Help, brake pressure.

[meyermobile]

Hello, I’m pulled over.

My bus started cycling. Build pressure for the brakes… release. Build pressure release.

The problem is when it releases excess pressure, it’s releases at 120, stays open until about 90 pounds, then closes. Gets to 120, releases until it’s90, then closes. It’s cycling about every minute or two.

When I’m parked, it’s the same thing. That release valve is staying open way too long.

Can I drive to the next town? I’m just about 30 miles north of Wichita, KS

Thanks in advance.
Lewis

[magnakansas]

I think as long as you keep enough air pressure to keep the brakes operating correctly... keep a goin. I am north of you two hours in topeka.


what is your direction of travel?

william

[Rwnielsen]

Someone here will have an idea/answer for you shortly. If it were me, I'd stay to the right and go for it. You should get an audible alarm before your parking brake locks up. 90 psi is still in the safe zone but I'd keep one eye on the gauges. It's probably something simple once you get to a shop
Good luck

[meyermobile]

We’re headed north. On 135 going to glen elder state park tonight. On to Nebraska from there.

[Brad_SwiftFur]

It sounds like your safety release valve is opening at too low of a pressure, typically these don't open below 140 or so. Either that, or your purge valve is sticking (more likely, these are a somewhat common failure item since they cycle quite regularly).

[meyermobile]

OK, if that’s helpful. Could I possibly shoot some WD-40 up there? We decided to keep heading north. I appreciate your reply.

[Brad_SwiftFur]

Originally Posted by meyermobile

OK, if that’s helpful. Could I possibly shoot some WD-40 up there? We decided to keep heading north. I appreciate your reply.

Hard to say. I would say it couldn't hurt, it might unstick whatever is sticking ... though if something has failed, it could stick in the 'open' position.
If it were me, I'd get to a safe spot and try it, prepared to replace something if it sticks open.

[magnakansas]

if you came up this way you could park in the driveway overnight .... I dont know how late the truck repair place is open... have tools.....

william 785 207 7600

[meyermobile]

Hello William, I appreciate that. We arrived at the state park safely. We’ll be headed to Nebraska later this morning. I’ll keep posting our progress.

I sent this response yesterday, but I guess the cell service want strong Enough to send it.

Thanks again,
Lewis

[Booyah45828]

Where is it leaking from? Which gauge is showing the pressure drop?

If it's leaking from the purge valve on the dryer, there should be not only 1 but 2 check valves to prevent what is occurring.

There is a check valve in the dryer that prevents air from the wet tank backfeeding into the dryer. There are also 2 more check valves between the wet tank and the secondary tank, and the wet tank and primary tank. This prevents a wet tank/accessory failure from draining either secondary or primary air pressure.

So, where's it exactly leaking from, and which tank needle is dropping?

[meyermobile]

Hey Booya,

I’m glad to see your post because I think there is more going on than what it seems.

I only have one pressure gauge. I’m guessing it’s to the main tank. I put a new Governor on the pump last year.

When the pressure gets to 120, the valve on the bottom of the drier stays open until it gets to 90, then closes. Pressure builds again 120, pressure is released to 90… over and over. This cycles in less than 30 seconds, a bit annoying.

I had the bright idea of seeing the Governor screw to stop at 110, in the hopes it wouldn’t trip the drier valve… hmm the drier valve then trips at 110, let it cycle a few times same result. I set it back to 120 again, drier trips at 120. I set it to 130, drier trips at 130. Set back to 120. All these changes by seeing the Governor screw on the pump.

There seems to be some communication between the Governor and the drier valve? Not sure why it stays open until pressure drops to 90. 90 is the lower end of the Governor though.

I ordered a new drier unit. Do you think this is the problem? I’m thinking I understand the system less than I thought.

[Booyah45828]

What kind of brake system is on this bus? Is it a hydraulic system with air accessories or do you have an air over hydraulic system? If it was air brakes you'd have to have 2 separate gauges/systems

The governor, compressor and drier valve are all connected through the unloader line. When the governor gets to it's set psi, it pressurizes the unloader line, which causes the compressor to unload and stop pumping air, and the drier switches into a purge cycle. With the drier in the purge cycle, it takes dried air from inside the drier housing, and backflushes it through the wet desiccant, drying the desiccant and pushing the moisture and air out of the exhaust.

You should never see a purge cycle on the gauges in the cab, because there is a check valve at the dryer outlet that prevents the air in the tank(s) from flowing back into and then out of the dryer during a purge cycle. Or you have a failed purge valve allowing unloader line pressure to leak.

What drier does your bus have? Being from the 80's it's likely an ad4, and I would recommend that you upgrade your drier to a modern unit, over replacing it with a "new" unit. That's because the ad4 is nearly obsolete anymore, and the quality of the new and reman units out there isn't very good. We install a lot of adip dryers instead.

Regardless, it sounds like the check valve in your dryer has failed, so yes, a new unit will remedy the problem.

[turf]

i had a similar problem once.

the air dryer would cycle every 30 seconds or so.. kept shooting off.

i believe the mechanic shop mis piped an air line when i had governor replaced. that cause the fail, imo. i ended up have them replace a compressor and the the air dyer to no avail. a second air governor installed correctly solved my issue.

good luck

[meyermobile]

Thank you. It is an all air unit. The only secondary possibility is a second hand on the meter. Stays on the high side. I bought a new AD9 compatible unit to arrive today.

I’vev been putting on on blaster each day on the components daily. Bendix told me it will be a direct fit.

I’ll keep you posted. I’m guessing it shouldn’t take too long. Kinda depends on the rust I suppose.

Thanks again,
Lewis

[flattracker]

When you have finished with the air dryer problem set your governor back to 120 psi.

[Booyah45828]

If it's an air brake vehicle, they've been required to be dual circuit since the 70's, so you should have 2 air gauges, or a dual needle air gauge.

Even so, the 2 systems should be separated from one another, with check valves to prevent air flow back into the dryer from both systems. With both systems flowing air back into the dryer, something seems very awry with what your describing. I'd be surprised to see all 3 check valves failed.

[Booyah45828]

Are you sure it's full air brakes, and not air over hydraulic. Air over hydraulic would only require one tank/gauge. And air over hydraulic brakes was a common system back in the day.

[cadillackid]

Originally Posted by Booyah45828

Are you sure it's full air brakes, and not air over hydraulic. Air over hydraulic would only require one tank/gauge. And air over hydraulic brakes was a common system back in the day.

he might have one of those 3-in-1 tanks that are common on busses.. it looks like a single tank but is actually 3 compartments..



this sounds like a check valve in the dryer.. my WABCO on the red bus is actually designed to drop 10 PSI out of the wet tank but its an accessory only setup so apparently wqhy that dryer is OK

[meyermobile]

Thanks Flattracker,

I set it back to 120 already, but thanks for the reminder.

- Lewis

[meyermobile]

Originally Posted by Booyah45828

Are you sure it's full air brakes, and not air over hydraulic. Air over hydraulic would only require one tank/gauge. And air over hydraulic brakes was a common system back in the day.

There aren’t any hydraulic lines I’ve seen. The tank may be as Flattracker mentioned.

A far as the Gauge. It does have a double needle, but it seems to indicate the high side of the pressure. Stays put as pressure drops.

I’ll take some pictures when the drier arrives tomorrow.

Air lines go directly to the brakes.

- Lewis