Rear disc brake issue- need advise

[Tbruz]

2004 Tahoe with 5.3 engine.
Longish stoty so be patient please.

Some background:
I'm a fairly decent home wrench and have performed brake replacements on about 75 different occasions and varied vehicles across many years ( family and friends) certainly not a professional though so I'm looking for some online advice /troubleshooting help here.

The current situation:
At first use the brakes perform perfectly.
After a rear brake job, using GM pads, new hardware, and having the GM rotors resurfaced by a very reputable machine shop I'm getting a weird rotational "thumping" when I apply the brakes at above 30 MPH. the new pads were bedded in by hard braking from 40MPH to 20 two times followed by 60MPH -20 two times .
The calipers were free, ie not stuck or sticking and the slide pins were removed, cleaned and re-lubed. wheels were properly torqued.
My issue:
At first use the brakes perform perfectly and are silent .
After about 15 minutes of use I can hear a rhythmic "thumping" at each application of the brakes. Its obviously rotational, heat related and coming from the rear brakes- NO shimmies or pulsating as you'd feel from warped rotors. I've taken everything apart again to see if anything obvious is wrong - all looks good.
I'm at a loss as to what to try next?

Thinking about replacing the new pads with the old ones(still have some meat left on them) just to see if the issue is still there and at least ruling out bad new GM pads.
Appreciate some suggestions as to what this issue can be
Next step I think would be popping for new rotors....even though these are non warped and within specs.
Thanks in advance
Tbruz


UPDATE with SOLUTION!
So i swapped back the old pads as I had them in hand still . Silent when cold but the same rhythmic thumping when the brakes heated up. I borrowed a runout gauge but could not get a clean reading seemed to be jumping.0008 to .0010 as the highest difference from being zeroed out on the face of the rotors; could not get reading off the backside so I moved on to swapping in new rotors....and back to the new pads I had installed yesterday- sanded lightly and re-bedded to the new rotors. PERFECTION! No thumping cold or after a good long spirited 45 minute run.
Thank you to all who helped with their guidance and suggestions- I truly appreciate the depth of knowledge and quick input I received from all.
Regards
Tbruz

 

[Fless]

Could it be something with the parking brake horseshoe? Perhaps some heat expansion related to that?

 

[Doubeleive]

so what's up with the front brakes then? the front brakes are 99% more likely to get warped than the rear. this can happen just from your bedding process with the rear.
Inspect the front and run your finger across the front rotor edge if it is not very smooth then that is your issue.
I'll add that there is some trickerey going on with these trucks, they like to "throw" sound, so a noise that may seem to be coming from one place is actually coming from another place.

 

[Tbruz]

Fless, I ruled out the parking brakes because the sounds appear ONLY when I'm applying the brakes. My thinking was if it was the parking brakes then I'd hear the noise all the time not just when braking.

 

[Tbruz]

Doubelive, sounds are DEFINATELY rear and were not present before my rear brake job; I will recheck fronts due to the bedding process

 

[OR VietVet]

Did you replace the pad hardware when you did the work or reuse the old hardware? As thin as the old rotors are when new and then to machine them, IMO, leaves them to thin to dissipate heat properly and should be replaced with every new brake job. Sounds like you are building heat and rotors are warped and causing the noise as the high spots slap thru the pads. Make sure the hardware is new, scuff the pads thoroughly to help seat again with new rotors, clean the new rotors completely with Brake Clean and clean rag, check runout of new rotors after install before you attach the caliper and pads, make sure you lube the contact points where the pads ride during use and then seat them in again. Plus, how do you know you don't have a caliper piston hanging up that releases when cooled? Last time the fluid was flushed/changed?

 

[Tbruz]

PNW Vietvet; I just added that new hardware was used. All metal slide/contact points were lubed.
Brake fluid is clean having been flushed maybe 30K miles ago.

Unfortunately for me I can't check runout, no dial gauge in hand. Warpage from the machine shop is a definite possibility, just thought i'd feel that warpage all the time after the initial incident; ie why/how are the rotors getting un-warped perfectly flat when cooled?
Again the rotors were WELL within spec and could have easily been reused without machining- so absolute minimum material was removed.
I will sand pad faces though- good idea and will rule out the pads.

Keep the ideas coming; I appreciate the additional concepts to check/rule out

 

[Scottydoggs]

did the new pads slide onto the new hardware easy? or was it a fight? i find myself grinding the pads hook ends open a bit more almost every brake change as i could see the pads getting hung up if they wont move freely on the hardware.

 

[Tbruz]

Scottydoggs, I did clean up the pad "hooks" a bit to clean up the paint flash/metal edges prior to lubing them. Pad fit did not appear problematic- not tight nor loose'
I'm leaning towards re-installing old pads shortly just to rule them out or in as possibilities.
Zero cost and quick enough to R&R the pads, now that the bolts have all been loosened via Pad/rotor R&R

 

[OR VietVet]

The warpage happens after they get hot and then dissipates a bit when cooled. Rotors on rigs nowadays are what I call "Bic Rotors". Never machine and replace them with quality rotors. Did the caliper pistons push in/collapse easily when you did it? Did you also check the back of the rotor where the hat lays against the axle hub? Check the back of each rotor and the face of the axle hub for any rust/grit/grime that is not allowing the rotor to lay flat against that surface.