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Old 08-13-2013, 06:54 PM   #1
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Brakes losing power under 4x4 Low Gear.

Today in Telluride we were heading up to Bridal Veil Falls on the narrow 4x4 Black Bear Pass Road. The turns are quite sharp and I had to go into reverse to make them. After several turns I noticed that after applying the brakes and stopping they went "dead", the brake peddle having lost its' "brake pressure", and I had to hold the brake down hard to get the van to stop. Needless to say we turned around and went back down.

On level ground I was able to replicate the break deadness after stopping... Continuing to have to push the brake pedal very hard to get the van to stop but after accelerating for a few seconds the van seem to shift gears and the brakes then function normally until I stopped again.

They seem to function normally when I'm in 2 wheel drive. Any thoughts?

Robert

Also I noticed my Atlas transfer case was leaking fluid but I can't imagine this is related?

THANK YOU in advance!
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Old 08-13-2013, 07:11 PM   #2
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Re: Brakes losing power under 4x4 Low Gear.

Can you elaborate on your symptoms? I'm not quite understanding. You say you lost pressure and had to press down hard. Did the brake pedal become harder to press? Or did you have to push it further but without more effort?

My first hunch is you are overheating the brakes and boiling the fluid. Are you holding the brakes while you travel in 4x4 low? There really isn't anything in the braking system connected to the Transfer case, but a large engine in low range has more than enough torque to over-power the brakes.
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Old 08-13-2013, 08:30 PM   #3
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Re: Brakes losing power under 4x4 Low Gear.

Do you have vacuum actuated hubs? If so, you probably have a vacuum leak somewhere in the lines to the hubs, or the hubs themselves.

In short, we need more information about your van:

- gas or diesel
- type of 4wd system installed (SMB, QuadVan, Quigley, etc)
- vintage
- does the problem also occur in 4wd Hi

Mike
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Old 08-13-2013, 08:31 PM   #4
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Re: Brakes losing power under 4x4 Low Gear.

Normally when I press the brakes down, the break moves easily, and the van comes to a quick stop.

The issue I'm talking about here happened after very little driving. I pushed the brakes down and the van stopped. But then the brake would not come back up and I could only push it hard into the floor to keep the van from rolling.

This did not happen when I was in 2 Wheel Dr. Every time I came to a stop. The brake would come up easily and I could push it down and stop appropriately.

I was able to replicate the problem on a flat surface while in 4 x 4 low. However if I accelerated the van seemed to shift and then the brake would function normally again. Obviously, I could not do this while on a steep dirt road making a sharp turn.

Does that help?

Robert
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Old 08-13-2013, 08:34 PM   #5
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Re: Brakes losing power under 4x4 Low Gear.

low revs and a bad vacuum pump?
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Old 08-13-2013, 08:45 PM   #6
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Re: Brakes losing power under 4x4 Low Gear.

Sounds like a Sportsmobile 4wd w/Atlas T/C.

Does SMB 4x4 have vacuum hubs? I don't know.

I also don't know if the vacuum pump is engine driven or electrical on your van, but I thought the 6.0's had an electrical pump. And I can't imagine why engine rpm would affect vacuum loss, but I can see how it could change vacuum if the pump is engine driven.

Try idling the engine in 4wd and push and release the brake until the brake pedal won't come back up. Repeat the test in 2wd and see if you get the same result with about the same number of push/release cycles.

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Old 08-13-2013, 09:01 PM   #7
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Re: Brakes losing power under 4x4 Low Gear.

It does sound like a loss of vacuum. I can't see how it could be connected to use of 4x4 on the SMB setup. Might be time to hook up a Voltage meter to the vacuum pump, and make sure it isn't losing power for some reason (only think I can think of is mis-wiring of the 4x4-low light).

Next time this happens, turn your heater control to VENT or A/C, then monitor it so see if the air starts blowing up to the windshield when you lose your power brakes. This would be a sure indicator of a vacuum leak somewhere.

Is it possible that you just happened to apply the brakes more often then normal while you were it 4x4-low? Just because it happened at the same time doesn't mean it has anything to do with using low range.
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Old 08-13-2013, 09:08 PM   #8
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Re: Brakes losing power under 4x4 Low Gear.

Or maybe something with the torque converter clutch which continues to pull the van when idling, applying mor torque when in slow gears?
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Old 08-13-2013, 09:37 PM   #9
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Re: Brakes losing power under 4x4 Low Gear.

Your problem sounds somewhat similar to what I experience in 4wd low as well. Except for me, pedal will go all the way to the floor and at times will not even hold the van from creeping forward. It usually becomes an issue on tight hairpin turns (like the last one going to Laurel Lake). I've often thought it is vacuum related and caused by the low RPM combined with additional torque of being in 4wd low.

Sorry, nothing to offer as far as a solution, but I'll follow this to see if you ever find your issue.

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Old 08-13-2013, 10:50 PM   #10
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Re: Brakes losing power under 4x4 Low Gear.

First off vacuum is highest at low RPM on a gasoline engine and a diesel engine has no vacuum at any RPM, thus the need for a vacuum pump.

Second, the hydraulic brake system is closed system unless it is leaking, brake fluid does not compress so a soft pedal can be caused by air in the system, failing Master Cylinder that is leaking internally, moisture in the system and failing brake hoses to name a few.

Third, the purpose of the vacuum boost is to assist you in compressing the Master Cylinder, which transfers brake fluid to the calipers. Losing vacuum boost will not cause the pedal to go to the floor, it will make it harder to push it to the floor.

So, since the brakes seem to be functioning normally 99% of the time, I'm guessing everything is OK with the hydraulic system and the brake assist system - so what could it be?

One of you provided a clue when you said it happened "on tight hairpin turns". I'm guessing that when crawling along in 4Low these turns are tighter and held longer than would normally be held at higher speeds and on the roads.

Two things I have seen before that cause problems like this are loose wheel bearings and caliper interference with other components.

Since these are what are called floating calipers, they can be pushed away from the rotor when the wheel bearings are loose.

More likely though in these cases it is the caliper being pushed away from the rotor because the caliper is contacting something at full steering lock and under suspension movement not experienced at higher speeds.

Without seeing the vans and testing them - this is just a wild guess - but a good one.

You might also want to check the soft brake lines and see if they are being stretched or pinched under some conditions.
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