View Full Version : Our Coach's Air Systems, Part 2

01-07-2017, 09:27 AM
Our Braking system is the fundamental reason our coaches are equipped with an air system. Clearly, braking is the most critical air system we have and one that cannot fail due to air system failures in our auxiliary air systems. Air braking systems are commonplace in the heavy truck industry. Therefore, Prevost didnít reinvent the wheel, thatís why our braking system is very similar to braking systems used in the heavy trucking industry. In part 1, I stated that the air supply system delivered air into the braking systemís main tanks through check valves at the inlet to the front and rear brake main tanks. Those check valves prevent air from escaping back into the supply system or the auxiliary systems. Once air is in the main brake tanks it can only flow downstream. I also stated that there is a protection valve between the air supply system and Prevostís auxiliary air system. The protection valve prevents air flow into the auxiliary air system until the braking system has been supplied enough air to function; typically around 75psi. Letís now look at the air flow and protection mechanisms as the air flows from the main brake tanks to the brake chambers.

Before we look at the air flow you should first understand some basic components in the air braking systems. Itís also important to know that our primary braking system is the rear and secondary is the front. Some literature refers to front and rear braking systems while others refer to primary and secondary braking systems.

Treadle Valve: This is attached to your brake pedal. Its sole function is to send braking control air to both the rear and front braking systems. It does not in any way mix air from the front and rear braking systems. Typically our coaches have a Bendix E-6 or E-10 treadle valve.
Relay Valves: In the braking system the primary functions of a relay valve is to allow low volume air to control the flow of high volume air and to ensure front and rear braking occurs almost simultaneously. To ensure maximum air is delivered to the brake chambers, these relays are located near the axle they control. Typically our coaches have Bendix R-12 relay valves.
Brake Chambers: These are axle mounted and located at each wheel location. They change air pressure to the mechanical movement of an arm that activates the associated brake.
Spring (Parking/Emergency) Brakes: These are the parking and emergency brakes. They activate drive axle brakes and possibly tag axle brakes. Unlike standard brake chambers that require air to activate the brakes, these require air to release the brakes. The spring brake chambers are typically piggy backed onto the standard brake chambers for the wheels that have spring brakes. The spring brake relay valve is typically a Bendix R-14.
Push-Pull Valve: This is attached to the yellow handled emergency/parking brake handle. It has two functions. The first we are all aware of is that of a parking brake that is engaged anytime we pull the yellow handle out. The other function is automatic. When both the front and rear brake system air pressure drops to a dangerous level, the yellow handle will rise and the spring brakes will be applied.

Letís use the above drawing to describe our braking system. Although not exactly the same as ours, itís 95+% the same and much easier to view.

The black lines represent the air supply system I described in part 1. The drawing shows a supply tank that in Prevost lingo is the Wet Tank.

The orange lines illustrate the front braking system related air lines, the green lines the rear braking system air lines and finally the yellow lines illustrate the spring brake system lines. Prevost air lines are color coded. Green lines are rear braking and supply, Red lines are front braking, Yellow lines are parking or spring brake lines, Blue lines are for suspension, and black lines are for auxiliary systems. Keep in mind that your converter may have added air lines that donít follow Prevost color coding.

Both the front and rear air system main tanks feed the treadle valve that is directly connected to the brake pedal. Although there is only one brake pedal and one treadle valve, the front and rear air lines are passed through the treadle valve isolated from each other. The front and rear air tanks also feed the front and rear relay or quick release valves. The reason for the direct feed to the relay or quick release valves is to ensure a high volume of air is readily available to the brake chambers

When the brake pedal is depressed the treadle valve opens proportional to the travel of the brake pedal. The further you press the brake pedal the more opened the treadle valveís two ports are that supply proportional brake control pressure to the relay or quick release valves. The relay or quick release valves open proportional to the supplied control air from the treadle valve. As the relay or quick release valve opens it passes air from the main tanks to the brake chambers through Anti-lock Brake System modulating valves. The brake chambers in turn mechanically move the brake shoes or brake pads. If the ABS sensors detect uneven wheel speed between wheels on the same axle, the ABS modulators will modulate or pulse the air going the slower moving wheel on that axle. Thatís really all there is to the front and rear braking systems with a couple of exceptions.

The front and rear main air tanks also supply air to the parking brake valve (the valve attached to the yellow handle we use to set the parking brake). The parking brake valve is responsible for suppling air to the spring brakes through the spring brake relay. When insufficient air pressure is supplied by the front and rear brake systems or when the parking brake valve is opened, there is not enough air supplied to the spring brake relay to keep the spring brake chambers from activating the parking/emergency brakes. Although not clear in this illustration, the front and rear air lines going to the parking brake valve do so through a double check valve. The double check valveís purpose is to supply the parking brake valve with the highest pressure available from either the front or rear air brake supplies and to maintain isolation between the front and rear braking systems.

The last thing a driver wants to happen is for their parking/emergency brakes to activate while moving. If this occurs, there is nothing a driver can do except try to get out of traffic before the coach comes to a stop. The spring brake relay is responsible for ultimately activating the spring brake The spring brake relay is also supplied air directly front the front main tank and from both the front and rear brake relays when they supply braking air to the front and rear service brakes. If service brake control pressure is present at the spring brake relay from the use of the brake pedal then the spring brake will release favoring service brake operation over spring brake operation. Most brake system pressure failures are gradual. A driver should have a warning light and sound alert when the air pressure in either front or rear braking system falls to a dangerous level. These alerts are typically triggered at around 65psi. At 65 psi you have a serious leak, lack or supply air, or have not used your brake correctly. Your first action when these alerts trigger should be to find a way to safely move out of the way of traffic. If your front and rear brake systems fall to around somewhere around 35 psi your emergency brakes are going to engage.

