'06 2500 Ls 300K: when pulling our camper (about 7500lbs) the truck losses its brake pedal. After a shot amount of miles, the pedal gets softer and softer, until there is little if any left. When it sets overnight, the pedal is hard again. I changed the fluid to DOT 4 and bled at all four wheels. No better. Is this a symptom of a bad master cylinder? If so, why does it loss the pedal only when towing. I have the trailer brakes set at a fairly light setting. Thanks -
Loss of brakes when towing
- Thread starter revsdiesel
- Start date
Braking systems depend on having a hydraulic fluid that does not compress and when brake fluid absorbs moisture, it does compress. When it compresses, little braking pressure is exerted on the brake discs/shoes. The brakes are said to fail.
Brake fluid that has absorbed moisture boils that moisture at a much lower temperature that fresh fluid that has no moisture in it. When your braking system heats up, particularly when towing, the moisture contained within the fluid vaporizes, and you have a compressible gas in your braking system. And your brakes initially feel quite spongy, then they just don't work. I have personal experience with this pulling a little 5th wheel trailer to a camp site near Mt St Helens, having my brakes 'boil' and loosing all braking from the truck. Fortunately, the trailer brakes were enough, but barely, to allow me to stop.
When you used DOT 4 fluid, you may have caused the problem to be somewhat worse than it was originally as DOT 4 fluid absorbs more moisture that DOT 3 fluid, given same operating conditions. While new DOT 4 fluid has a higher boiling point than new DOT 3 fluid, the downside is that you must completely change your DOT4 fluid more often than DOT 3 to eliminate the moisture from your braking system. Perhaps a 2 year change cycle would be appropriate for DOT 4 fluid.
Personally, I use only DOT 3 fluid, and at every oil change, I suction out the master cylinder and replace it with new, dry, brake fluid. Only takes a couple of minutes and helps to keep the moisture level low in the braking system.
Brake fluid that has absorbed moisture boils that moisture at a much lower temperature that fresh fluid that has no moisture in it. When your braking system heats up, particularly when towing, the moisture contained within the fluid vaporizes, and you have a compressible gas in your braking system. And your brakes initially feel quite spongy, then they just don't work. I have personal experience with this pulling a little 5th wheel trailer to a camp site near Mt St Helens, having my brakes 'boil' and loosing all braking from the truck. Fortunately, the trailer brakes were enough, but barely, to allow me to stop.
When you used DOT 4 fluid, you may have caused the problem to be somewhat worse than it was originally as DOT 4 fluid absorbs more moisture that DOT 3 fluid, given same operating conditions. While new DOT 4 fluid has a higher boiling point than new DOT 3 fluid, the downside is that you must completely change your DOT4 fluid more often than DOT 3 to eliminate the moisture from your braking system. Perhaps a 2 year change cycle would be appropriate for DOT 4 fluid.
Personally, I use only DOT 3 fluid, and at every oil change, I suction out the master cylinder and replace it with new, dry, brake fluid. Only takes a couple of minutes and helps to keep the moisture level low in the braking system.
Thanks - a few questions.
What is the hydroboost?
How in the world does new fluid collect moisture the day after its changed?
And I will check the pulley, however, what's the connection to the brakes fading away when pulling the trailer?
Thanks so much for the input! kip
What is the hydroboost?
How in the world does new fluid collect moisture the day after its changed?
And I will check the pulley, however, what's the connection to the brakes fading away when pulling the trailer?
Thanks so much for the input! kip
Hydroboost takes the place of vacuum booster on old gassers. Its tied into your power steering, so have you ever flush your power steering pump?
OK, I've googled and have a better understanding of some of this... still don't understand how moisture get into the fluid. And of course why this happens when pulling a trailer. And everything's ok without it.
Nope... haven't flushed the power steering pump. Something tells me this will not be as easy as it sounds.
Power steering pump pressurizes the hydro boost, if the pump pulley isn't spinning the power steering pump properly you loose braking power. The pulley is known to spin off or wobble before the pump itself goes out.
Checked the pulley with engine off and on, all seems fine. No visible leaks anywhere...
Is the engine tempt and the ambient air tempt under the hood when towing (also used the tow mode when this happened twice) what causes the fluid to boil, if boiling is the cause?
I would assume it is excessive use of brakes, but in both cases, it was mostly open road. And again, no real issues when not towing. thanks
When new fluid was put in when changed, there was no mention of the hydrobooster being touched (flushed or bled - if need be).
Is the engine tempt and the ambient air tempt under the hood when towing (also used the tow mode when this happened twice) what causes the fluid to boil, if boiling is the cause?
I would assume it is excessive use of brakes, but in both cases, it was mostly open road. And again, no real issues when not towing. thanks
When new fluid was put in when changed, there was no mention of the hydrobooster being touched (flushed or bled - if need be).
Take off the rubber line that goes into your hydroboost. Right behind the brake fluid reservoir. Attach a length of hose (I use a clear one) and run it to a bucket or container.
Now start the truck and pump the brakes. Turn the wheel lock to lock in the meantime also. Make sure to keep adding fluid to the powersteering reservoir. Its nice to have two people to do this part so one can fill. Continue until the fluid coming out of your hose is clean.
Should be done regularly. That fluid is going to be nasty if it hasn't been done.
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Now start the truck and pump the brakes. Turn the wheel lock to lock in the meantime also. Make sure to keep adding fluid to the powersteering reservoir. Its nice to have two people to do this part so one can fill. Continue until the fluid coming out of your hose is clean.
Should be done regularly. That fluid is going to be nasty if it hasn't been done.
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Here you go, new guy.
http://www.dieselplace.com/forum/71.../353950-power-steering-hydro-boost-flush.html
Its not difficult to do.
I think your master is going out myself, but do a brake fluid bleed first.
Before you start replacing parts hoping to discover the real problem/solution, buy one of these:
http://www.amazon.com/Brake-Fluid-Tester-calibrated-fluids/dp/B005HVG4GQ
Use it to see how much moisture is in your brake fluid. If it is 3% or more, you need to replace the brake fluid. If it's 1%, look elsewhere. IIRC, you live in Florida where the average humidity is pretty high and that makes the likleyhood of water presence much more pronounced than, say, for a the same truck/trailer in Arizona where the ambient humidity is 15% or so.
Look here for explanation about brake fluid and its characteristics:
https://en.wikipedia.org/wiki/Brake_fluid
Since you only experience the problem when towing, you might want to set your trailer brakes to carry more of the load and see if that helps. And you should also check your trailer brakes to make sure they are working as well as you THINK they are. Trailer brakes are notorious for poor performance.
http://www.amazon.com/Brake-Fluid-Tester-calibrated-fluids/dp/B005HVG4GQ
Use it to see how much moisture is in your brake fluid. If it is 3% or more, you need to replace the brake fluid. If it's 1%, look elsewhere. IIRC, you live in Florida where the average humidity is pretty high and that makes the likleyhood of water presence much more pronounced than, say, for a the same truck/trailer in Arizona where the ambient humidity is 15% or so.
Look here for explanation about brake fluid and its characteristics:
https://en.wikipedia.org/wiki/Brake_fluid
Since you only experience the problem when towing, you might want to set your trailer brakes to carry more of the load and see if that helps. And you should also check your trailer brakes to make sure they are working as well as you THINK they are. Trailer brakes are notorious for poor performance.