Odd intermittent brake problem - SOLVED!

[429CJ-3X2]

Roughly 6 months ago I was driving my '02 Sport Trac (114,000 miles) and as I braked at a stoplight, the brake pedal went almost to the floor. No warning at all. The brakes felt entirely normal on the drive to work and up to this point on this trip. I stopped and put some brake fluid in - it was really low. Brakes have worked fine since. No further issues until last week. As I braked at the end of our street, I heard a loud pop and felt it through the brake pedal, and the pedal went a little soft, but not nearly as bad as 6 months ago. The truck had sat for a few days, and we'd had several inches of snow during that time, so I figured the brakes had gotten some moisture on them and frozen stuck until I braked. That wouldn't explain the soft pedal though. Brakes worked fine the rest of this short trip. It was somewhat low on fluid, so I filled the master cylinder for the first time since 6 months ago.

It's repeated the pop and soft pedal several times since, although the pop hasn't been as loud or severe since that first time. Mostly I feel it in the pedal rather than hear it. Seems to be coming from the pedal/firewall area. When it happens, there's no pull like only one side's grabbing, and the next time I brake, even if it's immediate, everything feels normal. It hasn't leaked any fluid since I refilled it last week, but I've only driven it 50-60 miles, a mix of in town and on the freeway.

There has to be a leak somewhere, but I haven't found any visible signs of a leak. I haven't pulled the wheels and drums, but all the lines and connections I can see look good and dry, even the junction block at the rear axle housing.

It's puzzling that it happens so randomly, and the next 20-30 or more times I brake, it's fine. Ideas?

 

[CDW6212R]

That sounds like a leak from the master cylinder which wouldn't be immediately obvious. I also would be checking the brake hoses and lines carefully, those are much more likely to develop a leak. If you were driving a lot or the mileage on the pads was a lot, I wouldn't be that worried and say it needs pads. But it sounds like the brake pads are fine so the fluid level dropping will be something else.

Does your brake reservoir show fluid outside the top at all, being very dirty too? I have one truck now with a broken mounting tab on the reservoir, that sometimes allows a little fluid out. I have new caps on all of my 2nd gen's, their seals are common to leak if they are original.

If you cannot find any leak cause, then it may be time to replace the MC soon.

 

[429CJ-3X2]

I was thinking it might be the master cylinder given the way it feels and where the sound seems to be coming from. Probably won't hurt to check the rear brake shoes either, although I don't think that's the cause of this issue. I just don't remember ever changing them. The fronts were done about 3 years ago, pads and rotors. Hubs were done about 6 months before that I think. The system hasn't been opened since. And no fluid on the outside of the reservoir.

I drove about a mile to my part-time job at our church today, and the pedal was soft both going and returning home later. It's parked until I figure this out.

 

[CDW6212R]

Yeah, pull the wheels and check around the calipers closely. If there isn't any leaking you can find there and along the brake line going to the back(left frame rail and up to the rear end hose), then plan the master cylinder. I hope that's it, you don't want to be repairing any of brake line.

 

[429CJ-3X2]

Too cold to do much outside today, so I ordered a new master cylinder. The description of the Raybestos MC on RA includes symptoms of a failing MC, and they pretty well match my symptoms.

An embarrassing side story - embarrassing for the truck, not me - I've said it's almost impossible to get a Sport Trac stuck unless you're an idiot who can't drive, are trying to go someplace you shouldn't, or you're trying to get stuck. I needed to go to Urgent Care after cutting my wrist at work yesterday and intended to switch to the '01 ST for the drive, but it was stuck right where it was parked! The weather has been fluctuating between the teens to 70 and back, snow and rain, the last couple of weeks. When I parked it earlier this week, the front tires were in small depressions made when the ground was soft, and the back tires were in deeper depressions. The ground surface is frozen with ice under the tires and 4x4 isn't working right now. The truck wouldn't move. I had to pull it back a foot or so with the '02 ST last evening after driving my wife's '98 Escort to Urgent Care where I got 5 stitches to close the cut. The truck should be ashamed of itself!

 

[CDW6212R]

Sorry about that, the cut especially. I have had my Explorers just touch down into ditches or holes next to driveways etc, and I know that feeling of "WTF, stop, don't do that." I haven't gotten it stuck yet like that, but my black one without a front driveshaft I got stuck on simple grass. A 1" drop off from a paved driveway caught me, a not quite level spot but the two tires wouldn't bite and get up the asphalt. So I cherish the AWD for driving on simple dirt or grass etc, without a front LS or even a rebuilt rear LS. The guys here with serious trucks laugh at me if I tell that story.

 

[429CJ-3X2]

I drove the truck the mile to church today when I couldn't find the keys to the other one. Brakes felt and acted like it's terribly low on fluid, but the reservoir is full.

I got stuck on simple grass.

