A Few Ramblings About Brakes

[JackConrad]

    Recently there was a thread about what would happen if the toad became un-attached from the bus and the air line connecting the 2 vehicles for toad brakes was broken. The concern was about stopping the bus.  Well, I finally got a chance to test drive our MC-8 this morning after replacing a blown service diaphragm in the driver's side DD3.
    When I went for the test drive I put a male 1/4" MPT air quick connect in the fitting on the rear of the bus for the toad brake air line. No leaks until brakes applied, then, obviously, air leaked out this fitting. I had no problems stopping and continuing to drive. If I was in traffic or at a stop light/sign where I had to keep the brakes applied it would be a problem. When appling the brakes, the rate of air pressure loss was about the same as when I had the blown DD3 service diaphragm. We were able to drive the bus home with the blown DD3 by simply watching how we used the brakes. If we had to stop at a stop light/sign, we set the parking brake and released the service brake until we were ready to move. We ended up in a interstate detour due to an accident (almost an hour of stop & go at about 10-15 MPH). 
      Speaking of DD3 Brakes, the DD3 is made in a Type 30 and a Type 24 (MC-5 may have a Type 24).The Type 24 has a 24 sq. in. service diaphragm and a 17 sq. in emergency/aux. diaphragm.  Our MC-8 has the Type 30 DD3. The Type 30 has a 30 sq.in. service diaphragm and a 20 sq. in. emergency diaphragm. A normal brake application on our bus is 10-20 PSI. 20 PSI X 30 sq. in. equals a force of  600 PSI exerted on the slack adjuster.  I do not know how much pressure is exerted by the emergency air tank, but I think 85 PSI.  If it is 85, 85 PSI X 20 sq. in.=1700 PSI force on the slack adjuster. The front brakes on our MC-8 are Type 24 (24 sq. in.) and the tags are Type 12 (12 sq. in.) 
   There is a big difference in the price of diaphragms. Standard Type 12 and 24 diaphragms were purchased at a local big truck parts store $6.50 each.  They did not stock DD3 diaphragms and had a 3 week wait to get them. I called MCI and they had them in stock for about the same price. (service diaphragm $132.00 and emergency/aux diaphragm $29.00)
    One other problem we have had since we purchased this bus is a problem with the tag wheels locking up in a hard braking (such as when that idiot pulls out right on front of us or that light changes from green to red with a 1 second yellow.  I have checked free movement of the tag axle assembly and checked the air pressure in tag axle air bags (all within specs). I had noticed that the tag wheels would not lock up at less than about 30 PSI brake application pressure.  I was considering adding pressure regulators to limit the air pressure to the tag wheels to about 27 PSI. While looking through my MC-8 manual before rebuilding my DD3, I noticed a page listing all the air pressure regulators (page 4-63). One of the regulators listed was Trailing Axle Brake System (effective Unit Number 32587) and was to be set at 25-30 PSI. I installed an adjustable pressure regulator on each tag axle and set them at 27 PSI.  On my test drive this morning, I made a hard "emergency" braking application applying 65 PSI brake application force. Great stopping and no smoke!.  Jack

Note: Specifications quoted are from MC-8 service Manual and Bendix Service Parts Catalog 02-U-1 dated 4/79

[JohnEd]

Jack,

Thank you!

John