Soft brake pedal

[PMedina]

2004.5 4x4 with 260K Miles

Over a year ago, from one day to the next, my brake pedal was noticeably softer than before. I’ve done the following to try and improve it. It is better but it is not as firm as it used to be.

1) New master cylinder, which is aftermarket because the OEM has been discontinued.

2) New OEM hydraulic booster.

3) Bled the system several times with a Motive power bleeder. After the first bleed I took it to a mechanic with a Snap On diagnostic tool who was able to activate the bleed sequence for the ABS system. After that I bled the entire system again.

4) Inspected the Mopar brake pads purchased from Geno’s and they have plenty of life left in them.

5) Visually inspected the flexible brake lines and I don’t see any obvious issues.

I'm wondering if the hydraulic control unit of the ABS could be the problem and if anyone here has experience replacing it for the same reason. I don’t see the hydraulic portion listed on the any of the OEM part lists, but I contacted Mopar Parts Overstock and they said they can get the hydraulic unit and it’s $750. I've never had any of the issues with the electronic portion of the ABS that I have read about.

Thanks

 

[Darkbloodmon]

Did you check the calipers and their piston boots? if you live in a rust prone area with salt on the road; the seals can weep and introduce air in the system.

I did my brakes maybe two years ago now with Akebono Ceramic pads and Bosh ESI brake fluid using a motive bleeder, I didn't use a tool for the ABS. My pedal feels pretty good for 8800lb+, I still have a leaking hydro boost to replace and a leaking power steering pump.

With the truck off and pumping the brake pedal how many pumps does it take to get firm?

Where did you find a OEM New Hydroboost?

 

[PMedina]

Did you check the calipers and their piston boots? if you live in a rust prone area with salt on the road; the seals can weep and introduce air in the system.

I did my brakes maybe two years ago now with Akebono Ceramic pads and Bosh ESI brake fluid using a motive bleeder, I didn't use a tool for the ABS. My pedal feels pretty good for 8800lb+, I still have a leaking hydro boost to replace and a leaking power steering pump.

With the truck off and pumping the brake pedal how many pumps does it take to get firm?

Where did you find a OEM New Hydroboost?

Darkbloodmon,

Thanks for the response.

I have inspected the calipers closely for weepage but I see no evidence on the outside of the calipers that they are leaking. I will look at them more carefully. I'm in Arizona and only have had it on a salted road once.

I just tried pumping the pedal with the engine off and it took about 8 pumps to firm up the pedal.

The booster was purchased in Feb '22 from Larry H. Miller Dodge in Surprise, AZ. The part number is 5179891AC. $565

 

[seafish]

Are you hearing any hissing while you apply the brakes or use the power steering ??

I ask because the flouro liners inside the brake booster lines are known to slip out of position and cause problems.

 

[PMedina]

Are you hearing any hissing while you apply the brakes or use the power steering ??

I ask because the flouro liners inside the brake booster lines are known to slip out of position and cause problems.

Seafish,

No hissing and I have checked both lines at the top of the hydroboost for liner issues.

 

[Topzide]

The brake hoses to the calipers can collapse and cause restrictions not seen from the outside. I'd replace them do to age and mileage since you've developed an issue. Have you done a complete flush of the system or just a bleed? Brake fluid doesn't last the life of the truck. It attracts moisture and needs to be replaced at some point as a maintenance item. Pending on use, 50 - 100k is a good starting point.

 

[Darkbloodmon]

Darkbloodmon,

Thanks for the response.

I have inspected the calipers closely for weepage but I see no evidence on the outside of the calipers that they are leaking. I will look at them more carefully. I'm in Arizona and only have had it on a salted road once.

I just tried pumping the pedal with the engine off and it took about 8 pumps to firm up the pedal.

The booster was purchased in Feb '22 from Larry H. Miller Dodge in Surprise, AZ. The part number is 5179891AC. $565

I wouldn't worry about rust then if you're in Arizona. How did the brakes bleed? Strong flow out the bleeder or a slow flow?

From what I looked up in the FSM you followed the procedure to a T. According to the FSM the ABS system is independent of normal brake functioning braking. So it can be ruled out for the feeling you have in the pedal unless you've hit it hard enough to activate the ABS system.

@Topzide brings up a good point about internal hose collapse, looking through my logs I did a full flush, not just a bleed.

Not sure if this helps, but for comparison I went out side to pump my brake pedal to firm and it took about 2/3 pumps, they were short travel.

 

[PMedina]

The brake hoses to the calipers can collapse and cause restrictions not seen from the outside. I'd replace them do to age and mileage since you've developed an issue. Have you done a complete flush of the system or just a bleed? Brake fluid doesn't last the life of the truck. It attracts moisture and needs to be replaced at some point as a maintenance item. Pending on use, 50 - 100k is a good starting point.

