Parking Brake Won't Release

[mlh1936]

When I tried to depart Fred Hobe's yard in my '87 102A-3 brakes locked up. I had just backed up and then tried to move forward. I had full air pressure when I first started to move. After some investigation Fred found that I had air escaping from the top of both drive axle brake cannisters when the parking brake was off. When I pulled up the parking brake knob the air from the cannister tops would stop leaking. We thought I had a faulty diaphram in one of the cannisters so we(mostly Fred) replaced all the diaphrams in both cannisters. That did not fix the problem. The weather was warm so the brakes were not frozen. The transmission is shifting ok and trying to move the bus. What do you guys think could cause this problem? Could I have had both cannisters fail at the same time? Or maybe one failed before the other and I didn't know it? Any suggestions? Thanks.

[Ednj]

It might be the diaphram in the valve in the spare tire compartment.

Nick whats that valve called?

[mwf]

I had the same problem in my 88 MCI.  I had to replace the  valve under the driver (spare tire area and area by washer tank).  I got a replacement from Fleet Pride, it was made by Bendix.

[Nick Badame Refrig/ACC]

Yea, It's a Bendix Regulating valve. in the spare tire comp. under the driver..

But, it was leaking lots of air... and ended up locking up my park brakes [in front of Ed Skiba's house]

If your air leak is comming from the DD cans, then I suspect that there are 2 diaphrams in the DD's.

Or maybe you were just hearing your Unloaders leaking?

Nick-

[Stan]

Since the air brake experts haven't come up with any good ideas, I will kick in my suggestion. If you have DD-3 brakes, you should not have any air pressure at the brake cans when the park brake is off and the treadle valve is not pushed.

My guess would be a bad brake relay letting air go to the service brake chamber. However, that does not explain the leak at the cans. Unless the cans are rusted through, the only place air comes out, is the bleed holes which let air escape when the diaphragm is leaking.  The bleed hole is on the opposite side of the diaphragm from where you put air pressure. You have to find where the air is coming from at the cans. I am not sure, but I think it is possible to have a bad enough leak at the cans to prevent the inversion valve from shifting, since it is dependent on pressure differential to shift.

You will have to disconnect lines to track down the problem. If air is coming out of the relay valve, then disconnect the pilot line which comes from the treadle valve. If there is air pressure there, it is a faulty or stuck treadle valve. Have you tried pulling up firmly on the foot treadle?

Once you determine the exact location of the air leak at the cans and the what line has air pressure on it, Buswarrior will probably be able to theorize the exact cause.

[mlh1936]

Thanks guys. Assuming that my cannisters need replacing, what do you think of my replacing the DD3 cannisters with spring brakes. I have been told that all the newer coaches and semis use the spring brakes now. I was told that I could install the spring brakes with no modication required for only $70. each vs $600. for new DD3s. Also the spring brakes have a provision to release the brake manually if they become locked up such as my DD3s are. Sounds like a "no-brainer" to me. What is the down side of this? Thanks.

[Tom Y]

You can unlock manually but only to tow or move in a yard, not to drive. ( Spring Brakes) You may need to change some valves also. Spring brakes would not fit on my 5C,  but it is older. If you go springs you will want 30-30 I think but may be wrong. I'm sure no smaller than that.  Good luck. Tom Y

[JackConrad]

Unless the MC-7 is different than the 8 & 9, the spring brakes will not fit without a major modification of the bracket the brake mopunts on. This will also require re-positioning the slack adjusters. The 30-30 spring brake is much longer than the DD3.  Beleive me, if thay were a "bolt-on", they would be on our 8. Jack

[buswarrior]

Hmmm

Air leaking out of both chambers, suddenly, together, is quite a coincidence.

At no time under regular operation, would air escape directly from the chambers, with the exception of the small amount of venting for internal movement of parts.

The only part of the chamber that is pressurized when the parking brake is released is the locking port for the locking rollers. The air pressure forces the locking piston to move the locking rollers away from the shaft, allowing the shaft to move to the released position.

Replacing the diaphragms would not fix a leaking locking piston grommet.

In English, on the face of it, the locking portion sounds like it is leaking. That requires a fairly extensive tear down of the chamber. You may want to be trading them in, but let's fool around a bit first to be sure.

You have been following the proper procedures for releasing the DD3 parking brakes? Press in the button and make a full service brake application?

My schematics are not at hand...

I wonder if you had one already failed, and the loss of the second one put you over the top, leaking more air than the compressor could keep up with. Was the air system cycling on the trip to Fred's?

Drivers should always note the movement of the air pressure gauge, noting when the air dryer purges, staying familiar with the air consumption of the coach.  We really don't want to be leaking enough air for that compressor to cycle very often out on the open road.

And we absolutely want to notice when what is "normal" changes.

I cannot remember off the top of my head what pressure should be present at the locking port. The parking application is supposed to have a regulated 85 lbs applied to it, (or close to that, yours is an '87 and might be a little different)  If the regulator has failed, and the locking ports are supposed to be under the regulated supply and full tank pressure is being applied, that would hasten a grommet failure.

I'm still curious as to the failure of both at the same time...

As far as swapping to spring brakes, the others have it covered. They won't fit without a monumental re-working of the mounting points and associated camshaft and slack adjuster positioning. As well as some serious re-plumbing. Not impossible, but not cost effective. Once you have new chambers and valving, you are done for a decade or three... at lower cost!

By your description, not directly related to your current predicament, but DD3 owners often neglect to change out their inversion valve as preventive maintenance. Just for fun, be sure there is no air escaping from it's exhaust port in both cases, parking brakes applied, parking brakes released. It is mounted on the top of the differential on the MC7/8/9, not sure for your A3. Anyway, it will be at the end of 4 of the 6 hoses that connect to your brake chambers.

Check to see if any air is leaking out the exhaust of the relay valve that the other two air lines connect to.

Under the driver, find the brake pedal valve, and again, check for air leaking out the exhaust on the bottom under both cases, parking on and off.

Your bus will also have the shuttle valve to further complicate matters. It's job is to manipulate the brake system under a loss of air scenario, to help get it stopped, to fulfil a FMVSS requirement. This isn't the place for that discussion!

Have you ever put any grease to the zerks on the DD3 brake chambers?

Be sure that you get core value for your DD3 chambers, if you trade them in. They are quite expensive. And retrieve those new diaphragms before you trade them in!

I can't think of a better place to break down than at Fred's...!

Please keep us up to date!!!

happy coaching!
buswarrior

[Lin]

I replaced my DD3 chambers recently.  They were $495 each at US Coach plus $150 each core charge, so I am sure you will not forget to ship them back.  When you get them, make sure that they mount so the exhaust/drain is on the bottom.  We did not realize this until they were installed and one of the exhaust was positioned at 3 o'clock.  Apparently that chamber was for an application that had horizontal  mounting bolts instead of vertical like mine.