I'd forgotten that my '89 Bronco II got stuck on grass once. The parking lot at a swap meet I went to was a hay field that hadn't been mowed in awhile. I parked the BII almost at the top of the crest, but the 4x8 trailer hooked to it was downhill from it. It took some maneuvering to get it going. Fortunately, the crowd had thinned out and I think I ended up going across the slope at an angle rather than head-on. The BII probably wouldn't have had a problem without the trailer. Wish I still had it for a daily driver, but a near head-on collision on a slick street at 25 mph with a full-size Chevy extended cab pickup driven by a drunk driver put an end to it. Not a straight panel on it, but it gave as good as it got, shoving the cab of the Chevy back into the box.

 

[CDW6212R]

Good times, memories of our old cars. I guess you have to do the MC, and let's hope that is it.

 

[boominXplorer]

Check for a bad front wheel bearing. The play can be pressing in the caliper as you drive giving you a squishy pedal with no fluid loss.

 

[429CJ-3X2]

The wheel bearings are only about 2-3 years old, which doesn't rule out one being bad. There has never been a pull or wobble like I would expect if the problem was at any one of the brakes or bearings. Actually, last Thursday as I eased it home before going to Urgent Care, the right rear drum locked up when I stopped at the end of the church drive. The street was a little slick from some very light snow, so that didn't concern me. That it didn't fully release for 2 blocks was concerning. That's the only time it's done anything like that.

NOTE: ABS does not work - I pulled the fuse in the power box under the hood because it wasn't working right about 2 1/2-3 1/2(?) years ago several months before the left front wheel bearing failed.

When I last drove it Sunday, and when I moved it to work on it earlier this week, it essentially had/has no brakes at all. The pedal feel is exactly the same as when I am bleeding the brakes and have a bleeder open. New master cylinder had no effect whatsoever. The new M/C did not include the pressure valve(?) that screws into the outlet for the rear brakes. Thinking that could be bad, I took one from an '02 ST at the salvage yard, but haven't put it in yet. But if that was bad, I'd still have front brakes, right?

Although it did lose an ounce or 2 of fluid over 6 months, there's no evidence of an external leak. This seems to me like a blown seal or leak within the brake system, but that would most likely be in the M/C, right?
OR
Given the loud pop I heard, the way it felt, and where it seemed to come from, (the firewall area), I'm wondering if the rod from the booster to the M/C is broken or misaligned. Engine off, the pedal is hard like it should be. Engine running, there's no resistance at all. I did get fluid flowing at all 4 corners when bleeding the brakes after installing the new master cylinder, but for the entire pedal travel or only the bottom? If that rod is broken or misaligned, could it be moving before it engages the M/C? I know it's a longshot, but I've had enough "...possible, but that like never happens" car problems over the years to not rule anything out. I've not been motivated to dive more into this this week partly because I'm at a loss here.

 

[Josh P]

When the pedal started to go soft in my van, it was the vacuum booster. I also had a small leak on the back side of the master cylinder so the engine was eating some brake fluid. The symptoms were not exactly as you describe, but the pedal felt different at times, like good days and bad days.

 

[429CJ-3X2]

Thanks. I came across info last night that says a failed seal around the rod where it exits the booster can cause the secondary seal in the M/C to fail. That allows vacuum in the booster to draw brake fluid into the booster. The fix is to replace both M/C and booster.

 

[429CJ-3X2]

Haven't resolved or given up on this yet, not that giving up is an option. A stiff back on the one nice day last week and weather conditions (cold, rain, high winds, in various combinations) the rest of the week kept me from doing anything on it last week. Mostly though, it was reluctance on my part to tackle a problem I don't have clear solution to.
I put a new(reman) brake booster in yesterday. When I took the new master cylinder off to swap the booster, I bench bled it again and found I didn't have anywhere near all the air out of it when I first installed it. When I bled it again yesterday, I kept at it until well after it took considerable effort to push the piston 1/2 an inch. Once I got the new booster in and the m/c bolted to it, I jumped in and pumped the brake pedal several times before I realized I hadn't connected the brake lines to the m/c. I connected them and tried it again. Pedal still goes to the floor, but it does feel a little different than before the new booster - like stretching a new rubber band vs a worn out one - but I haven't bled the brakes again and I obviously introduced air into the m/c again. I'll be pulling it off and bleeding it again, then bleed the lines.

I came across a YouTube video where a guy/mechanic had a similar problem with an S10. He searched the web and didn't find any answers. After spending a lot of time and brake fluid trying to get a firm pedal, he eventually isolated his problem to the right rear brake cylinder. He said the bleeder was allowing air to be sucked past it without losing fluid. I can't see how a bleeder valve can suck air through or around it, but not leak pressurized brake fluid out. Anyway, a new brake cylinder fixed it. He also was trying to bleed the brakes with the engine off, so ???

