Air getting into brakes

ChryslerKid

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So not b body related. I picked up an 02 dakota about a month ago. I dont like "newer" vehicles but the guy that owned it couldnt drive anymore and sold it to me for $700. The trucks in real nice shape but the brakes were spongy as all hell. I would get nothing out of the pedal and then all of it at once. I pulled the tires and all pads/shoes, rotors/drums in good shape. All the soft and hard lines i can see until it goes into the frame are in good shape and dry. Wheel cylinders and calipers dry on the line side and on the piston side. So i replaced the master cylinder and bled all the fluid in the system through. While bleeding the pedal goes to the floor for every side except the rear driver. The pedal hardly goes half way down. Anyhow i get a nice rock hard pedal when finished. Over the course of a week or two the pedal slowly goes right back to being mushy and having a hard time stopping. Feels like air. So i bleed ONLY the rear driver side and it goes back to a hard pedal feel. Right wheel cylinder is dry but i enternained the idea that its somehow sucking in air so i pump up the brakes and crack the line on the back of the cylinder. Still the pedal only goes down halfway. Ive never lost any fluid from the system. Sorry for the longwinded question but im stumped as to how air is getting into only this one side of the brakes.
 

Sam69sat

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I would replace the rear driver soft line. Sounds like it's blocked or partially blocked internally not letting the fluid flow back and forth freely. Might have an air bubble between the line and wheel cylinder.
 

5.7 hemi

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Does it have ABS? If so, you’ll need to cycle the pump.
 

Crowbar

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I agree with Sam69sat I’ve seen the inside of the soft line swell on the inside while outside looks perfect RC
 

ChryslerKid

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I agree with Sam69sat I’ve seen the inside of the soft line swell on the inside while outside looks perfect RC
Well shoot no sense in doing just one i might as well do all three. Is there any chance the wheel cylinder could be drawing in air without leaking. Ive seen a caliper do this once with the piston seal acting like a one way valve but never with a wheel cylinder.
 

Crowbar

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I’d start with all new soft lines and go from there RC
 

4406bbl

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Well shoot no sense in doing just one i might as well do all three. Is there any chance the wheel cylinder could be drawing in air without leaking. Ive seen a caliper do this once with the piston seal acting like a one way valve but never with a wheel cylinder.

Wheel cylinders can do this.. in the old days the spring inside had metal expanders on the ends to expand the cups to prevent this so it can happen. Wheel cylinders are cheap so why not swap them and the soft line.
 

RemCharger

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Wheel cylinders can do this.. in the old days the spring inside had metal expanders on the ends to expand the cups to prevent this so it can happen. Wheel cylinders are cheap so why not swap them and the soft line.
I agree.
 

65wgn440

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I don't believe replacing flexible hoses will cure this problem. If a flex hose has an internal rupture, it will not cause a soft pedal. Either it would not flow fluid at all (rare) and that would not cause soft pedal or it would flow one way to cylinder, but not allow fluid to flow back towards master (that brake would drag and be obvious). Sounds like air/or problem in ABS modulator, if equipped. I have also ran into issues with some Chrysler's that have inline proportioning valves on rear lines. Have had to replace a few after brake repairs to get a good pedal/ brake operation. Not sure, again, if yours is so equipped.
 

5.7 hemi

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Rubber will swell.
 

pnora

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Fact-------------------Brake systems do not suck air. Period.
 

4406bbl

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It can happen. If the cylinders do not have expanders and the master cylinder residual pressure valve no longer holds 10 pounds, or has a 2 pound valve, or the cups are old and shrinking, or the rear abs valve on pickups and vans fails to let that 10 psi thru. Dirt in unbled old systems seems to cause it.
 

MoparLeo

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Obviously the brake system has been completely flushed out with new brake fluid first. Correct? Did the correct bleed pattern ? Master is new , not rebuilt, right ? Guy that had the truck before had the same problem, right ? Rear brakes have been adjusted, right ?
 

BB BELLA

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Fact-------------------Brake systems do not suck air. Period.
you've never owned a mid 70's corvette,those oe calipers did and no leak!caused by caliper runout worked the piston like a pump I use to bleed them weekly!
 

ChryslerKid

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Obviously the brake system has been completely flushed out with new brake fluid first. Correct? Did the correct bleed pattern ? Master is new , not rebuilt, right ? Guy that had the truck before had the same problem, right ? Rear brakes have been adjusted, right ?
New centric master, flushed entire system with new fluid, guy before didnt mention the issue but it was the first thing i noticed getting inside. Bled rear passenger, rear driver, front pass front driver. And the rear brakes are adjusted properly nice and snug. Im going to replace the wheel cylinders and all three soft lines then ill move to the calipers if all else fails but i hate getting rid of oe calipers. All they sell is reman aftermarket. Called dodge they discontinued factory calipers. Might even replace the front bleeders first just to try and avoid pulling off factory parts that might be perfectly good.
 

MIKESPOLARA

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Fact-------------------Brake systems do not suck air. Period.


Fact--------some do. Early 4 piston Corvette caliper will suck air in without displacing fluid if rotor runout is excessive, usually more then 4-5 thou. The side by side pistons oscillate drawing in air. I never believed it until it happened to me.
And I have heard of drum brake cylinders that can suck air thru the rubber seals
 

pnora

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Fact--------some do. Early 4 piston Corvette caliper will suck air in without displacing fluid if rotor runout is excessive, usually more then 4-5 thou. The side by side pistons oscillate drawing in air. I never believed it until it happened to me.
And I have heard of drum brake cylinders that can suck air thru the rubber seals
Explain how the pistons can oscillate and become air pumps? That's something I have a hard time believing. And why should a Corvette calipers be any different than any other car? Its just a caliper. I understand excess rotor run out can cause certain issues. To me a couple of rotor rotations the pistons should slightly go back and stay back. That should only cause a lower pedal on the first pump. The fluid is also heavier than air so that should also make it harder for air to enter.
 

ChryslerKid

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Does it have ABS? If so, you’ll need to cycle the pump.
Well now i feel stupid. I assumed it didnt have abs because i didnt see any wheel sensors and while bleeding them just now i noticed the abs module sitting right infront of my face. I wonder if someone before me ran the old master dry at some point. Either way i have to get that pump to cycle through now regardless. Then move on to the next step if its still soft.
 

Basket case

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where does the exhause exit? I had a W150, that around town it was fine. But on the road for several hours on a road triip the brakes got soft. Pulled off the road and bledd the brakes, went another hour and soft brakes. Under the truck to bleed agai, and figured out the since ther was no tailpipe, the muffler was blowing hot exhaust on the looped rear hard line the PO had installed. Ran a new line properly and solved the problem.
 

MIKESPOLARA

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Explain how the pistons can oscillate and become air pumps? That's something I have a hard time believing. And why should a Corvette calipers be any different than any other car? Its just a caliper. I understand excess rotor run out can cause certain issues. To me a couple of rotor rotations the pistons should slightly go back and stay back. That should only cause a lower pedal on the first pump. The fluid is also heavier than air so that should also make it harder for air to enter.


Google it, it's well documented and I had my car do it on one wheel when which had about 8 thou runout. They make tapered shims especially for Corvettes so you put a tapered shim under the rotor that is half as much as the runout and your rotor now runs true. Which is exactly what I did to solve the problem
 
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