park brakes will not release

mccarlk · April 26, 2009, 05:15:45 AM

hi i just bought a 76 mci5b for parts, when i try to release the brakes, it just hisses out the rear, can i cage the brakes so it will move?

JackConrad · April 26, 2009, 05:29:58 AM

Quote from: mccarlk on April 26, 2009, 05:15:45 AM
hi i just bought a 76 mci5b for parts, when i try to release the brakes, it just hisses out the rear, can i cage the brakes so it will move?

This bus probably has DD3 drakes on the drive axle. Short answer NO. DD3 brakes use air pressure to set the parking brake, rollers to hold the brake on, then air to release the rollers. Does the air leak with parking brake applied or released? This will helpm us determine the problem. Jack

mccarlk · April 26, 2009, 06:41:59 AM

when trying to release,

buswarrior · April 26, 2009, 07:42:49 AM

Is the source of the air leak coming from the valve mounted on/above the differential? It has 6 hoses attached, one in the end, 5 out the sides.

I'd blame the inversion valve being stuck before the locking ports/mechanism in a chamber for a disabling leak.

Just keep building the air and releasing the parking brake, and applying sharp full brake pedal pressure on and off, before, after and during in some mad sequence of trying to get whatever is stuck to move. This will require a large consumption of air, and require re-building a number of times.

You are trying to get a stuck valve to move, leaning on it in one direction with air pressure and then, the other it's internal springs.

You may also be suffering from the brake shoes stuck to the drums, lean on them backwards and forwards with the transmission to break it free.

happy coaching!
buswarrior

Lin · April 26, 2009, 08:30:25 AM

You probably already know this, but just to make sure. If they are DD3 brakes, they require a full pressure, hard pedal application to release them even normally, after the valve is pushed in.

Tony LEE · April 26, 2009, 10:35:28 AM

Could also be a hole in the diaphragm.

Depending how much pressure is being applied to the actuating rod, you may be able to slacken off the brake adjusters enough to release the brakes.

JackConrad · April 26, 2009, 11:18:20 AM

When making a service brake application AFTER PUSHING IN THE PARKING BRAKE KNOB, make a FIRM application and HOLD this pressure at least 5 seconds. This gives the small spring time to move the roller collar so the rollers cannot bind against the shaft. Jack

buswarrior · April 26, 2009, 10:01:21 PM

leaking diaphragm won't make an audible leak for release of parking brake.

It makes a leak for applying the parking brake for one, and makes a leak for applying the brake pedal for the other.

Yes, safely get under the coach and try to back off the slack adjusters. Block the wheels, block the suspension at the bump stops and under you squeeze!

You might be lucky and have the rare quick release yokes on your set-up?

happy coaching!
buswarrior