Originally Posted by WARGEAR
How long has it been sitting in one place? Could just be rust on the drums holding it still.
Very likely. I second WarGear's suggestion.
However, are you absolutely positive this bus has air brakes? Not to be condescending, but some still do have hydraulic brakes, and most assume they have air when they may not.
The reason I ask is that many older Ford and some Navistar chassis skoolies (the older, the more likely) have the dreaded Lucas-Girling system, which is known for a slew of problems, hard to service, and even harder to find parts for, let alone find a shop that will touch them. My previous Ford B-700 bled down the parking brake chamber after sitting for too long and had to be scrapped due to inconvenient timing.
If you have THIS to (dis)engage the parking brake, it is air brake.
Air Brake Knob.png
If not, you have hydraulic...
If they are indeed air, first order of business is to check system air pressure on the gauge. If it is below 60, that is likely causing the problem. Air-brake buses engage their spring brakes much sooner (60 psi) than trucks (20-40 psi). Their low-pressure warnings come on sooner, too, at 85 psi vs the 60 psi that trucks are set for. System pressure should hold between 90-120 psi on most systems, there is a blow-off valve to keep it from exceeding 150. Slow leaks are quite common in older air systems, which will cause them to bleed down slowly when sitting. I've seen severe leaks lose pressure completely in a matter of a couple hours. Leaks will also allow more moisture to enter the system, which is not good for it.
If air system pressure is not at least 60-80 psi (which still indicates a problem), start the engine and let it run for about ten minutes. The system should build air pressure back up within a few short minutes, but if it has bottomed out, it will take longer. Especially if there is a moderate leak to start with. If air system pressure is sufficient, generally only two things will keep the parking brake from disengaging -- chamber / linkage malfunction, and brake linings rusted to the drums.
If you are not aware, it is necessary to open a drain cock on the air supply tank periodically. If I remember correctly, DOT rules state once a day -- reason being that compressing air always results in a bit of moisture in the system, and a small bit of oil from the compressor can make its way to the tank as well. Hopefully THIS is not your problem, if the lines are clogged with oil and water, it will take a bit of work to purge. Even worse if it has made it to the chambers. However, if it was working fine when it was parked, I doubt this is likely.
One of you will have to get under the bus and watch the brake chambers when attempting to disengage. CHOCK THE WHEELS FIRST! If the chamber is releasing and moving the brake linkage, then it is likely rust on the linings and drums.
I do not recommend attempting to service a brake chamber, if that is the problem. Many reasons behind this, all for safety.
On both air and hydraulic systems, the semi-metallic brake linings will indeed rust and bond with the drums. Clearing rust on the drums and linings is a bit of a job, as removing the wheels and drums will be necessary, but it is something the average person can do. Keep in mind the wheels and drums will be much heavier than anything you are used to. Standard 2 - 1/2 ton floor jacks and jack stands are not going to safely support your bus for very long, either.
Only undertake this if you think you can handle it. Make sure any jack / jack stands used are rated for the weight of your bus (likely at least 6-8 ton). DO NOT ATTEMPT THIS WITH JACK STANDS OR JACKS THAT ARE NOT APPROPRIATELY RATED - IT IS NOT SAFE. Start by properly chocking both wheels on the other axle, and release the parking brake. Then begin lifting the rear. Chase the axle housing tubes with your jack stands, which is to say every time to you can raise them and latch them, do it. It will minimize the fall if your jack slips.
Once the wheels are removed, you can heat the drums with a propane torch, tapping on them with a hammer to work them loose. Brake cleaner may help with this, I recommend wearing some sort of mask or respirator to prevent inhaling brake dust -- it can cause cancer.
Spray the brake linings well with brake cleaner, lightly sand the friction surface, as well as the drum contact surface. Reinstall and you should have no problem.
If you don't have the proper equipment for this, or do not feel comfortable tackling it, you might call a truck repair shop and see if they have a road service available. It's cheaper than towing (usually $100 hookup and $10 a mile).