Air brakes question

[JenR]

Good morning from Oregon!* I just purchased a Skoolie. Its a 2001 international dt466.* I have a question about air brakes.

I notice when I release the emergency brake it doesn't discharge very much at all. Also, the warning paddle (don't know the technical term) does not stay up when air pressure is built up to safe levels. The gauge reads 110 and the bus will move.*
When I apply the brakes I get nothing, nothing, then they grab hard. Also, its work to get the bus up to speed and it won't coast, like maybe the emergency brake is still applied?* When I turn the bus off, the air gauge reduces way too quickly.
I also noticed that I will occasionally hear the discharge sound when driving and not applying brakes.

Any help would be greatly appreciated

[Johnny Mullet]

Sounds like you have a pretty bad air leak so I would start there. Chock the wheels, start engine, build air. Release the brakes and shut engine off and find that leak.

[o1marc]

The compressor will build and then release when it's full during your driving. I think anything above 60psi will release the brakes to drive and build to around 120-130psi

[JenR]

Thanks! That's where I'll start. Got a friend coming over in a while to look through it with me

[Brad_SwiftFur]

Please don't take this the wrong way, but it sounds like you have a lot you should learn about air brakes.


There's plenty of excellent Youtube videos covering this but I'll try for a quick text version.


When you first start the engine (and let's assume the air pressure is zero) it should begin building air pressure. You'll have a "Wet", "Primary", and "Secondary" air tanks (sometimes combined in one tank, with two or more compartments). Many systems will also have an air dryer. So the air leaves the compressor, through the air dryer (if equipped) and into the "wet" tank. The primary and secondary tanks are supplied from that. Many vehicles have a "protection valve" which will not open until around 60 PSI, this supplies most of the vehicle, air suspension, air seat, door, stop sign and bar, etc. The brakes *ALWAYS* get full pressure, no matter how low it is (thus, not on the "protected" portion).


Once the system reaches 110-120 PSI, the air compressor "governor" will disengage ("cut-out")and most systems it will "purge" (a quick hiss of air like you described), completely normal and is intended to purge built up moisture from the system. As you drive and the air pressure drops, typically around 90 PSI or so, the compressor will "cut in" and resume building pressure. Again, once it cuts out, it will purge again.


Now that you've let the engine idle and have full pressure, you'll have a knob which is usually pushed in to release the parking brake (some *OLD* vehicles had knobs which had to be pulled out; it was standardized to a push-in style somewhere in the 70's). Air will be supplied to the parking brake chambers and your air pressure will drop a bit but this is not supposed to be a constant flow - if it is, you have a problem (likely causes are a blown diaphragm in the brake chambers or a failed air line.) What this does - inside each rear brake chamber is a huge spring which is under a *LOT* of pressure - this is your parking brake (it's also why you *NEVER* disassemble the parking side of these chambers! Most are sealed these days but older ones could be disassembled). So, supplying air to this side pushes a diaphragm against the spring, compressing it and releasing the parking brake. Parking brakes are generally forbidden on steering axles, for what should be obvious reasons.


So now you have air pressure and released parking brakes and you begin driving (we will assume you've done proper pretrips and found no problems). You found that the foot brake (foot valve, treadle valve, it goes by more than one name), it's basically a foot-controlled air pressure regulator. They typically have very little travel, relying on application pressure to regulate how hard you stop. The harder you push, the more air it supplies. When you release, it should release all the air (and there are often relay valves to augment this.)


It sounds to me like you are getting good air pressure with the bus running. You have 2 air systems, gauges for both, make sure both are full pressure. So far, so good? Release the parking brake, see if the bus will roll relatively freely (it should). If not, listen for air leaks (As noted above, you will want to chock the wheels, or better yet, have a helper for this, you do *NOT* want that bus to move uncontrolled!) At this point there are two likely things - if you hear an air leak, you likely have a parking brake that isn't releasing fully - either fix the air line, or replace the brake chamber, as appropriate. No air leaks? The brakes may be dragging (adjusted too tightly), they will need backed off on the adjustment (less likely but you never know....) Either that, or ...



