Braking at High Altitude

[TDBrown]

Does altitude somehow effect braking performance? A few years ago (during COVID), my wife and I drove the Raptor off-road across the country on the Transamerica Trail. The trail took us up over Engineer Summit (?) which, as I recall, was over 13,000 ft elevation. When we started down the back side of it, going into Silverton, CO, we lost almost all braking. Like we'd be going 5 mph and it would take forever to get it to stop. It was a scary situation with me pumping and standing on the brakes and getting almost no bite. When we dropped elevation, the brakes started working fine again.

We checked everything I new to check and all looked fine. We chalked it up to freak occurrence. The next day we drove up over Imogene Pass and the exact same thing happened. At the top I could stand on the brakes and get almost no braking. We limped down the mountain. As soon as we got to a lower elevation, the brakes worked fine again.

I ran all this past my auto shop and all they could think to do was change the brake fluid. It has bothered me all these years, so I thought I'd ask the forum. Any idea what would cause something like this? And has changing brake fluid truly solved the problem?

 

[mprice1234]

The only thing I know that greatly effects braking in high altitude is overheating the brakes. With all the switch backs and downhill driving in the mountains overheated brakes greatly lose braking power. Coming down hill on Pike's Peak they have a check station halfway down were they check wheel temp for overheated brakes. If they show to hot, they will have you pull over till your brakes cool down before they let you continue down the mountain.

 

[TDBrown]

The only thing I know that greatly effects braking in high altitude is overheating the brakes. With all the switch backs and downhill driving in the mountains overheated brakes greatly lose braking power. Coming down hill on Pike's Peak they have a check station halfway down were they check wheel temp for overheated brakes. If they show to hot, they will have you pull over till your brakes cool down before they let you continue down the mountain.

That's what I thought too. But I stayed off the brakes almost constantly and used engine braking to limp. Unless brakes heat up incredibly quickly at altitude, that wasn't the issue.

 

[stevenstommyboy1]

Does altitude somehow effect braking performance? A few years ago (during COVID), my wife and I drove the Raptor off-road across the country on the Transamerica Trail. The trail took us up over Engineer Summit (?) which, as I recall, was over 13,000 ft elevation. When we started down the back side of it, going into Silverton, CO, we lost almost all braking. Like we'd be going 5 mph and it would take forever to get it to stop. It was a scary situation with me pumping and standing on the brakes and getting almost no bite. When we dropped elevation, the brakes started working fine again.

We checked everything I new to check and all looked fine. We chalked it up to freak occurrence. The next day we drove up over Imogene Pass and the exact same thing happened. At the top I could stand on the brakes and get almost no braking. We limped down the mountain. As soon as we got to a lower elevation, the brakes worked fine again.

I ran all this past my auto shop and all they could think to do was change the brake fluid. It has bothered me all these years, so I thought I'd ask the forum. Any idea what would cause something like this? And has changing brake fluid truly solved the problem?

So how are the brakes since? Why did you go back for a 2nd time if they were performing like that? Glad nothing happened on the 2nd run. Have you had the issue since? I have had brake fade in several trucks towing in the mountains. Probably my fault for riding the brake instead of using the tranny. Never had the brakes pedal Go to the floor.

 

[TDBrown]

So how are the brakes since? Why did you go back for a 2nd time if they were performing like that? Glad nothing happened on the 2nd run. Have you had the issue since? I have had brake fade in several trucks towing in the mountains. Probably my fault for riding the brake instead of using the tranny. Never had the brakes pedal Go to the floor.

Because I'm old and stupid. When they started working perfectly coming down, and I checked brake fluid levels, I chalked it up to a freak incident.

Since then the brakes have been fine. But I haven't gone up to altitude like that since. I've replaced fluid, pads, and rotors.

The brake pedal didn't really go to the floor. There was resistance, just no stopping.

 

[downforce137]

likely a vacuum issue with the booster if the pedal was very hard and had little assist..

if the fluid boils the pedal will go to the floor because there is now air in the lines..

these trucks seem to hold the throttle open a bit, so theres little engine braking at lower RPMs so Im guessing you ran out of vacuum in the booster after a couple of pumps..

 

[TDBrown]

likely a vacuum issue with the booster if the pedal was very hard and had little assist..

if the fluid boils the pedal will go to the floor because there is now air in the lines..

these trucks seem to hold the throttle open a bit, so theres little engine braking at lower RPMs so Im guessing you ran out of vacuum in the booster after a couple of pumps..

Thank you! That's the best idea I've heard yet. How does one fix that?

 

[Bigt3141]

Thank you! That's the best idea I've heard yet. How does one fix that?

Perhaps installing a vacuum reservoir would help with this, I used them on cars I built with a large cam.

 

[downforce137]

Thank you! That's the best idea I've heard yet. How does one fix that?

not too sure.. either a lower gear and maybe more RPM will get the throttle blade to close and get some vacuum build up, or just stopping and idle for a short time should take care of it..

theres a check valve in the big hose to the booster, so you will want to check that out as well.. Never really came across this, unless the check valve was bad or the booster maybe..

you'd need to put a vacuum on the booster and see if it holds vacuum for a period of time to test that out..

come to think of it, if you shut the truck off you can press the brake pedal and it will feel normal for maybe 2 or 3 presses, right?

try it after the truck sits for a bit and see if its stiff or soft and that will tell you if the booster or check valve is leaking down..

 

[TDBrown]

not too sure.. either a lower gear and maybe more RPM will get the throttle blade to close and get some vacuum build up, or just stopping and idle for a short time should take care of it..

theres a check valve in the big hose to the booster, so you will want to check that out as well.. Never really came across this, unless the check valve was bad or the booster maybe..

you'd need to put a vacuum on the booster and see if it holds vacuum for a period of time to test that out..

come to think of it, if you shut the truck off you can press the brake pedal and it will feel normal for maybe 2 or 3 presses, right?

try it after the truck sits for a bit and see if its stiff or soft and that will tell you if the booster or check valve is leaking down..

I'd have to take the truck up to 13,000 feet to test it. When I got to lower altitude, and for the four years since, the brakes have been entirely normal. But it sounds like it might be worth having the shop replace the check valve and the booster just in case.