I did a complete flush. I've been thinking about replacing all the flexible lines. OEM lines are pretty expensive but it looks like you can get a set of stainless steel braided lines for much less.

 

[PMedina]

I wouldn't worry about rust then if you're in Arizona. How did the brakes bleed? Strong flow out the bleeder or a slow flow?

From what I looked up in the FSM you followed the procedure to a T. According to the FSM the ABS system is independent of normal brake functioning braking. So it can be ruled out for the feeling you have in the pedal unless you've hit it hard enough to activate the ABS system.

View attachment 136815
@Topzide brings up a good point about internal hose collapse, looking through my logs I did a full flush, not just a bleed.

Not sure if this helps, but for comparison I went out side to pump my brake pedal to firm and it took about 2/3 pumps, they were short travel.

I would say it was a slow bleed/flush with about 15 psi to start if I remember correctly. I pushed close to three quarts through it. When pumping pedal they were long strokes. Before considering the HCU again, I will start with the flexible brakes lines, will inspect the clailpers closely and will do another flush.

Thanks for all the help. I will report back. Those brake lines were special order so it may take a while.

 

[Ozymandias]

Is it soft without the engine running to?
Because mine always felt soft but is rock hard without power assist.

And it went even softer with the new power steering pump, can stomp it all the way down to the floor board. Was always like that.

 

[PMedina]

Is it soft without the engine running to?
Because mine always felt soft but is rock hard without power assist.

And it went even softer with the new power steering pump, can stomp it all the way down to the floor board. Was always like that.

It is soft with the engine running too.

 

[Ozymandias]

I meant how does it feel without the engine running after pushing the pedal several times.

 

[PMedina]

I meant how does it feel without the engine running after pushing the pedal several times.

While running it pumps up, as much as it is going to, in two or three strokes.

 

[petersonj]

While running it pumps up, as much as it is going to, in two or three strokes.

I don't have any personal experience with the hydraulic part of the anti-lock brake module, but I do have a strong background in the field of hydraulics. If you are able to pump up the pedal, this means that there is air in the system or it means that the brake fluid is going elsewhere and then returning. If the system is operating properly, you should not be able to raise the pedal height by pumping the pedal. Since you have performed brake bleeding as required, I would be leaning toward a faulty hydraulic module.

The one thing you mention in your first post is that the "soft pedal" came on suddenly. That doesn't sound like an air issue.

There is a pump and an accumulator, as well as some valving, in the hydraulic part of the brake module. I am wondering if it is possible that one of the valves is leaking and some of the brake fluid is going into the accumulator as you apply the brake pedal. Then when the pedal is released, the fluid returns to the master cylinder. This could also explain why you can raise the height of the brake pedal while pumping it.

Again, I have no personal experience with diagnosing the operation of an anti-lock brake module, so I am just throwing the idea out there.

- John

 

[Ozymandias]

While running it pumps up, as much as it is going to, in two or three strokes.

And then? Is it hard with the engine OFF after a couple strokes?
Howe far can you push the pedal in this situation - 2"?

 

[PMedina]

And then? Is it hard with the engine OFF after a couple strokes?
Howe far can you push the pedal in this situation - 2"?

With being engine off for at least a half hour, the first stroke goes almost to the floor. Then with 3 more strokes the pedal firms up and travels about 4". If I pump the pedal continuously it stays firm and travels about 2". Then if I stop with pressure on the pedal it moves in about 2" more.

 

[Ozymandias]

Yes that's definitely not good, I'll think about that.

 

[Ozymandias]

Aside from that, did you check for rotor run-out?

Have the calipers been out/replaced or untouched?

Just try to follow the troubleshooting tree here.

 

[PMedina]

Aside from that, did you check for rotor run-out?

Have the calipers been out/replaced or untouched?

Just try to follow the troubleshooting tree here.

Thanks Ozymandias

I have not replaced the rotors but had them turned once at 150K or so. I'm at 260K now. They appear to be in great shape, no surging on the pedal and the truck brakes nice and straight. I will check the run-out.

 

[PMedina]

Update - My brakes are back to functioning like new.


I changed my mind on replacing the hoses and focused again on the ABS hydraulic module after learning a bit more on how the ABS system works. Under normal braking conditions the ABS module is supposed to be isolated from the base hydraulic brake circuit and there should be little to no bleed through to the ABS system. If the valves, which are supposed to isolate the ABS system under normal braking, are not functioning properly there will be bleed through into the accumulator of the ABS system. This was my problem and was causing the soft and low pedal. From what I see, it’s not a common problem so I felt comfortable buying an ABS unit from a salvage yard. It was an easy change out.

I found this Youtube video on how to check for bleed through on a Kelsey-Hayes module.

“Brake Pedal Goes to the Floor I ABS Pump Test I FORD I No Codes“




Thanks for the help