Help with sporadic brake issue.

[NIevo]

We have a 2017 Yukon XL that we just earlier this summer installed the full Z36 kit on. Lines were bled and proper seating directions were followed. Brakes perform great around town and normal driving. However, twice now when going down a good hill we have had problems. Once was pulling our boat and the other was with just 4 people in the car and not towing. Both times when we got to the bottom the brake smell was horrid and they didn’t want to function. I almost had my foot to the floor to get them to grab and it was incredibly spongy.

What could this be due to? I would think it would be air in the line maybe but that doesn’t explain the smell and it’s only been those couple times, no weird braking in any other driving. We’ve put well over 1000 miles on it with the brakes so I’m not sure what would be causing the smell as well.

 

[OR VietVet]

First of all...how steep a hill? You said "brake smell and didn't want to function". What do you mean by didn't want to function? Just the pedal going to floor and spongy or that and they did not want to slow the rig down either? Sounds to me like you are riding the brakes and overheating them but I wasn't there and don't know the hill steepness or length. Sounds like you were smelling hot brakes. Is the brake system vacuum boost or hydro-boost?

 

[NIevo]

Didn't function as in didn't want to stop the vehicle. I said in the OP that my foot was dang near to the floor.

First hill was just right around a mile I'd say at about an 8% grade. Second was maybe a little longer but less grade. I'm not new to driving nor towing, wasn't riding the brakes excessively at all. I've taken both grades with and without trailers with numerous vehicles and never had a problem, including this one before the brake change.

Not sure if the 17's are vacuum or hydro to be honest. I'm guessing vacuum.

 

[Thrust]

Have you cycled the antilock?
I had air in mine after swapping to Brembos.

 

[Jimmyy]

I would start carrying a thermo gun to check each wheel. You could have a caliper hanging up.

 

[NIevo]

Have you cycled the antilock?
I had air in mine after swapping to Brembos.

What do you mean? Are you just referring to bleeding them when installed? If so then yes, I have a bleeder and did all 4 corners.

 

[Rygrego]

What do you mean? Are you just referring to bleeding them when installed? If so then yes, I have a bleeder and did all 4 corners.

If you allowed air in the system you must purge cycle the ABS system via a capable scan tool. It take a few seconds but I know of no other way.

 

[OR VietVet]

Didn't function as in didn't want to stop the vehicle. I said in the OP that my foot was dang near to the floor.

First hill was just right around a mile I'd say at about an 8% grade. Second was maybe a little longer but less grade. I'm not new to driving nor towing, wasn't riding the brakes excessively at all. I've taken both grades with and without trailers with numerous vehicles and never had a problem, including this one before the brake change.

Not sure if the 17's are vacuum or hydro to be honest. I'm guessing vacuum.

Listen, I asked because you didnt say and just because your "foot was dang near to the floor" does not mean it wasn't stopping. You could be stopping but have a strange feeling. I ask because I dont like to assume. For that reason, I will bow out and let someone else deal with it. If you read my responses you should know I ask all questions for a reason.

 

[B-train]

I agree with OR VIETVET. I suspect the length and steepness of the grade had them overheated. Just "covering" the pedal slightly still builds up INTENSE heat when slowing down 6500 lbs@ 8%. Try manual mode with tow hall activated and find a gear that holds you close to the speed desired. From there use your brakes in a PLUGGING type motion - hold to slow 5 mph below the speed in a short distance, then release and let engine braking take back over. This will give the rotors time to cool again. Repeat as speed gets too high again.

Brakes work on the principal of friction and heat exchange. Cooler brakes can absorb more heat, therefor being more effective. Once hot or overheated, there is no more difference in heat exchange/rejection and that's where your pedal was on the floor and not slowing you down well. Your engine and Trans will do wonders for engine braking, the rpm will probably be at 4k on a grade like that, but just let her work.

As for the ABS. If you didn't drain the system completely at any corner (ie: line clamped off) while replacing components, then the likelihood of air in the ABS is slim. If you did evacuate any part of the system, then yes cycling the ABS is beneficial. If you don't have a tool or want to pay for one, go to a gravel road and do some ABS stops. Then get home and Crack the bleeder and see what comes out with gravity and the cap on the master cylinder loose or off. You'll probably see some bubbles at first. Then have an assistant pump the pedal 3 times and hold while you open the bleeder, close the bleeder, and then have them release the pedal. Repeat as needed until only fluid comes out.

And, your truck is vacuum operated. There is a belt driven vacuum pump where the power steering pump used to be on the older yukons.

 

[OR VietVet]

I agree with OR VIETVET. I suspect the length and steepness of the grade had them overheated. Just "covering" the pedal slightly still builds up INTENSE heat when slowing down 6500 lbs@ 8%. Try manual mode with tow hall activated and find a gear that holds you close to the speed desired. From there use your brakes in a PLUGGING type motion - hold to slow 5 mph below the speed in a short distance, then release and let engine braking take back over. This will give the rotors time to cool again. Repeat as speed gets too high again.

Brakes work on the principal of friction and heat exchange. Cooler brakes can absorb more heat, therefor being more effective. Once hot or overheated, there is no more difference in heat exchange/rejection and that's where your pedal was on the floor and not slowing you down well. Your engine and Trans will do wonders for engine braking, the rpm will probably be at 4k on a grade like that, but just let her work.

As for the ABS. If you didn't drain the system completely at any corner (ie: line clamped off) while replacing components, then the likelihood of air in the ABS is slim. If you did evacuate any part of the system, then yes cycling the ABS is beneficial. If you don't have a tool or want to pay for one, go to a gravel road and do some ABS stops. Then get home and Crack the bleeder and see what comes out with gravity and the cap on the master cylinder loose or off. You'll probably see some bubbles at first. Then have an assistant pump the pedal 3 times and hold while you open the bleeder, close the bleeder, and then have them release the pedal. Repeat as needed until only fluid comes out.

And, your truck is vacuum operated. There is a belt driven vacuum pump where the power steering pump used to be on the older yukons.

Not saying that riding his brakes was the problem but was a guess because I have heard/seen where so many times people say they know how to use brakes going down a hill, towing or not, and then when you ask them to explain their process of that, they don't know how. What slows you down with brakes is friction and friction builds heat and when enough heat transfers to the fluid you got problems and the pad surface itself crests that temp threshold and stops wanting to grip that rotor and that rotor is fighting back the whole time. Having ran shops, I have seen the aftermath of that problem. He may have a whole other problem and what I was asking about may not be the problem but I draw the line at trying to help and then a response assumes I did not pay attention to what was said in the post. I do pay attention and ask questions accordingly to clear up the foggy areas of the story. I hope he gets the problem solved.