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Air bubbles only visible on rear brakes when bleeding

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Got around to the 30k mile interval so it was time to flush the brake fluid.  Followed the procedure as per the FSM.  I use an air powered tool that pulls the fluid through back from the master cylinder to flush.  I couldn't help but notice that both rear brakes were producing air bubbles no matter how long I was running new fluid through them (rear passenger was more noticeable than rear driver).  Both front brakes had zero air bubbles which is I why I'm a little confused! I did not get around to pulling the drums off to inspect for leaks, but is it safe to condemn the wheels cylinders being bad?  I'm going to double check with tracing the brakes lines for the rear end but I haven't had any loss of fluid in the master cylinder or on the ground.  I think the brake pedal feels normal (quite firm/resistance is present) but I honestly don't know what a "spongy" pedal is like.  

 

I know there's an upgrade that can be done with the wheel cylinders as I searched on a couple other threads.  Although it's early on a diagnosis, but could someone post a part number just in case?  I understand they are relatively inexpensive.            

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Posted (edited)

when you pull suction from the bleeder nipples you get air coming in via threads. this is why i always bleed positive pressure from master cylinder.

 

one thing over anything when bleeding hydraulic assisted brakes... DO NOT PUSH THE PEDAL to push fluid through, you may bottom out plunger damaging seals inside and then you get to replace the whole thing.

 

i have one of these, it works perfect and only need 1 person. it is worth the money and super easy to use!

 

https://www.motiveproducts.com/collections/domestic-bleeder-kits/products/chrysler-dodge-mopar-bleeder

Edited by CUMMINSDIESELPWR
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Weird ive always full stroked the pedal to make sure push all the trash out. Still OEM master at 416k miles no seal issues. 

 

On super black systems I disconnect the hoses from the calipers and drums. You don't want to push the trash in the bottom of calipers and wheel cylinders. What happened to me is the debris piled up in the calipers binding up the piston causing brake drag. Ended up getting a seal kits and rebuilding each caliper for about $12 an axle. Dark brake fuild is sign of moisture in the fluid which oxidizes the seals in the system causing leaks and master cylinder failure.

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Pressure from master cylinder end is my preferred method.

 

If you want to vacuum bleed the system try pulling the bleeder nipples and coat the threads with paste type anti seize compound (helps to keep the nipples free and prevents air infiltration during bleeding process.  Also be sure to replace any missing bleeder nipple covers (keeps junk & moisture out).

 

Joe

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Posted (edited)

I no longer use vacuum bleeders. I've got a old mayo jar with a piece vinyl tubing. Leave just a bit of old fluid in the jar. Open the bleeder screw, then hook up the vinyl tubing. Jump in the cab slowly pump the pedal full strokes to the floor. About 3 good long strokes you need to check fluid level. Then walk back and see if the fluid in the vinyl tubing is cleaner looking and air bubble free. Refill the reservoir and continue till the fluid looks clean and bubble free. Start at the passenger rear the driver rear. Then passenger front and then drivers front. Usually takes me about 2 quarts to flush and bleed the system. 

 

No need for a second man to open and close the bleeder screw. The fluid in the jar prevents back flow of brake fluid or drawing air. Works just as good as a pressure bleeder without the cost!

 

dodge-rear-axle-disc.jpg.1812d8588feab18

Edited by Mopar1973Man
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Also, you may also need a scan tool (DRBIII, Genysis, OTC, ...) capable of activating the ABS module to flush old fluid out and any air trapped in module.  Not as simple as the old days.

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I vacuum bleed the brake fluid and wrap the threads of the nipples with teflon tape.

 

On 7/23/2020 at 12:18 PM, Mopar1973Man said:

Weird ive always full stroked the pedal to make sure push all the trash out. Still OEM master at 416k miles no seal issues. 

I've seen 3 brake master cylinders go bad from full stroking the  brake pedal; do it at your own risk.   

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Posted (edited)
20 hours ago, Joe_Pool said:

Also, you may also need a scan tool (DRBIII, Genysis, OTC, ...) capable of activating the ABS module to flush old fluid out and any air trapped in module.  Not as simple as the old days.

you need to do this with front wheel abs systems. rear only abs no need.

 

and the full stroke pump of pedal will work if the bore isnt rusted or corroded, like IBM said do at own risk.

Edited by CUMMINSDIESELPWR
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Hence why I make sure to change fluid every 30K miles before the moisture content of the brake fluid is rotting out the master cylinder. Hence why I've got 18 year old master cylinder that I can full stroke yet after 417k miles. That why I had OEM calipers for 300k miles because I kept the moisture out of the system and flush the system regularly. Heck I just did some brake work last week and full stroked it again to build up brake pressure again. I'll even full stroke the 1996 Dodge no issues that is a 24 year old master cylinder with 187k miles and still doing good. Again I keep up on fluid changes! 

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3 hours ago, Mopar1973Man said:

Again I keep up on fluid changes! 

Not everyone does their maintenance in as timely a manner as you and may only bleed the brake system when replacing a brake component; hence the warning about full stroking the brake pedal when flushing the system.  

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11 hours ago, Mopar1973Man said:

Hence why I make sure to change fluid every 30K miles before the moisture content of the brake fluid is rotting out the master cylinder. Hence why I've got 18 year old master cylinder that I can full stroke yet after 417k miles. That why I had OEM calipers for 300k miles because I kept the moisture out of the system and flush the system regularly. Heck I just did some brake work last week and full stroked it again to build up brake pressure again. I'll even full stroke the 1996 Dodge no issues that is a 24 year old master cylinder with 187k miles and still doing good. Again I keep up on fluid changes! 

you full stroke a lot of things dont ya mike....   :think::stuned::doh::poke:

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I don't get freaked out by scare myths. Yeah :lmao:Have a chuckle with your stroke comment. I heard that don't press the brakes to the floor you ruin the seals in the master. I better not push my clutch to the floor. It might ruin the seals in the clutch master. Right??? :whistle:

 

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5 hours ago, Mopar1973Man said:

I better not push my clutch to the floor. It might ruin the seals in the clutch master. Right???

Wrong!!! You full stroke that thing all day long.  No problem with rust in the bore; just worn out seals.

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Posted (edited)

Same... Bore can't rust if it made from aluminum. As long as there is no moisture there is on oxidation.  I don't have a cast master cylinder. :wink: Just like the current clutch cylinders are aluminum as well.

Edited by Mopar1973Man

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