As for pushing the pedal, nothing, nothing, then grabbing, I had a truck that did this. The foot valve was on the floor (common with cabover trucks and flat-front buses) and they get a lot of dirt and crud in them, they begin to stick. And in your case, I wouldn't be surprised if it wasn't releasing fully either. That's my suggestion, check the foot valve. Another member here had similar issues and that's what his problem ended up being.


Oh, and air loss after shutdown? Mine does this as well, I need to replace my suspension leveling valve (fairly common wear-and-tear item and they're not expensive).

[WIbluebird]

Air loss after shutdown can be really weird. We have brand new buses that lose all their air after parking overnight and then we have 6 yr old buses that hold air for months while sitting unattended.

[JenR]

Originally Posted by Brad_SwiftFur

Please don't take this the wrong way, but it sounds like you have a lot you should learn about air brakes.


There's plenty of excellent Youtube videos covering this but I'll try for a quick text version.


When you first start the engine (and let's assume the air pressure is zero) it should begin building air pressure. You'll have a "Wet", "Primary", and "Secondary" air tanks (sometimes combined in one tank, with two or more compartments). Many systems will also have an air dryer. So the air leaves the compressor, through the air dryer (if equipped) and into the "wet" tank. The primary and secondary tanks are supplied from that. Many vehicles have a "protection valve" which will not open until around 60 PSI, this supplies most of the vehicle, air suspension, air seat, door, stop sign and bar, etc. The brakes *ALWAYS* get full pressure, no matter how low it is (thus, not on the "protected" portion).


Once the system reaches 110-120 PSI, the air compressor "governor" will disengage ("cut-out")and most systems it will "purge" (a quick hiss of air like you described), completely normal and is intended to purge built up moisture from the system. As you drive and the air pressure drops, typically around 90 PSI or so, the compressor will "cut in" and resume building pressure. Again, once it cuts out, it will purge again.


Now that you've let the engine idle and have full pressure, you'll have a knob which is usually pushed in to release the parking brake (some *OLD* vehicles had knobs which had to be pulled out; it was standardized to a push-in style somewhere in the 70's). Air will be supplied to the parking brake chambers and your air pressure will drop a bit but this is not supposed to be a constant flow - if it is, you have a problem (likely causes are a blown diaphragm in the brake chambers or a failed air line.) What this does - inside each rear brake chamber is a huge spring which is under a *LOT* of pressure - this is your parking brake (it's also why you *NEVER* disassemble the parking side of these chambers! Most are sealed these days but older ones could be disassembled). So, supplying air to this side pushes a diaphragm against the spring, compressing it and releasing the parking brake. Parking brakes are generally forbidden on steering axles, for what should be obvious reasons.


So now you have air pressure and released parking brakes and you begin driving (we will assume you've done proper pretrips and found no problems). You found that the foot brake (foot valve, treadle valve, it goes by more than one name), it's basically a foot-controlled air pressure regulator. They typically have very little travel, relying on application pressure to regulate how hard you stop. The harder you push, the more air it supplies. When you release, it should release all the air (and there are often relay valves to augment this.)


It sounds to me like you are getting good air pressure with the bus running. You have 2 air systems, gauges for both, make sure both are full pressure. So far, so good? Release the parking brake, see if the bus will roll relatively freely (it should). If not, listen for air leaks (As noted above, you will want to chock the wheels, or better yet, have a helper for this, you do *NOT* want that bus to move uncontrolled!) At this point there are two likely things - if you hear an air leak, you likely have a parking brake that isn't releasing fully - either fix the air line, or replace the brake chamber, as appropriate. No air leaks? The brakes may be dragging (adjusted too tightly), they will need backed off on the adjustment (less likely but you never know....) Either that, or ...



As for pushing the pedal, nothing, nothing, then grabbing, I had a truck that did this. The foot valve was on the floor (common with cabover trucks and flat-front buses) and they get a lot of dirt and crud in them, they begin to stick. And in your case, I wouldn't be surprised if it wasn't releasing fully either. That's my suggestion, check the foot valve. Another member here had similar issues and that's what his problem ended up being.


Oh, and air loss after shutdown? Mine does this as well, I need to replace my suspension leveling valve (fairly common wear-and-tear item and they're not expensive).

All good information. Thank you. Ive had three buses in the past that had air brakes and i didn't have any issues with them. So I kinda know how they work, just have never had to troubleshoot them before