ABS Hydraulic Control Unit Leaking Dump Valve

[SafariGoneWrong]

Hi All,

Earlier this week after starting the Expedition for the first time in a few weeks, I noticed the brake pedal going to the floor (more easily than in the past...it's been getting worse) with the engine on / vacuum to the booster. I replaced the master cylinder though I knew that probably wasn't the problem as there were no signs of leakage. The pedal is firm--no sponginess and it doesn't pump up. With the engine off, I can't push the pedal to the floor (not even close) no matter how hard I push. I'm thinking ABS Hydraulic Control Unit as there's no other place to have an internal leak. Today I tried multiple panic stops and couldn't get the wheels to lock up, the pedal indeed wants to move towards the floor in a panic stop. I'm thinking the dump valve for the rear wheel circuit is the culprit, as braking was even during the test drive. Here's the question: Has anyone had their ABS Unit rebuilt, and if so, any recommendations? I'm leary of a junkyard unit.

It's frustrating because I've been good about flushing the fluid every 3 years or so. The shop manual only talks about bleeding the HCU after replacement. I'm thinking I should have done the HCU bleed routine in FORScan each time. Oh well. Of course the Expedition is different than the other cars, I've flushed my Passat's fluid every 3 years for 26 years and no problems, never bled the HCU independently.


Thanks, Dave

 

[whtbronco]

Well this is gonna be interesting. I did swap an ABS solenoid module in one of our Monte Carlo's, the replacement came from a car we bought and stripped for parts and I had no issues. I've done nothing with the Expedition ABS module.

 

[SafariGoneWrong]

Time to follow up after 6 months of the XLT in the garage on jack stands waiting for me. It's still doing the exact same thing--brake pedal goes to floor slowly but only when the engine is running; the pedal is firm and doesn't pump up when the engine is off. Suspecting the ABS hydraulic unit, I replaced that with an eBay purchase; however, I transplanted the original ABS module onto the eBay hydraulic unit because there was nothing suggesting it was at fault. Also replaced: master cylinder, front rotors, front calipers and pads and flexible brake lines. (The left front caliper bleeder screw was slow to let fluid out so I went ahead and did everything brakes up front.) After everything was re-assembled, I bled the brakes first with a pressure bleeder. After that, I ran the ABS service bleed procedure in FORScan and pressure bled the brakes again. After an engine running test with the same pedal to the floor result, I repeated the ABS bleed procedure followed by a pressure bleed. That's where I'm at right now. Forgot to mention the master cylinder was bench bled using old school tubes into reservoir setup. There are no signs of leakage anywhere and a visual observation of each caliper while the brake pedal is depressed shows normal at all 4 wheels.

I'm at a loss right now, one more thing I'm going to do is have my helper depress the brake pedal while I crack a bleeder screw at each position, and see if by chance any air bubbles come out perhaps indicating air is getting sucked in somehow, especially at the rear calipers.

If anyone has been through this before, I'd be interested to hear your story. Also, one last nugget, the Motive Products 1107 pressure bleeder adapter worked great on my Raybestos master cylinder reservoir--no leaks. I'm not sure it would work on the OE reservoir as the distance from the top surface to the 3 tabs isn't the same as aftermarket.

 

[whtbronco]

I think bleeding the brakes the old fashioned, 2 person method, is a great idea. If that doesn't present the issue how could it be anything aside from the master cylinder though. I mean if the brake fluid is getting by the plungers inside the master cylinder the pedal would keep moving. Without hydraulic assist there might not be enough pressure to induce the leakage.

The one thing I will add is that if I push with say medium or harder pressure the brake will slowly go to the floor in my 2004. I never liked it, but it's been this way since new. I was told it was something to with a soft pedal or some crap like that, pressure doesn't change just the pedal slowly goes down. Shut off the engine and the pedal is nice and firm.

 

[SafariGoneWrong]

I'm with you @whtbronco on both points and thank you for all you do for this forum. To add, when doing the ABS Service Bleed this time, I thought about what was happening. With foot on pedal, it drops (dump valve opens) and then pumps up when the ABS motor runs and it does this cycle for a total of 3 times--once for each circuit I'm assuming. (My lowly XLT has only one circuit for the rear brakes.) If a dump valve was hanging open, I'd think there would be an issue with the pedal pumping up but there was no difference in behavior between circuits. I've ordered calipers and flexible brake hoses for the rear as it's now an 18 year old vehicle and I have to rule out any air getting sucked in through the caliper piston seals. Also arriving today are the Motorcraft BRT3 and BRT10 brake lines from the master cylinder to the ABS unit.

