Emergency Air Up

[bevans6]

Emergency brakes come on at 25psi. 65psi is when the low air pressure warning comes on. 100psi is the recommended minimum for moving but... if you're over 25psi, as long as you don't use air, you should be able to move.


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No, to put it short form.  Part of this is correct.  A typical DD3 system will auto-apply the parking brake when the air pressure in the emergency/parking brake tank falls to around 25 psi - the pressure setting in the push-pull valve spring.  A spring brake will auto apply when the air pressure falls to around 65 psi, or the pressure where the spring overcomes the force of air on the diaphragm in the brake canister.  To release a spring brake you need to build air pressure over the 65 psi level to force the spring to move to take it's pressure off the push rod.

DD3 are a different beast from spring brakes.  Typically the parking brake diaphragm is fed through the inversion valve and through an 85 psi air regulator which is supplied from the emergency/parking brake tank.  As well, the parking brake diaphragm is smaller than the service diaphragm so it develops less push for a given air pressure.  If a bus is parked normally and the parking brake is set, it's set with 85 psi of air pressure in the parking brake section of the canister.  At the same time, the locking clutch on the pushrod locks.  To release the parking brake, the first thing is the push pull valve is operated and the inversion valve removes air pressure from the parking brake section of the DD3, and applies air pressure to the locking port to release the push rod locking clutch.  Normally that will not release the parking brake.  To release the brake, the second step is a service brake application of greater force than the parking brake force needs to be applied to move the push rod out slightly farther than it was locked at, so the locking clutch can actually release the push rod.  Only then is the parking brake actually released.  Typically this requires around a 90 psi application of the service brakes, so you need at least 90 psi in the air system to release the brakes.

[TomC]

135psi is a bit high for your brake system. 120psi is more like. Believe it or not, those extra 15psi is extremely hard on the compressor. But-you choose. Good Luck, TomC

[gg04]

We carry a bottle of dry nitrogen, argon, and co2.  For tig ,tires and tools.  Most dangerous is still propane, more deaths and property damage worldwide.  Best accelerant and enhancer . rdw

[BlueScarecrow]

Thanks all for replying. I did look into the legal transport of a high pressure tanks and you can transport a few... I think it's 4 scuba tanks simultaneously. Beyond that I think it's illegal. Anyway I would only have a single tank mounted in the center of one of the bays to avoid any possibly of a mishap during an accident. When I was plugged into the wall at the service station, they had two huge air compressors in the back and the hose itself was larger than normal. I'm thinking about a 60 CF Steel cylinder permanently attached. If I fill it with helium, would my bus become lighter on air-up? 

[kyle4501]

A small 120V air compressor and a bendix DV2 drain valve should be able to keep the air pressure at the right levels without the risk of compromising any parts of the system.
Depending on where you tie in to the system, you may need to set the pressure below the coach Bendix d2 governor so it doesn't vent thru the air dryer exhaust.

On a spring applied park brake, 30 psi may start to release the brake, but it takes 60 psi to fully retract the brakes.
Then, if you have an air throttle, you will need air for that . . . .

My 80 gallon shop air compressor at 150 psi can air up the coach (from 0 psi) to 120 psi in around 5 minutes (If it is at 150 when I start)

[Scott & Heather]

In all reality we used a pancake compressor for years to keep our first bus aired up in rv parks. Worked perfect. Nothing needs to be exotic here. Simple it probably better. But I'm all for new ways.


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[belfert]

I like the idea of a small compressor that always keeps the bus aired up versus a high pressure canister.  My bus will hold air for a long time in the summer, but not long in the winter.

[buswarrior]

Instead of work-arounds...

How about make your air system tight?

The new stuff is start and go. For the last 20 years, no leaks overnight.

Our shutters, belt tensioners, air wipers, can all be isolated from the "brake tanks".

By design, some older models already are.

You might be quite surprised to see your own bus release the parking brake and go, with the air gauge very low.

The "brake tanks" can't be seen on the dash in many older models, the gauge is plumbed into the auxiliary.

Catch up the preventive maintenance, check valves and pressure protection valves.

happy coaching!
buswarrior