Mountain descent brake failure - next steps?

Donphillipe

Active member
Finally did it. I came down a mountain and I thought I was doing fine because I was in 1st gear (Winnebago over 2005 Sprinter chassis). All was well and the brakes worked OK, I thought. It was a very long drive, a couple of hours. I felt the wheels at the bottom of the mountain when I parked and checked the temperature with an IR gun and they were well over 480 degrees. I left the vehicle parked for about 15 minutes and then I tried to move it.

Ouch! No brakes and the pedal went all the way to the floor. I used the emergency brake to stop me, then left it parked for 30 minutes, the brakes pumped back up then and while they appeared to be working, although not perfect, I am afraid to drive the vehicle now. No mechanics down where I am so the task is going to be up to me.

I looked under the chassis and I didn't see anything had burst with fluid running out. I am wondering if I should perhaps get someone to pump the brakes for me while I work at each wheel to bleed the lines with the objective of replacing the fluid.

Ideas?
 

Old Crows

Calypso 2014 View Profile
Hate to tell you...."By FMVSS116 standards, DOT 4 fluids must have a minimum dry boiling point of 446°F and a minimum wet boiling point of 311°F. DOT 4 is the grade applicable to most race engineered brake fluid in the world today, especially with regard to viscosity limit."

Depending on where you measured the brake temps, you exceeded the fluid's temperature range....more likely than not. Since brakes are still wonky....you might look into flushing fluid and inspecting the system.

(coming down the mountain....you have air blowing over the disks & calipers to cool them. After using them hard, it is sometimes a great idea to NOT stop to let them cool but to keep moving at a reduced pace to keep the air circulating and more effectively remove the heat. That avoids "heat soaking". Hint: look at a video of a Formula 1 car's pit stop. You can see the waves of heat, carbon dust and smoke boiling out of the vents due to no air circulation.)

???? Did you "ride" the brakes? Or, did you use them intermittently to keep the speed at an acceptible range.
 

Midwestdrifter

Engineer In Residence
You need to determine what the cause of your failure was. The calipers can have an external leak, or air in them. A gravity bleed it not difficult, and could resolve your issue (give it a google). The master cylinder can leak internally causing issues. The test for this is to (with the engine off) pump the brakes to remove any vacuum assist. Then press the pedal down hard and hold it. The pedal should stay firm, and not sink after a minute or two.

Snubbing, or part time braking has no real value. You should brake constantly in order to maintain a safe descent speed. This speed is one at which you brakes won't overheat. The only factory determining brake temps is descent speed, and any engine drag/braking available. For the same average descent speed snubbing will produce higher peak brake temps than constant braking. On some modest grades going faster, and letting aerodynamic drag keep your speed down works. In most cases you will be better off dropping your descent rate quickly with a sharp braking maneuver, and maintaining that slower rate with the brakes. This is basically what trucks do. In the end is is physics, the faster you loose altitude, the faster heat is dumped into the brakes.
 

Old Crows

Calypso 2014 View Profile
A brief explanation of proper downhill breaking recommended today. You may read past the business of air brakes/systems, valves, etc., as the Sprinter is an all hydraulic system. It is all about keeping the vehicle at a safe descent speed AND avoiding excessive heat build up in the brakes by allowing it to dissipate. On firmly & and off.


http://www.crashforensics.com/papers.cfm?PaperID=36
 

calbiker

Well-known member
You may have some water in brake fluid that turned to vapor, which caused your pedal to go to floor. You should be OK to travel once fluid cools. Get fluid flushed ASAP.
 

SneakyAnarchistVanCamper

Reading till my eyesbleed
With your life on the line, i think it is worth it to replace master cylinder and brake caliper seals. I wouldn't feel right about it otherwise. JMO, ya get what ya pay for.


Edit: I'm wrong, nevermind.
 
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JBR

New member
Finally did it. I came down a mountain and I thought I was doing fine because I was in 1st gear (Winnebago over 2005 Sprinter chassis). All was well and the brakes worked OK, I thought. It was a very long drive, a couple of hours. I felt the wheels at the bottom of the mountain when I parked and checked the temperature with an IR gun and they were well over 480 degrees. I left the vehicle parked for about 15 minutes and then I tried to move it.

Ouch! No brakes and the pedal went all the way to the floor. I used the emergency brake to stop me, then left it parked for 30 minutes, the brakes pumped back up then and while they appeared to be working, although not perfect, I am afraid to drive the vehicle now. No mechanics down where I am so the task is going to be up to me.

I looked under the chassis and I didn't see anything had burst with fluid running out. I am wondering if I should perhaps get someone to pump the brakes for me while I work at each wheel to bleed the lines with the objective of replacing the fluid.

Ideas?
I agree with Old Crows, the brake fluid boiled.

The best way to avoid that is to pick a safe descent speed, select a low enough gear that the engine is providing reasonable breaking, then, when you reach your target speed plus 10% or so, apply the brakes firmly until you slow to your target speed minus 10% or so. Then foot off the brakes until you reach target speed plus 10% again. Then repeat. This lets the brakes cool.

Do not use light constant pressure on the brakes!

Sent from my Pixel XL using Tapatalk
 

BaywoodBill

pre-Yuppiedom
In reading through this thread I see proponents of both braking techniques. Both schools of thought have proponents. I guess I just don't know enough to change my own technique: don't go down long really steep hills
 

Nautamaran

2004 140” HRC 2500 (Crewed)
Go fast enough that you have enough air over your brakes to cool them, and slowly enough that the brakes do not generate more heat than they are dissipating.

Long durations of light constant brake pressure tends to glaze the disk surface (just as engine idling will glaze cylinder walls) so it’s better to use short, moderate braking cycles (like the +10% -10% method described above)

-dave
 

icarus

Well-known member
Finally did it. I came down a mountain and I thought I was doing fine because I was in 1st gear (Winnebago over 2005 Sprinter chassis). All was well and the brakes worked OK, I thought. It was a very long drive, a couple of hours. I felt the wheels at the bottom of the mountain when I parked and checked the temperature with an IR gun and they were well over 480 degrees. I left the vehicle parked for about 15 minutes and then I tried to move it.

Ouch! No brakes and the pedal went all the way to the floor. I used the emergency brake to stop me, then left it parked for 30 minutes, the brakes pumped back up then and while they appeared to be working, although not perfect, I am afraid to drive the vehicle now. No mechanics down where I am so the task is going to be up to me.

I looked under the chassis and I didn't see anything had burst with fluid running out. I am wondering if I should perhaps get someone to pump the brakes for me while I work at each wheel to bleed the lines with the objective of replacing the fluid.

Ideas?
I’m curious to know how long the grade was and how steep? To come down and overheat (severely!) the brakes all while using 1st gear must have been very long, very steep.

Icarus
 

YoungRovers

New member
As someone already mentioned..brake fluid is hydroscopic, which means it will absorb moisture from the air, severely lowering your boiling point, brake fluid should be changed and flushed every 2 years and with your added weight I would recommend a very good quality of fluid like the ATE racing fluid dot4 and maybe going to ventilated cross drilled rotors. Your heat readings were somewhat affected by heat soak and residual temps rising, as mentioned, it’s best to continue driving at a slower rate to allow airflow to cool them..or if possible stop at a couple of locations on big downhill runs for 10-20 mins breaking the long decent up into a couple of sections.
 

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