One major difference our air brake system has that is not illustrated is brake operation with the tag axle in the raised position. The tag axle service air brake line has an air control relay in its path that is controlled by an inversion valve. When the tag axle is raised the same control air line that activates the tag lift chambers is fed to the inversion valve. When this occurs the inversion valve doesnít allow the flow of air to the service brake for the tag axle brake chambers even when the service brakes are applied. If the tag axle didnít have this feature the tag axle tires would lock up and in almost every case scuff the locked tire when the raised tag tire makes contact with the road.

Bendix Corporation is the leader in air brake system components and the supplier to Prevost. The drawing and descriptions in this document came from public documents from the Bendix Corporation. You are encouraged to visit Bendix Corporation (CLICK HERE) (http://www.bendix.com/en/) to read firsthand about the Bendix products installed on your coach. Bendix recently launched a new site (CLICK HERE) (http://http://safertrucks.com/) to provide training material to the trucking industry which you may find useful. Finally, you may consider attending Bendixí on-line air brake school (CLICK HERE) (http://www.bendix.com/en/servicessupport/brakeschool/brakeschool_2.jsp).

You may also want to download the air drawing for your coach. You can find your coach's pneumatic drawing (CLICK HERE) (https://techpub.prevostcar.com/en/pneumatic-diagrams). These drawings are a bit hard to read, but they have the resolution to be printed on a poster scale to make them easier to follow. I find it best to have both the air drawing and the parts manual. Some details are a bit hard to see on the drawing. The check valves at the main air tank inlets is one example.

The next part in this series will discuss the auxiliary air system.

dale farley
01-07-2017, 10:19 AM
More great info Gil. Thanks

02-08-2018, 09:23 PM
This was SO VERY Helpful. Thanks Gil

02-09-2018, 08:08 AM
Good info Gil. Side note on the air gauges on the dash. I had the top gauge on My dash that would go down overnight considerably compared to the other. It was intermittent and has been fixed. I feel like it is relevant to Gilís description of how the brakes work. After getting some understanding of the process , the only way a driver has to monitor his air brake health is by these air gauges on the dash. If I have a failure on one of the systems , I would like to know which one is failing and which one is operable. How many of You know which gauge is for PRIMARY and which is for SECONDARY on Your coach ? Pretty important Iíd say. They are not marked at all, Just two gauges one over top of the other. These gauges were placed by Prevost when the shell was manufactured. This is not a converter item. I thought they should have been placed by matter of importance with the top being primary and the bottom secondary. Makes sense right ? Not so on My coach. The top is the secondary or front brakes, and the bottom is primary or back brakes. Are all prevost dashís setup this way, How were Yourís placed ?:confused::confused::confused:

02-11-2018, 09:15 PM
Gil, can I please stay with you and work on my Prevost internship?

02-11-2018, 09:24 PM

I'll have to see when the gauges actually got labeled. Here's another question. Which braking system is the primary?


Too funny...

02-11-2018, 09:32 PM

You might have been ripped off. Here's a dash picture of a factory dash on a XL. Although it may be a different year, the air gauges are labelled PRI and SEC.

Joe Camper
02-12-2018, 07:20 AM
I'm of a different opinion. What difference does it make if u aren't aware of what gauge is primary or secondary. If rolling along and 1 drops off regardless of what one drops, knowing the 2 isint going to help or change anything, u will be pulling over. IMHO there is a point for the normal owner when the info u need to know gets beyond someone.

Know how to do your pretrip air tests drive easily and have a good mechanic.

It's been my experiance most of the mechanics that I encounter don't know what Gill is passing on that's what scares me. If as owners this is overwhelming, it's ok that would be understandable.

02-12-2018, 09:52 AM
Joe's point is spot on. A problem in the primary OR secondary air braking system needs immediate attention. I'm sure those on this forum realize the secondary system is not a back-up to the primary system, but not all do. I personally don'like the primary and secondary labels as they are both critical. IMO, Front and Rear are better labels, which is how Prevost labelled gauges on my 03 chassis.


02-12-2018, 10:38 AM

I'll have to see when the gauges actually got labeled. Here's another question. Which braking system is the primary?


Too funny...
Gil, Primary is Rear and secondary is front.

02-12-2018, 03:30 PM
No Markings on Mine , Yep Ripped off ! I got the cheap version of the Prevost I guess ? :confused:14825
Naturally if there is a problem with either system it will be time to stop and repair like Joe said. But when I had one gauge leaking down much faster than the other. At least I will know which system to look at or have looked at.

02-12-2018, 05:15 PM

You don't need any markings, just wait to see if the drive axle tires smoke. If so, it's the primary/rear brakes that lost pressure.

Joe Camper
02-13-2018, 06:13 AM
When you pump the brakes down or if you use the brakes hard the gauge that's dropping quicker is the rear brakes because it's feeding two axles, not just 1,and with bigger chambers than the steer axle too. That's the easy way to decipher which gauge is which.