1973 Blue Bird

[Rudy]

Sharkey, thanks for the knowledge.

[Jones'n4chrome]

Here are two questions.

Why do the brake lights go on and stay on when I switch the parking lever to park when the motor is running? I'd sure like to disconnect this function. It sounds like a tricky task.

On the bottom of the placard, it says Emergency Stop. Does that mean if my brake pedal malfunctions, I can use this lever to stop the bus?

If the parking lever is actually a gear selector, then you could leave it configured as is. Whenever you need to leave the engine running but you don't want the brake to stay lights on, then set the emergency brake to "park" and then set the gear selector to "neutral"
On my bus the emergency brake is the only park brake.

[Rudy]

There is no "park" on the gear selector. When the engine is running, the brake lights stay on if I put that red lever to park, regardless of whether the shifter is in neutral or in gear.

[Jones'n4chrome]

When you put the lever in park do you hear it dump the air out of the rear brakes? You may need to turn off the engine first to hear it.

[Rudy]

When I put the lever on, it sounds just like when I press the brakes.

Psshhh

[Jones'n4chrome]

Is there a separate valve to apply the spring brakes then?

Look at the back side of the of that park valve and see if there is any kind of switch or wiring going to it.

[Rudy]

Spring brakes? I have air brakes.

I managed to crane my neck in order to look at the back of that switch. There is just an air line. No wires there.

[Sharkey]

Spring brakes? I have air brakes.

No, you have air-actuated spring brakes.

In order to understand how air brakes work, you should go download the Bendix Air Brake Handbook and read through it.

Your rear brakes have two cannisters nested one on top of the other, one pair each at the ends of the rear axle. One cannister contains air-operated diaphragms, whie the other contains a powerful spring. The spring has enough tension to actuate your rear brakes fully and lock the drum solid. The actuator cannister is connected to your air brake system and is used to ~counteract~ the pressure of the spring. When you step on the brake pedal, a second section of the diaphragm cannister applies pressure to apply the brakes in proportion to how hard you press the pedal.

The reason for using a spring is so that if you lose air pressure while driving, the brakes are applied and you stop to find out what the problem is. This is the reason that your bus has to sit and idle for a while before you can drive it anywhere after starting, without air pressure, the brakes are locked.

The "parking" lever you pictured, removes air pressure from the diaphragm cannister, allowing the springs to lock the brakes.

Front brakes are somewhat different, they don't have a spring cannister, only a diaphragm cannister. You don't want the front wheels to lock up after a compressor failure while you are driving. It would be difficult enough to control the bus with locked rear wheels, but at least you'd still be able to steer.

The actual stop light switch is somewhere else in the system, connected to a tee fitting in an air line somewhere.

[Rudy]

I will read that Bendix brake manual. I suspect it will mention adjustment of brakes as well.

Thanks

[Jones'n4chrome]

Rudy,
You do have spring brakes, but it's just a small part of your air brake system. Spring brakes will apply the rear brakes if you loose to much air pressure. So if you ever hear your air buzzer or see your air gage below 60 (I think) get to the side of the road ASAP because your bus will soom come to a stop.

Anyway, spring brakes can also be used as parking brakes, by dumping all the air out of the "spring brake chambers" via an air valve usually on the dash. Do you have only one valve on the dash?
Here is some basic info. But I studied the bendix air brake manual that Sharkey posted. Many of the truckers here could explain the system better then I can.

http://www.ehow.com/how-does_5747300_do ... work_.html

[Jones'n4chrome]

Yeah, what Sharkey said...

[Sharkey]

I will read that Bendix brake manual. I suspect it will mention adjustment of brakes as well

For that, go read the Slack Adjusters document.

[Rudy]

Another mechanical task I will tackle. Thanks, you guys.

[Jones'n4chrome]

The "parking" lever you pictured, removes air pressure from the diaphragm cannister, allowing the springs to lock the brakes.

So how does that apply the brake lights when the engine is running? Is there a separate switch on the "spring brake" circuit that is though the key only?
Would that be a school bus safety requirement?

If it is that way, it would be very easy to disable it, then the switch on the "service brake" circuit would apply the brake lights under normal braking.

[Mark R. Obtinario]

I won't add anything to the discussion about air brakes because everything that is needed to know has already been covered.

When working on the rear brakes, never, ever, and I really mean never, ever open up the spring chamber. That is a good way to get yourself dead. The spring inside is large enough that one spring working will stop the vehicle. If you release that much energy all at once you risk losing pieces of your body, the most significant part being your head.

Once you have figured out how to adjust your brakes you will find that they are relatively easy to do. At that point you will wonder why you waited so long to tackle that task.