Park brake problem

[thomasinnv]

Calling all experts, need some direction.

For quite some time now, once in a while when pushing the button to release the park brake, the brakes will not release (yes, even after doing a full brake application). I always just pumped the brakes down to activate the compressor and then tried it again once the dryer discharged, assuming it was just a problem of not having enough pressure to release the lock pin. Yesterday morning however was a different story. Took several attempts to get rolling. I had moved the bus about 30 feet to hookup the towed with no issues. Then when I tried to pull out and hit the road, no good. Some attempts would result in the park brake light going off but the brakes not released, other attempts would result in the light staying on but the brakes were actually released. Every attempt when pushing in the push/pull valve air would continuously run through the valve. When the brakes did finally release properly and the light was off, there is no air rushing through the push/pull valve.

One other issue I have noticed is that sometimes when I push the valve in, the brakes don't release right away but after about two full seconds brakes release and the bus will roll.

I'm leaning towards a sticky inversion valve and possibly too high of pressure on the park brake regulator. I welcome your thoughts and advice. They are DD3 btw.

Wow the text ended up being huge on my phone. I'm posting from my phone and I don't know what happened sorry about that.

[bevans6]

My bet would be the sprag clutch on one of the push rods is sticking, but it might be the inversion valve.  If it's the inversion valve the brakes will lock on when in service mode, or the parking brake won't apply in the first place.  All the inversion valve does is toggle air back and forth between the locking port and the parking brake port when told to by the push/pull valve.  Have you greased the locking clutches lately?  Couple of sparse pumps is all, then exercise it with a few applications and releases.  If it is right, you should be able to apply and release with just the push valve, if you do it right away so the air pressure has no time to leak out of the parking brake chamber.  It could be the pressure regulator (set to 85 psi normally, from memory) but the parking brake diaphragm is 80% the size of the service brake diaphragm so will develop only 80% as much push from the same application pressure, so you should always be able to release the brakes with a full service brake application, if nothing is sticking.

[luvrbus]

Do like I did toss the DD-3 and go spring brakes plenty of room for spring brakes on a DL3,they are not your fathers Oldsmobile

[chessie4905]

I'm with Bevan on this one. If that doesn't work, remove lock port hose from dd3 and attach pressure gage to hose to verify you are receiving pressure. The lock port is the one closest to push rod. When the parking button is in release position,down, air is constantly supplied to lock port. When button is in apply position, up,air is cut off from lock port. The rollers in the lock port have a small piston at one end that move to release rollers from locking shaft when air is applied to lock port.
Due to age of your coach, you should consider removing dd3s and reconditioning while parts are still available. GM recommends to remove, clean, and repair or rebuild every 3600 hrs.,100,000 miles, or 1 year, in revenue service, whichever comes first. Yours is way over regardless.
Oh, and block wheels if you are under it along with body.

[buswarrior]

The pneumatic description suggests an inversion valve problem.

It's the commander of the ship, and rarely/never gets changed out as it should for preventive maintenance.

Dash indicator and air noises are all pneumatic. Erratic release timing can be pnuematic and mechanical.

When was a little grease put to the locks last?

Use anti-seize, carefully, on all MATING SURFACES of new airlines, not just the threads... often, all those airlines have to be cut to remove, as the fittings have rusted solid within themselves and won't spin. Threads and line are fine, more expense down the road...

Happy coaching!
Buswarrior

[luvrbus]

The DL3 has 2 inversion valves I know the one at the rear controls the parking brake but haven't figured out exactly what the the one in the spare tire compartment does guess I should study on it.
I do know also when a quick release valve goes bad the air will travel back through the the system and parking valve looking for a exit that only took me a 1/2 day to figure that one out   

[bevans6]

The front one is to make the front brakes work with a modern(ish) dual tank air system, from what I read in the MCI release.  With the old dd3 system the front brakes don't work in emergency braking at all.