A friend came to me w/ his low miles (65k) 4WD 2008 Dakota with a smoking hot RF brake, The pads were nearly worn out on the RF.

The LF was NOT dragging and was not hot and the pads were half worn only on that side.

I couldn't get the RF pistons to retract while still connected to the brake lines, so bought a rebuilt caliper and changed the RF caliper and all F pads.

I had trouble bleeding the new RF caliper by gravity, so used a MityVac to suck fluid through the caliper to bleed (I have a lift and it was inconvenient to try to get through the drivers door as the lift post was in the way). It was tough to get fluid out and I had to use 25" vacuum to pull fluid through the open bleeder screw (I coated the bleeder screws with anti-seize to seal the threads so as to not suck air past them during bleeding).

I pulled the LF caliper to replace its pads as well. One LF piston retracted to within 1/8" of full retract, but the other LF piston would only go half way back leading me to believe that was was potentially sticking.

I obtained another caliper for the LF and installed it. I bled that one using the brake pedal and a hose on the bleeder screw (I had a skinny guy climb through the door tpress the brake pedal).

The truck seemed fine on a test drive (no dragging), so I called my buddy to take his truck and he drove away.

Today he came back with BOTH front brakes dragging (170 deg F using an infrared thermometer). I lifted the truck and both wheels were HARD to rotate.

I pulled the RF wheel and caliper and tried to retract the pistons one at a time, and retracting either (squeezing it back) made the other piston extend, so I knew they were free, but fluid was NOT going back to the master cyl as it should.

To see if it was a blocked RF flex line I broke the connection between the RF flex line and the RF hard line, and fluid immediately ran out, allowing those pistons to retract easily and also the LF brake now released as well. Retracting either RF piston did not now cause the other to extend as before.

I went around the other side (LF) and that wheel now was no longer dragging (though it had time to cool down).

That implied to me the fluid pressure was trapped INBOARD of the flex lines, maybe by the ABS system or by the master cylinder.

I asked a friend with a professional shop and he said the rubber flex lines hands-down based on hundreds of cars he's done, but to me that doesn't make sense as one side would not affect the other if it was a flex line.

I have never heard of a bad M/C causing dragging brakes, and I don't know enough about ABS systems to know if one could cause both front brakes to drag (though remember initially only the RF was dragging). My shop friend brought over his "high-end" scanner and said the ABS showed no faults, and the Check Engine was NOT illuminated anyway.

SO.... Do I follow his advice and have my friend buy 2 flex lines to try ($65 w.tax) from Autozone to try first, or have him buy a rebuilt master cylinder (for $97 w/ tax).
Though I don't know how the M/C could cause this, it seems to me that something in common with both F brakes was causing a problem (though originally only the RF brake was dragging hard and wore out it's pads), and that would involve the ABS or the M/C.

Call me confused,
Bob.

 

If I was the truck owner, I'd tell you to start with the flex lines. Could be that the RF was just about to fail when you looked at it the first time.

Either way, if you're not charging him labor, the worst he's doing for both parts is a $200 repair.

Just my two cents...FWIW

 

Bad flex hose. They collapse inside and either the brake pedal is rock hard and no brakes or it will apply the brakes but since there's no reverse pressure, they stay applied. Replace both hoses and I'd check the rear end at the differential. They are rubber and do age. My '02 Ram snapped a line when I had the FEA done so I replaced the other one. They were the same age so the other was about to fail. If the rear hose has any cracks from dry rot, it's about to go too.

 

The only thing that makes me think it's NOT the flex line is that when I broke the connection between the caliper and the hard line, fluid came out and both calipers released.

By breaking that connection (cracking the flare nut open a bit), fluid passed back through the RF flex hose, and apparently released the pressure no only from the RF caliper, and also the LF caliper on the opposite as well.

If the flex lines were bad, both calipers would have stayed locked up since the fluid would have been trapped on the caliper sides of both calipers, wouldn't it?

But instead opening the inboard connections between the wheels allowed the calipers to release.

