The pedal in my 05 started going to the floor when stopped. The truck will stop mostly normally then as I'm sitting at lights the pedal will slowly sink and the truck will start inching forward. I'm not losing any fluid can't find any leaks. I checked all the hoses while a helper pumped the brakes non are noticeably swelling. So I swapped out the master cylinder and its doing the same thing. Capped off the master cylinder and I have a solid pedal with it capped. With the MC tied back in I went around with vice grips and clamped all the hoses and still have a sinking pedal. I'm under the impression its the ABS module at this point. I can't find any threads on a truck doing this with a bad ABS module I've talked to a few mechanics and get differing opinions from sounds like the abs with out me mentioning it to it can't be the abs module. My shop manual does not list the abs module as a cause under symptoms of a sinking pedal only air and bad MC.

I'm wondering how to test the ABS module, my mechanic buddy that thinks its the abs module says to bypass its like it'd be so easy but the way the lines are bent it'd be a huge pain without destroying all my factory lines. So I'm thinking of capping off the outputs like I did the MC and testing that way. Anyone got any other ideas?

I did restest the old MC with the caps and its solid so it is another issue and not air. Went through and bled a gallon of brake fluid through the system when I first changed the MC. I did not bleed the entire system again before clamping the hoses after capping and reattaching the new MC. So it is possible I have a little air in the system and my test on the hoses isn't solid.

 

Unfortunately all your symptoms strongly suggest the MC is the problem---I've almost never seen the ABS fluid module be an issue on vehicles equipped with that system.

Has the brake booster chamber been checked for vacuum or leaks of any kind?

I know the prospect of re-forming hard brake lines isn't first choice but you could be facing buying another ABS module or making new lines---neither one all that enjoyable to think about.

 

With the ports on the MC capped the pedal is rock solid with the truck started, so I don't think its bad. I should of done the test originally as I saw it in the shop manual but thought it had to be the MC too.

If something was wrong with my brake booster wouldn't I have a hard pedal.

Yeah I was thinking more like a junkyard abs module or something to test as I'm not trying a $1000 one. Any reason capping off the output of the ABS module wouldn't tell me anything?

 

If the booster has failed completely then yes you'd have the hard pedal---its maybe possible its leaking internally causing your sinking pedal.

Its entirely possible capping the ABS module would indeed say if that's leaking or bypassing internally.

I feel ya on the cost of ABS parts---my '03 E250 control module took a dump and needs replaced---to the tune of $550 my cost plus installation. Considering this van is headed to the scrap yard hopefully this year that's not a problem I'll be addressing anytime soon. I myself hate the whole process of using scrap yard parts---simply too much hassle and not worth my time.

You might also post this same info in the Brakes sub-forum here: Brakes Ect

Good luck with your ABS---hope its nothing too costly!

 

So I capped off the abs output ports and had a firm pedal. Then decided I must have bad hoses so swapped all four out. Still sinking pedal figured it had to be air trapped in the abs module after bleeding all around again twice. Got my mechanic buddy over this morning with his computer to bleed the abs. No go still soft pedal. He had a test where there are these ports under the lines on the abs module. You stick something in there and press the brakes if it pushes out more than 1/2" the dump valves are bypassing. First one only pushed out a little, second one pushed out over an inch. So now its officially the hydraulic portion of the abs module. I'm going to track a junkyard one down tomorrow figure since they rarely fail, its the way to go. Should be around $100 vs $1000.

 

So your earlier suspicions are confirmed---hope I didn't lead you down a rabbit hole of bad advice.

As you mention that is a somewhat rare occurrence and one I'll keep in the back of my mind for future reference.

Best of luck on finding a good part in the yards and thanks for your update too!

 

I ran across a You Tube video that describes perfectly what the ABS HCU test is: In the comments below that, there's a guy who 'fixed' the problem on a few vehicles by threading those test ports with a 1/4x20 tap, then inserting 1-3/4 threaded bolts (approx.) with thread locker to keep the bad valve from moving. If anything, it buys some time right? Not here to start a debate on messing with brake system safety components, just sharing what I found and letting people come to their own conclusions.
2007 F-250 Super Duty

 

That might be a bit late to this party BUT its VERY useful anyway. Do I gather if the valve block fails this test its to be replaced? That the service bleed doesn't solve the problem?

 

Yes -- replacement of the HCU assembly is required if it fails the test. Service bleed by cycling the pump may not cure the problem, although some commented they got lucky doing that as it did bring back function.
My situation finally revealed itself last night when I noticed a wet spot under my truck that wasn't there before when I parked it. Corroded or rubbed brake line on top of the fuel tank. Brake reservoir got to the low mark before I saw anything. I just figured my pads were worn. Looks like a nasty job....

 

Thanks Dexter---BTW loved your TV show!

I've had a few brake lines that formed rust or rub holes, mostly the fronts as they seem to be subjected to the most road muck. For that reason I keep extra coated brake line, tube nuts and a flaring tool in the tool box. Also use those DOT-approved straight line splicing fittings---those are amazing.

Thanks again---reps sent!

 

Ha! Yeah, I get that about my name sometimes....thanks for the brake line tips!