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jillie · 08-12-2012, 06:58 PM

I'm studying hard for this Non-commercial Class B test. Reading the CDL manual. I came across this caution: Never push the brake pedal down when the spring brakes are on. If you do, the brakes could be damaged by the combined forces of the springs and the air pressure.

So in what order do you do these when starting your engine?
Release spring brake?
Press on brake pedal?
Put it in gear?

Perry White · 08-12-2012, 07:04 PM

What do you need a Noncommercial CDL for?

jillie · 08-12-2012, 07:17 PM

Quote:

Originally Posted by Perry White

What do you need a Noncommercial CDL for?

It's a Non-commercial Class B. I need it because I live in PA and our motorhome weighs more than 26,000 pounds. Since the coach has air brakes, I also have to do the air brake test. The only manual available from the state to study is the CDL manual.

targaboat · 08-12-2012, 07:21 PM

It is the law in Texas.

You have the order correct. Release park brake and then push on the service brake. Do not matter when you shift the transmission.

Rotten-Red · 08-12-2012, 08:25 PM

The question was! What order would you do these things before starting engine. Why would you do any of the three. I would start the engine with the parking (spring) brake set. Then when ready to move release parking brake, apply service brake, put in gear.

Perry White · 08-13-2012, 08:33 AM

You know anybody that don't have there foot on the brake pedal when they push the emergency brake knob? Didn't think so. Maybe I"m wrong but don't you have to have air to release the brakes? Am I backwords?

jillie · 08-13-2012, 06:51 PM

Yeah, more confusing stuff from the CDL manual. I always put my foot on the brake before releasing the spring brake but that caution concerned me. Thought I'd check with the experts here who've been around longer than me.

Foxhaven · 08-13-2012, 07:00 PM

Forget everything but what what the book says and the answer that they want...

Mr_D · 08-13-2012, 07:17 PM

Quote:

Originally Posted by jillie

I'm studying hard for this Non-commercial Class B test. Reading the CDL manual. I came across this caution: Never push the brake pedal down when the spring brakes are on. If you do, the brakes could be damaged by the combined forces of the springs and the air pressure.

So in what order do you do these when starting your engine?
Release spring brake?
Press on brake pedal?
Put it in gear?

That "advice" is absolutely wrong for modern air brakes (since at least 2000). The modern units use a double acting diaphragm and can't be hurt by full pressure of the service brakes.
But to pass the exam you need to answer the way they want!

Stillwater · 08-13-2012, 07:19 PM

The OP's question was about combining forces. Nothing else, not about why you need the air brake certification or anything else.
What happens when the spring brakes are applied, the air that was keeping them released is exhausted and the springs apply a manual spring tension on the brakes. It is in the range of 35-45# psi. If the spring/park brake is applied and you apply air pressure by way of the foot brake pedal then whatever pressure you apply with your foot is "compounded/combined". So if you apply full tank pressure of 130# and the spring brake is on you now have 35-45psi plus 130psi so therefore you now have 175# psi applied to the brake system which could lead to bent parts.
You would not normally apply that much foot pressure so if you only apply enough foot pedal pressure to keep the coach from rolling you would only have maybe 40#psi therefore you would only have maybe 90-95# combined pressure. It is good to be aware of the compounding/combing issue just be aware that you should not apply full foot pedal pressure until the park brake is fully released. BTW it is an issue to be aware of but I've been dealing with air equipped vehicles and machinery for 40 years and never seen any damage attributable to compounding. I always just lightly apply the foot pedal pressure and release the spring/park brake. Hope that clears up your question I know it's long winded!

Lug_Nut · 08-13-2012, 08:30 PM

Most, if not all, air brake equipped vehicles employ an anti-compounding valve to prevent this damage from happening. You therefore can apply service back pressure while the spring brakes are engaged. I always use lighter pressure than normal for this task. The reason this is still taught is the fact that you must be able to operate any vehicle year, including ones that may be old and may not be so equipped.
Never the less, it is still a good practice to not apply heavy pressure to the service brake while the park brake is engaged.

Peter

Route 66 · 08-14-2012, 05:17 AM

Quote:

That "advice" is absolutely wrong for modern air brakes (since at least 2000). The modern units use a double acting diaphragm and can't be hurt by full pressure of the service brakes.
But to pass the exam you need to answer the way they want!

If it was a problem, there would be countless MHs with brake problems and there is not, as most owners know very little about air brakes.

I don't ever recall hearing of a problem.

Skip426 · 08-14-2012, 08:30 AM

Too bad the clown writing the manual , and the test questions for that matter; couldn't state " Apply service brake lightly, while releasing park brake". A lot of difference, between a 10% application; to keep the coach from rolling, and a 75>100% application that would damage a system without anti-compounding features.