As far back as I can remember, I too have noticed the pedal going down slowly when pushed hard with the engine running. It seems worse now, and a road test months ago showed the old XLT was unable to panic stop--couldn't get the ABS to activate--and stopping distance was poor.

 

[whtbronco]

Yikes, yep that is not the slow descension of the brake pedal I was referring to. Clearly you are correct that is a problem. I will say that if the caliper piston seals were leaking they would leak brake fluid due to the high pressure, long before they would suck significant air in. I did replace all 4 brake hoses, the flex hose at each wheel, and that sure did help improve brake pedal feel. I would like to replace them with braided stainless hoses since I like a nice firm pedal with good feedback.

I know you replaced the master cylinder, as did I 6yrs ago with a Raybestos unit. Anyway, if the pedal is going down like you reported I suggest you either have an external leak that you would very likely notice and the reservoir would be low on fluid or the master cylinder is leaking fluid internally past one or both seals. If there's another possibility I sure can't think of it.

 

[SafariGoneWrong]

No change in reservoir level and no signs of leaks, even along the frame rail. It’s weird. Hate to admit I looked at YouTube (I’m 60 and that didn’t exist when I had to figure out problems the old fashioned way), a guy traced his very similar problem to a wheel cylinder that was sucking air but not leaking. Lip seals on caliper pistons, transmission clutches, etc are great at handling pressure in one direction but not so much the other. And since the XLT has been a hangar queen much of the last few years my new theory is air being sucked in by a rear caliper. Got to have a hypothesis… I’ll report back. Oh, last alibi is the first Raybestos master cylinder spurted brake fluid out the rear when bench bled. Had to try another.

 

[whtbronco]

Um wow, the master cylinder failure on the bench is odd.

Yeah really, we didn't have the internet at all. Heck we didn't have cell phones and yet we managed to grow up and repair stuff.

Well I reckon it is possible to not leak one way and yet leak the other. I'll say it's odd, but certainly could happen. Let's hope it's just a caliper. Good luck.

 

[SafariGoneWrong]

To follow up...rear calipers replaced (new UltraPower from RockAuto...we'll see how so-called new calipers go), rear rotors replaced, rear brake hoses replaced along with the lines from the master cylinder to the ABS unit. Front calipers are reman AC Delco along with new brake hoses in the front. It still does the same thing with the pedal eventually going to the floor but maybe a little bit better panic stop action. It rained yesterday and I had a chance to do multiple panic stops on a wet access road at about 55mph. I can get the front wheels to lock up (ABS does it's pulsing thing) as the Expy gets down below 30mph or so but not sure about the rears. Test drive on Sunday night down an almost empty 4-lane road I couldn't get wheel lockup action at 60 - 0 mph, even though stopping action felt decent with significant nose dive. That was on brand new Bendix pads at all 4 corners but still unacceptable despite the lack of pad bedding down. I'm going to have to go with replacing the master cylinder again as Raybestos is now suspect to me since the first master cylinder leaked while being bench bled. The only explanation I can go with is your theory there's internal leakage in the master cylinder. It's hard to accept it's the ABS HCU since the eBay replacement didn't change anything.

 

[whtbronco]

Agreed. Sucks to replace the master cylinder again though.

I used Bendix Fleet Metlock pads/shoes for many years and really liked them. I have now found that, believe it or not, Raybestos makes a fantastic break pad line. The Element 3 pads are really sweet and not expensive. Most are rated GG and man do they bite. I have them on 3 of our 4 vehicles now and each of us likes the feedback and stopping power from them. Aside from these pads I have been less than impressed with Raybestos in recent years. I got 3 sets of rear rotors from Raybestos and all were seriously out of round and had significant lateral runout, before getting a set of near perfect Motorcraft rotors.

 

[SafariGoneWrong]

The last time I used Bendix pads prior to this debacle with the XLT was a year ago in my Kia Stinger (sold last month...) because Kia's OE pads for the Brembos are notorious for leaving deposits on the rotor causing the dreaded pedal pulsing/out of round feeling. There was a very noticeable difference going to the Bendix pads--they just didn't bite like OE but I didn't care because there was still great stopping power and brake action was finally smooth. I'm going to a better pad in the XLT, I have to considering my situation. I'll join the Element 3 pads club--thanks for the vector! The rear Bendix pads I put on the XLT have an extreme leading and trailing edge bevel--very little surface area of the pad contacts the rotor with a fresh pad. I've never intentionally, until now, driven the XLT on a test drive with the intent to stand on the brakes at highway speed and attempt to induce wheel lock/ABS action. Makes me wonder what kind of stopping power I had a few years ago...