I wish I understood more about how the ABS ties the fronts together. I know if the ABS is totally de-energized, fluid should pass back and forth easily and transparently. Of course the scan tool showed no faults in the ABS, so I guess I should just accept that the ABS could not be causing it.

No one answered whether it's possible for the master cylinder to block the return path of fluid from the calipers, so I guess that is a very unheard-of thing as well, pointing o both flex lines becoming restricted within a day of each other? They ARE 11 years old, I guess, but I have much older vehicles myself and never had a line restrict, but I guess it's luck of the draw, as they say.

 

Anything is possible. An overfilled master cylinder can cause this - I've been told, never seen it. It happens on motorcycles that sit for long periods, especially old fluid that's never changed and breather port gets blocked in the master cylinder, but I've never heard of this on an automobile, but could happen.

 

Your flex lines are probably old enough to be due for a change anyways. Have them trace the solid lines, too. My truck had to replace the metal lines at about 55K miles because they were corroded enough that they were about to fail. Although, I would expect the solid lines to corrode from the outside and then leak, rather than corroding from the inside and creating a blockage. But, depending on long it sits or water in the fluid, I suppose that could happen.

 

As an experiment I took the truck for a drive, and sure enough both front brakes got overly hot even though I used very modest braking (you could feel it radiating) and the IR temp showed about 170F on each side pointed at the "hat" part of the brake rotors, and you could barely feel the truck dragging when pushing it (It rolled very easily when they were fully released manually at one point).

I took a 12mm flare nut wrench with me, and when stopped I cracked the front master cylinder flare nut, expecting to see fluid come out. NONE did come out at all, which I found interesting since usually some fluid will come out by gravity w/o pushing a brake pedal. I did the same w/ the rear connection and fluid freely ran out but not under any real pressure). I guess the front piston is the front brakes, and the rear, the rear? Afterward, the Frt brakes still dragged.

I drove it a few more miles, and did the same thing, but had a friend press the pedal while I cracked each fitting open (essentially bleeding the MC connections). This time fluid spurted out of BOTH connections when the flare nuts were cracked.

After this, I drove the truck and checked the front rotor temps, and they were only 120 degrees (both sides w/in 1/2 degree F of each other).

This was a non-scientific test, so because I didn't drive the same exact route each time, I can't be sure the lower temps after bleeding the MC connections was due to bleeding it, but it's interesting.

I am going to contact the owner and ask if he wants me to buy and install new hoses as an experiment. Meanwhile, I'll drive his truck more to see if it stays better after the informal MC bleeding I did.

Bob

 

Interesting.

The exterior of the lines (and the rest of the truck top to bottom) is absolutely like new with not a spot of corrosion, even surface rust, anywhere so I didn't give a thought to the insides of the hard lines, though the fluid that came out of them when bleeding looked almost clear as new, so hopefully no rust inside them. I once lived back East and everything rusted there (forget even getting the bleeder screws to turn), but here in the PNW it's amazing how nothing rusts that is exposed to the air (though again inside lines could be another matter of course).

I'll do the flex lines tomorrow and report back.

 

DOT 3 and 4 brake fluids are hygroscopic. They love water and will absorb it out of the air. That's why unless you plan to use a lot, you should buy small bottles of brake fluid instead of the larger ones and you shouldn't leave it open more than you need to. I won't use DOT 5 synthetic because while it is hydrophobic (hates water) water can puddle in the lines and freeze in cold water. This makes for interesting ddriving in cold weather as well as older equipment is NOT designed for it. I've seen the old pressure brake light switches start leaking.

 

assuming there are no deffective parts, and if the dakota is the same as a ram I had the exact same issue. the rubber flex lines go through a metal bracket. the brackets around the hose get packed with road grime dirt rust etc and then swell. the swelling squeezes the rubber line off. when you use the brakes there is enough pressure to force oil through to apply the brakes but the oil can't pass back out to release the brakes, and it acts like a check valve. pry open the metal bracket around the rubber flex lines. it's a zero dollar fix.