Even if the new master cylinder and booster don't solve the problem, I don't think replacing them was a mistake or a waste. The original m/c did show some signs of a leak at the rear, and where the rod from the brake pedal enters the booster looks nasty, so it seems like changing them was a good idea regardless.
I think if I have to change another booster I'll invest in an air ratchet. The job isn't all that bad, but it's awkward as can be. Have to lay on my right shoulder while working the ratchet with my right arm to get the nuts off. At one point, my 4 yr old grandson said, "Good job, Grandpa! Only one more." I turned to see him lying upside down on the seat with his head under the dash looking over my shoulder. If he was strong enough to turn the ratchet, I would have gladly let him sit on the floor and take over!

 

[429CJ-3X2]

Just watched a video of a guy with an Expedition who had a similar problem. He fixed it by replacing the ABS pump with a used one, and didn't have to bleed it. A commenter on the video mentioned in my previous post said that fixed his issue as well. Might have to give that a try if bleeding everything again doesn't do the trick. Changing the ABS pump in a Sport Trac or Explorer would be a breeze compared to an Expedition given the ST/Explorer ABS unit is right up top and the Expedition ABS is close to the frame under some stuff.

 

[CDW6212R]

I hope it's not the ABS module, but you already have some air in or above the ABS module already. Getting air out of the ABS is harder than the rest, you can get a device to force the unit to function, or work it out over a long time with hard braking and more bleeding. I added ABS to one of my 95 Crown Vic's, and it had a somewhat soft pedal for a year until I did another brake job and bled the brakes.

 

[429CJ-3X2]

My ABS has been inoperable since I pulled the fuse several years ago when it was malfunctioning, so I've been thinking/hoping fluid would just pass through it. I suppose if an internal seal failed it could be a problem. I'd do away with it if I could. I just don't like it.

 

[CDW6212R]

The only issues I've had with ABS is when stopping hard on an uneven slick surface; gravel or leaves on pavement are the worst for ABS.

Any air that gets inside the internal ABS passages, will basically stay there until the ABS is activated. Under normal use the brake fluid passes through the ABS module unhindered, through a different path. the ABS function is a combination of valves that stop flow through that main path, and divert it while absorbing some of the downstream fluid(which lets the caliper pistons recede). What matters is that there are passages inside the ABS that are not used virtually ever, and air can get in them. Any air creates a cushion for the fluid force, even a tiny amount.

So you may want to swap the ABS just to fix the problem if that is part of it. I would continue to watch the brake fluid level constantly and verify everything is in great condition. With everything in top condition, it's easier to locate a problem.

 

[429CJ-3X2]

That confirms what I thought about how ABS works, and got me thinking about why I don't like ABS and why I pulled the fuse for it in this truck. Also has me almost convinced it's the cause of this current problem. I started driving in 1970 and didn't have a vehicle with ABS until I bought my '89 Bronco II in '05. It only worked maybe half the time. Same with the '94 Explorer that replaced the BII - ABS light was on as much as it was off, with no pattern as to why. On for one drive, off the next. The ABS in both vehicles almost put me through the front of convenience stores. The parking lots had patchy ice, and the ABS kicked in and applied the brakes on the ice and released them on a clear patch. Only the curb kept me from hitting the building. It always seemed to work at the worst possible time, and not work when it might have been helpful. The ABS in my '01 Sport Trac hasn't worked since I bought it 8 years ago.

ABS did work in this '02 Sport Trac. I pulled the fuse when it started kicking in as I tried to stop at stoplights, and as I eased into a parking spot in the parking garage. Instead of coming to a complete stop, the truck kept rolling and it felt like I had no brakes, but it would only do it once in awhile. Research on the forum said it was the ABS malfunctioning. A the same time this happened, I had to brake fast on the freeway, ABS kicked in as I braked exactly as the front tires were on the rain-slicked steel seam at the end of a bridge and released the brakes on the drier pavement. Thank God I never hit anything. Pulling the fuse to disable the ABS solved those issues. Also ended the severe vibration I'd had for several years when stopping from highway speeds (but not always), which was likely related to the front hub that failed a few months after I pulled the fuse rather than an ABS malfunction.

All the problems I've experienced with ABS have been intermittent, as was this problem at first. Maybe the pop at the start of this was a valve in the ABS opening when it shouldn't have, and it eventually stuck open? That would maybe tie in with the problem that caused me to pull the fuse.

 

[CDW6212R]

Yes the complexity of the ABS internals do worry me a little about what might happen in the worst case. The age of these is getting up there now, so some new odd things may appear soon. I think the ABS light displaying that the system is disabled, may be a good thing, or useful for a switch to shut it off when needed.

 

[429CJ-3X2]

A switch would be nice.
I watched a couple more videos where the ABS was causing/allowing the pedal to go to the floor. All early 2000s Fords - E-series, F-Series, and Expedition. Not that it doesn't happen with other makes, but Fords are what came up after the first one. Maybe it's not as uncommon as people thought.