Air over Hydraulic versus Full Air Brakes

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btizzle84

btizzle84

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Central Illinois
I know air over hydraulic braking has been around for a long time, used a lot in military applications, etc. However, how does the RV owner community feel about them - from a user perspective, maintenance perspective, etc. ? I dont know how much I like all the moving parts involved, versus standard air brakes (pretty simple set up if you really compare/contrast). I am going to be doing cross country trips probably twice a year, at least once, and regional trips consistently. Some manufacturers i'm looking at have air over hydraulic on their big coaches  (ie. 38-40'), and I just dont know how I feel about them being responsible for all that weight  :eek:. Thanks in advance for your valuable feedback!
 
I cannot speak to air over hydraulic from an engineering perspective at all, but can from a consumer perspective, and having been in the hydraulic brake business in a previous life.

Having owned two air brake coaches in the last three years, the simplicity of use and ease of maintenance has to be a plus. They are pretty bulletproof and you have to look at that as why they are so widely used in this application. The complexity of hydraulics on such a large frame would seem to invite a lot of maintenance and make them expensive, both in money and possibly weight.

I feel sure there is a value proposition to the air over hydraulic system, but it may be lost in the general RV usage world. I guess my question is: if they are the next best thing to sliced bread, why don't all coaches have them?  8)

A totally unscientific answer to your question.
 
We have air-over-hydraulic brakes in our 40', 32,000 lb MH and they stop the vehicle just fine even in panic situations.  I have the complete service history for the vehicle and the brakes have required some maintenance but not an excessive amount.  There were some issues in the mid-2000's relating to the Bosch brakes but those appear to  have been resolved.

As for the use of the brakes on long descents, we have a 2-stage compression brake (Jake brake) and between it and manual control of the transmission I can pretty much take most descents with little or no use of the service brakes.  No matter what braking system you have, the preferred approach is to use engine/exhaust braking to take the load off of it during long downhill runs.

If you have any specific questions about the brakes, feel free to ask.
 
Air over hydraulic was a feature of the Peak chassis (Western RV), and I think the Bluebird (Wanderlodge) chassis. Beaver had them at one time too. Basically it is a hydraulic brake with air power assist instead of the vacuum assist used in cars. The primary advantage of air-only is that air at the brake shoe/caliper is not subject to boiling if overheated, as hydraulic brake fluid is. That's a possible safety issue when braking heavy loads on steep grades, but it's also largely an antique issue. Modern brake systems, assisted by engine or jake brakes, rarely have the overheating issues that were common 25+ years ago. Still, air-only has an extra measure of reliability in that regard.

But air isn't really any simpler. You have numerous air lines that can leak, an air reserve tank, and an onboard compressor to supply it all. And air driven drums or calipers have pretty much the same wear and maintenance as hydraulic ones.

I wouldn't be put off by a good motorhome that has air-over-hydraulic brakes, even if I might prefer air-only.
 
Gary RV Roamer said:
Air over hydraulic was a feature of the Peak chassis (Western RV), and I think the Bluebird (Wanderlodge) chassis. Beaver had them at one time too. Basically it is a hydraulic brake with air power assist instead of the vacuum assist used in cars. The primary advantage of air-only is that air at the brake shoe/caliper is not subject to boiling if overheated, as hydraulic brake fluid is. That's a possible safety issue when braking heavy loads on steep grades, but it's also largely an antique issue. Modern brake systems, assisted by engine or jake brakes, rarely have the overheating issues that were common 25+ years ago. Still, air-only has an extra measure of reliability in that regard.

But air isn't really any simpler. You have numerous air lines that can leak, an air reserve tank, and an onboard compressor to supply it all. And air driven drums or calipers have pretty much the same wear and maintenance as hydraulic ones.

I wouldn't be put off by a good motorhome that has air-over-hydraulic brakes, even if I might prefer air-only.
Click to expand...

Bingo, there is the answer.  Brake fluid is still subject to boiling and must be changed out periodically due to being hygroscopic, meaning it absorbs and retains hold of water.  Studies have shown the the average hydraulic brake system can absorb 1 to 2% of water per year, effectively lowering the boiling point.

Mike.
 
It's a bit more basic than that, I think. Hydraulic fluid, being a liquid, can boil into a gas and put bubbles in the lines that are readily compressible. Air is already a gas, so it can't "boil" and the system is made to use a compressible gas. Besides, hot air works as well as cold air in an air brake system.

Both air and hydraulic fluid can gain water and it is detrimental to good brake perfromance. Air systems are very prone to water retention, and modern ones have air dryers built in to compensate. They also have air tank drains, and older air systems required daily draining of the water from the air tanks.
 
Gary RV Roamer said:
Air over hydraulic was a feature of the Peak chassis (Western RV), and I think the Bluebird (Wanderlodge) chassis. <<<<<Snip>>>>>
Click to expand...

I can't speak for very early the Blue Bird Wanderlodges but since at least the late 1970's all Wanderlodges have a robust commercial air brake system, not air over hydraulic.
 
Upon reflection, I realize you are correct. It was the Magnum chassis (Safari and Beaver) that i was thinking of, not Bluebird Wanderlodge.

Many school buses have either plain hydraulic or air over hydraulic systems as well.
 
SMC was inconsistent when it built the Magnum chassis.  In the 2000 timeframe the Beaver Patriot Thunder had air-over-hydraulic on the Magnum "C" chassis, but the top of the line Safari Panther had full hydraulic brakes on the same chassis.  Can't imagine why they would design in two different systems.
 
I know this is an old thread - but i just came across it searching for info on Air-over-Hydraulic brakes.

Since some of you have/had this style braking system, can anyone tell me the manufacture and model number/name used for the "Air-over-Hydraulic" brakes on the 1996-2000 SMC and Beaver coaches with the Magnum B-series (Magnum Air) chassis?

More specially the Master Cylinder Booster setup. I'd like to locate a service manual and parts list. But first I need to know who made the system.

BTW, this is not used on my coach (so I cant just go out to coach and look for myself).

Thanks
 
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