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Purplejchild

Dragging brakes

Question

I have a 2011 Chevrolet Silverado 2500hd 4x4. It has developed a dragging brake issue.  I have changed all 4 calipers,  brake pads,all brake hoses,master cylinder, and hydro boost, flushed with new fluid. It shows no check engine or Abs lights or codes. I can drive it between 10 to 20 miles and you can start to feel the brakes applying till it locks up. Then I get out and pop the brake line and relieve  the pressure  then we're  fine for awhile till it starts all over again. I've  taken a temperature gun to the calipers when its locking up and the front can be up to 180 degrees and the back up to 150 degrees, or it can be the back hotter then the front. I've  taken it to a dealer and they have scanned it and with no codes they are saying it has to be a mechanical issue, but I have changed all the mechanicals except the hard brake lines. I am starting to think it's the Abs module or pump,  but with no lights or codes I hate to put another 1000in this gamble. Any advice or help would be much appreciated.  Thank you!

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What locks up? The front, back, one caliper in particular, all 4?


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Posted (edited)

It's only locked up once and was the rear. Once I feel the brakes start dragging,  I pull over and open the bleeder valve on the caliper  and it's  ready to go.  As your going down the road you can feel  the breaks start dragging. 

Edited by Purplejchild

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The flexible lines can fail internally and act as one-way valves, perhaps something like that is going on.

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Someone mentioned that to me so I changed  all flexible lines out. Nothing changed. 

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You had contaminated fluid prob from hydro boost going bad leaked into the master.
Anyways when oil gets into the brake fluid it causes this issue.
So when you started replacing parts you didn’t replace them all at the same time so the master cylinder got contaminated again.
Replace master cylinder again, but be SURE you’ve flushed out everything before putting another one in.


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Is there anyway of flushing the system and flush the master cylinder I have now?

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If the fluid was contaminated enough to cause something to fail, such as a seal to swell up or something like that, just flushing in new fluid (which you indicated you had already done) isn't going to fix those bits, you have to find and replace them.

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If the fluid was contaminated enough to cause something to fail, such as a seal to swell up or something like that, just flushing in new fluid (which you indicated you had already done) isn't going to fix those bits, you have to find and replace them.

^ what he said
Replace the master again.


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Let me ask you this.  The hydro boost wasn't leaking when I replaced it. It was dry inside. The reason  I replaced it was, after driving  the truck about 15 miles and the brakes  just start to drag the brake paddle would  might  a high pitch squeak then after I release  the pressure in the brake line the squeak would go away but then redevelop when the brakes start to drag again.  Now after replacing they just start to drag with no squeaking. 

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One guy was telling me he thinks it might be the abs module.  Since it not effecting the same wheel it bounces around one time it might be the back dragging  and the next time it's the front. He recommends pulling all 3 abs fuses and driving around and slowly putting in each one back in till I can narrow it down to the right system. Any thoughts on this or do you think it's still in the master cylinder?

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Typically if oil gets in the master cylinder (most common way for oil to get in master is someone topping the brake fluid up and using the wrong fluid) the seal on the cap will swell up.  BTW, avoid topping off the brake fluid reservoir, there is more than enough brake fluid in the master to account for pad wear.  Its not like it gets used up.

 

You say you open a bleeder, which one, and is it always the same one?  There are two circuits in the brake system, if you only open one bleeder, and it is always the same one, then you have narrowed it down to just half the brake system. 

 

Since you have already replaced the master cylinder, that should have taken care of a plugged compensating port in the master.  This port allows brake fluid to "leak" back into the master cylinder as the fluid expands when driving on the highway (long instances of no brake application).  If that port gets blocked, pressure will build up in the disc brake system, eventually applying the brakes enough to stop the vehicle.  That port can also get blocked by an incorrectly adjusted brake pin that goes between the brake pedal and master cylinder.  But, if the master was not removed, or no one had touched the brakes prior to this happening the first time, it will not be that pin misadjusted.  Touching the brake pedal while driving(aka riding the brakes) will also block that port.

 

Personally I have no idea if the ABS can cause any of this, I "retired" before ABS became common.

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I usually open the drivers front and when I do the pressure is relieved and the truck rolls foward a bit

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When I replaced  the old master cylinder the cap on it wasn't  swelled or anything 

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Contaminated hydroboost systems aren’t common but they do happen.
I’ve also heard abs modules staying ‘on’ and holding brakes.
Also,
Double check pushrod length/adjustment, make sure there’s free play in the pedal, if the rod is too long, or adjusted too far in, it’ll hold the pistons inside the master beyond the compensator ports and never let pressure bleed off after applying brakes

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5 minutes ago, tlaw91 said:

Contaminated hydroboost systems aren’t common but they do happen.
I’ve also heard abs modules staying ‘on’ and holding brakes.
Also,
Double check pushrod length/adjustment, make sure there’s free play in the pedal, if the rod is too long, or adjusted too far in, it’ll hold the pistons inside the master beyond the compensator ports and never let pressure bleed off after applying brakes

I could understand the pushrod being the wrong length if this issue started after changing the master cylinder, but it didn't.  The rod can't change lengths on its own, and even if it could, it would get shorter, not longer.   It really is acting like a compensating port being covered or plugged.  Easy way to test is to remove the cap from master cylinder, and using a clean rag, make a funnel shape around the open cap and have someone tap the brake pedal.  The brake fluid should spurt out the opening as the brake pedal tapped.  If nothing comes out, try the other cap, assuming it has two caps.  

 

I went through a bunch of cars with oil in the master cylinder when I was working at Firestone in the 80s.  A local gas station attendant used oil to top off the master, this was back when they would actually open your hood to check oil and fluids.  Every rubber part that the oil hit swelled up.   

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