Manager Forum

Got this 2008 Dodge Ram 1500 with a 5.7 engine. Auto and 4wd.
Issue: brake pedal fades to the floor. If the pedal is released immediately and then reapplied, it has great brakes. We replaced the brake master cylinder, bled the brake system, all to no improvement. We can go for a drive, get to where we need to do an ABS stop and the ABS system functions fine. Has new brake pads all around, no leaks, and no ABS codes and no ABS lights on in the dash. We have bled, bled and rebled the brakes. Nothing changes.

What are we missing? We are extremely frustrated. This has been tormenting us for several days now.

I forgot to mention that when we try and run the ABS pump with our scan tools, we get an "INVALID ERROR" message. However, as mentioned, the abs system does function on the road.

Does the pedal fade when blocking of the master? Why was the original master replaced? If it doesn't fade with the master blocked off its by-passing in the hydraulic control unit.

If I block off the hoses going to the wheels, I have a solid pedal. If I release the blocked hose going to the rear wheels, I still have a solid pedal. If I let the rear wheels unblocked and release either one of the hoses going to the front wheels, then I have a fading pedal. My theory is that by blocking off the hoses at the wheels I have just proven that the problem is not in the master cylinder or the brake booster, neither is it with the hydraulic control unit. Or am I wrong?

That should be proving out the master, probably some air yet. Do you have a pressure bleeder, where it pushes from the top? It would be nice to activate the pump during bleeding, but not sure why you get that error. I regret not buying my pressure bleeder sooner after getting one, they work great,even on the fun Chevys.

How are you able to block off the lines ?

Brake hose clamp from MAC Tools.

Yea, I know, some techs will say thats a no-no, and I understand but it does work and so far, in the years that many of us have been doing it we have not had a problem with them.

Sorry I missed this yesterday. I agree with crimping off hoses as being a no no since the inner wall of the line can and will crush. But in doing that I think you answered and solved he issue. The first thing I would do after that is make sure there is no play in the front wheel bearing then remove calipers check slides and make sure pads are free in the carrier. Common issue with these. I've seen wheel bearing weak enough allowing the rotor to lean into the caliper collapsing the piston and slides frozen keeping caliper from sliding snug to pads and frozen pads so tight that it stops the piston from pushing it in making the caliper have to slide more than it should to satisfy the application. Being is the North East I see a lot of brake issues like this and for the most part involve what I described. By the way after you figure it out please replace the brake lines. Being almost 10 years old dictates that anyway. Hope you get it but have a great weekend anyway.

Had an ES350 a few weeks ago that had a very similar issue. Once we blocked off the front calipers, the pedal felt great. We looked every which way at the front calipers and saw NOTHING wrong with them. Pistons moved, slide pins were great, pads were perfect, hoses to the calipers were in perfect condition. We got tired of trying to understand it and threw 2 front calipers and 2 hoses at it. It fixed it. Still to this day, me and the tech don't know what/why it was doing it. Sounds to me like you have a similar issue.

Yeah, I have had the thought of doing this to this one. One thing, this truck only has rear wheel antilock brakes. THerefore, the fluid and pressure goes directly from master cylinder to the calipers.

We first 'pinched' off the front brake lines with hose clamp pliers and didn't get much of a change, so we threw a master cylinder at it (rebuilt aftermarket). Same issue, so we put a new aftermarket one on. Same issue. So we put a new factory one on. Same issue. So we blocked off the ports at the master cylinder and the pedal felt great. So we went to the ABS module/actuator (where the lines came out) and blocked off all four brakes. Pedal felt great. So we reconnected the back brakes and pedal felt great. So we reconnected the front brakes and the pedal went to the floor again. Made absolutely no sense! Calipers, brackets, pads, and lines all looked great. We even tried vise grips on the front lines (I know, shouldn't do it, but we were pulling our hair out). Made no sense, but we had to do something. So we put front calipers on and the problem was fixed. Hope this helps.

Looks to me that there has been a lot of talk about since all was good when lines were crimped it's time to suck it up and replace them. If I for some reason decided to next step it I would take calipers off and clamp them closed with vice grips to see what happens. From my point there are only a few issues that can cause this one is bad calipers, another is restricted flex lines, and lastly a bad wheel bearing allowing rotor to lean into and collapse the piston. With any issues involving hydraulics especially front intensified by if the vehicle sees highway speeds I replace the lines and calipers on both sides. Customer may balk but I don't want to hear about the vehicle veering off the road or into oncoming traffic while braking at high speeds. Good Luck

How did this one go?

Ok. Customer brought it back for us to resume the challenge.

I just got back from another drive around the block and same issue: brake pedal falls to floor. If released and immediately reapplied, it has a solid pedal. I'm gravitating to agreeing with the faulty caliper issues. We will recheck the front wheel bearings in the morning and if they are ok, then the vehicle gets new calipers on the front. Will keep ya posted.

Have you done any pressure testing?

We replaced both front calipers and hoses. Same fading brake pedal problem.

Now where do I look???????????

I've seen the ABS Hydraulic Control Unit (HCU) cause this. Looking on identifix shows others have fixed soft pedal issues with the HCU also.

Does your scan tool have a bleed capability to make sure all the air is out if the HCU?

This particular vehicle only has rear anti lock brakes. There is a little valve/solenoid (not at all like a typical HCU unit we normally see) on the line going to the rear brakes, however there is wheel speed sensors on each wheel. The scan tools we have do not give us an option to cycle a 2 wheel ABS system during bleeding.
We have bled, bled, bled and rebled all to no improvement. We have essentially wasted many dollars of parts as well as a gallon or two of brake fluid.

Do you have a gauge to isolate?

brianp87 wrote:Do you have a gauge to isolate?

No I do not. If I block off the front wheels, the pedal only goes in a little then is hard and stays that way. If I remove the hose block off of either front wheel or both, the pedal goes to the floor, leading us to suspect the front brakes as the cause of the fading/sinking brake pedal.