Brake Stuck

cheechako · May 23, 2012, 10:27:53 PM

I went to drive my bus MC5A the other day and it started and built air fine but when asked to move she would not budge, I suspect a spring brake is not releasing. I have not had time to get back to it and do the proper air check but based on what I experienced it acts like a spring brake. My question is should I be experiencing a major air leak with this or not? I will do a leak down test soon.

Dave5Cs · May 23, 2012, 10:36:56 PM

Is the governor kicking out at about 112- 120 PSI ? I had a leak after using the brakes around town and then they will lock up as well they should at 30 PSI. I have to change and clean out Air dryer and find the leaks. Does your Air raise up in about 10 to 15 minutes? Try putting 100 lbs of foot pressure on the brake to see if it releases? Just some things I have had to do. check out leaks at your inversion valve, levelers, and/or quick release valve.
Dave5Cs

bevans6 · May 24, 2012, 03:13:43 AM

MC5A, 99% of the time, will have DD3 brakes - not spring brakes. You need 100 psi, a full brake application, a working push/pull valve, a working inversion valve, R8 valve, and non-stuck DD3 push-rod locking mechanism for the brakes to come off. I swear when I think about all this, the miracle is they work so well so often...

Brian

cheechako · May 24, 2012, 06:13:35 AM

I have always depressed the brake pedal after pushing the parking / emergency brake valve in because I am not sure if it has DD3 type brakes. I suppose it would roll without depressing them if they were spring brakes. Thanks for the input I will post the outcome.

Ed Hackenbruch · May 24, 2012, 07:58:25 AM

I had that happen a couple of times when we first got our 5A. What i found was if i fan the brake pedal a couple of times to drop the air pressure before i set the brakes, it is easier to release them later. You have to have more air pressure to release them than when you set them. If you set them at 100 or 105 you need at least 110 to release them,....i usually drop my air pressure to about 90-95 before i set them.

bevans6 · May 24, 2012, 08:31:35 AM

There is supposed to be a pressure regulator mounted on the front wall of the rear axle bay that should be set to 85 psi (from memory, I don't have my manual handy). That limits the pressure that goes to the parking brake portion of the DD3 chamber. Also, the diaphragm size of the parking brake chamber is a little smaller than the service diaphragm, which also reduces the effective pressure. This is per my MC-5C manual, anyway.

Brian

Don Fairchild · May 24, 2012, 08:39:51 AM

If it has been setting any length of time with a lot of moisture it may have the shoes rusted to the drum see if you can rock the coach back and forth to get them to release and if not, Take all the safety precautions and then get under it and beat on the drum with a hammer. You might also grease all the fittings and see if that will help.

Don

bevans6 · May 24, 2012, 09:19:40 AM

Also (again, per the setup of my MC-5C, your bus may be different) the parking brake air pressure is supplied by the parking/emergency tank, not by the service brake tank. There are one-way check valves so that once air is in the parking brake tank, it will not go out when the service brake pressure is fanned down. The service brake pressure can be at zero, and the parking brake tank will retain it's full pressure. So fanning the service brakes down shouldn't make any difference - if the setup has the check valves and they are working!

While I am at it, my understanding of the way the diaphragms are set up in the DD-3's is such that they are in series, not in parallel. Springs in parallel add their force, springs in series add the inverse of their force (like resistors in parallel, 1/RT = 1/R1 + 1/R2, etc). So when the parking brake is set, the highest pressure on the push-rod will be with the service brakes at zero. Adding pressure to the service brake chamber (stepping on the pedal to "set the parking brake") actually reduces the pressure on the push rod. This is different from how spring brake chambers work, which use an anti-compounding valve to make it impossible to have both service pressure and spring pressure on the push rod at the same time. DD-3 do not have an anti-compounding valve. Clever folk these Bendix engineers...

Brian

qayqayt · May 24, 2012, 05:01:19 PM

On the DD3's, there's a grease fitting on the brake actuators that is often missed when the coach is serviced. The manual calls for a lithium based grease because of the rubber components. We had the same problem and after talking to an old mechanic he suggested I try it, and it worked. Check the manual for the location of the fitting. I'd try this first. For the cost of a bit of grease, your problem might be solved.

Bryan

Sam 4106 · May 25, 2012, 10:02:31 AM

Hi qayqayt,

Thanks for the reminder about the grease fittings for the brake locks on the DD3. I greased the bus this week, in preparation for our month long trip and forgot those 2 fittings. Took care of them this morning. I remember a bus mechanic telling me to grease those fittings sparingly so I only give them 1 pump of grease, and that may be too much.

Good luck, Sam

MCIGenes · May 26, 2012, 09:09:47 AM

I had the same problem with my MC5B a few years ago. Mine turned out to be a bad service brake valve that was not letting air back to the spring brakes to release them. For starters you should make sure you brake peddle is pulled up all the way. Mine had a problem coming back up all the way and that was keeping the spring brake on the right side locked up. CJ's in Minn replaced the main service brake valve and several cans and I have not had a problem since July 2006. Worked for me. Before CJ's fixed it I had it in a shop in Elkhart Indiana and then again in Portage Wisconsin and again in Unalaska Wisconsin. CJ's was the only ones that knew that type braking system to look in the front of the bus and find the problem. Every other shop was looking for a cause in the rear, because it was a rear brake locked up. It was a real sick feeling to stop at a rest stop in Indiana and not be able to take off again. AAA then sent a wrecker out that was too small and I watched the front of his wrecker come off the ground as he tried to pick my bus up. Good luck finding your problem.

Gene

mikelutestanski · May 27, 2012, 04:05:01 PM

Hello
If you make a brake application when the push pull valve is in the set position you will drive the locking rods in harder and set the brake rod tighter against the locking wedges in the dd3 can. then to release you need to overcome the initial pressure.
SO be careful when making brake applications (i.e. make sure the push pull valve is released) because the DD3 operates differently than the spring brake.

The reason that mci buses do not have spring brakes is that the cans will not fit in the space under the bus.. Too much stuff in the way.

regards mike

cheechako · May 27, 2012, 09:57:55 PM

I finally got to the bus today and found the problem, when I tried to move it I was in a hurry to get to my destination and did not notice the bus had settled into the dirt from several rain storms. I did a full pre trip air check and found the system tight, then and only then did I look at the wheels and noticed the settling. After a brief shovel job it rocked right on out. Boy do I feel stupid and now know do not hurry a pre trip. I had a gig down in Dragoon with my band and was running a bit late.

Thanks for all the troubleshooting info.

belfert · May 28, 2012, 05:41:29 AM

I had the same thing happen with the rear wheels settling over the winter into a gravel parking lot. I thought the parking brake was stuck on too. I finally realized by looking in the mirror that the wheels were turning, but I was stuck. I don't recall how I got out, but it wasn't too difficult once I realized the brakes weren't frozen on. The wheels had only sunk three to four inches, but it was enough to cause issues.