sanjuan.edu/webpages/jasonkeith/files/Ch 7 Power Assist Units TB.pdf

Page 4 shows a cross-section of brake boosters for anyone not familiar with their construction.

 

[Pacfanweb]

Vacuum boosters are more prone to intermittent failure. You typically have disconnect one by laying in driver side footwell to unbolt it from brake pedal and firewall.

However, the vac booster is like 3rd in line of things to check and I should not have blindly recomeended replacing it. I apologize. There are several checks to do first to determine where the problem lies. check for master cylinder leaks, air bubbles by pumping brakes then opening brake fluid cap, caliper leaks, pinching vac booster line, etc.

Vacuum booster would lead to a hard pedal, not the pedal going to the floor.

IMO, without seeing the symptom firsthand, the most likely culprits would be, in this order:

ABS
Master cylinder

That's it. Can't think of anything else that would really cause that particular concern.

 

[Pacfanweb]

I would think it would have had to have been a mechanical problem, not a vacuum problem with the booster. The pedal cannot physically go all the way down in a hydraulic system without a leak somewhere or air in the system.

I would have been very interested in taking that bad booster apart to see what the problem was.

http://www.sanjuan.edu/webpages/jasonkeith/files/Ch 7 Power Assist Units TB.pdf

Page 4 shows a cross-section of brake boosters for anyone not familiar with their construction.

ABS. It can cause intermittent issues like this with no air in the system and full of fluid. It's the big wild card here.

 

[Ferzerp]

Maybe she's just a more fun driver than you are and boiling the brake fluid

 

[railer]

I said the same thing and was told I had given useless advice.

Uh, asking "Do you feel OK with her dying tomorrow?" does not really give me a lot of insight into my intermittent braking problem, believe it or not.
But it is a valid point that brakes should not be taken lightly, especially when it's the wife, so if I can't get this figured out while I'm driving the car, I will take it to a garage.

In reading over on the Mitsu 4g forums (Mitsubishi Eclipse owners, at least the ones who choose to post on the internet, do not seem to be the most mechanically savvy people), it seem as though a lot of Eclipse 4g owners have had issues with the ABS system causing them grief, so that's something that I'll keep in mind.
So I'm thinking:
1) Bleed brakes / check brake fluid for funk/air bubbles, etc.
2) Check out master cylinder
3) Possible ABS issue....will probably take to garage for that one. Possibly take car to Autozone so they can tell me if there are any ABS codes that are hiding in the ECU but haven't tripped the warning light yet.

 

[nerp]

It did sound a little finger waggy. Sorry.