Hey guys,
I've got an issue on my 102C3 - the bus airs up to approx 90psi, then stops and goes down to approx 70psi, then airs up to 90, back down to 70, and on and on it goes. It won't air up enough to release the parking brake. The governor I believe is ok, I think there is supposed to be a check valve somewhere that might be stuck, but I have no idea where or what it is to try to replace it. Any help would be appreciated. Luckily I'm finding this out at my house and not on the side of the road.
A couple weeks ago I had a similar problem but eventually (after 45 minutes or so) it aired up fully and ran great for 200 miles. This time it won't air up after an hour of the bus running. I've pushed down the brakes multiple times, tried to hold the parking brake down, and nothing gets it to go past 90psi.
Strange problem. I'm really curious to see what the cause is. Sorry not to be more helpful though.
Jim
Air dryer needs servicing or a broken unloader valve on the compressor, but I would start by changing the governor first
Quote from: luvrbus on February 14, 2026, 10:18:05 AM
Air dryer needs servicing or a broken unloader valve on the compressor, but I would start by changing the governor first
Thanks Cliff. Should I try to adjust the existing governor first or forget that and just buy a new one?
Try adjusting the governor remember it turns left (up) to increase the pressure the opposite of most regulators
Quote from: luvrbus on February 14, 2026, 01:30:25 PM
Try adjusting the governor remember it turns left (up) to increase the pressure the opposite of most regulators
OK, and can I do that while it's running or only when it's off and then turn it on and see what happens?
Maybe isolate the compressor and governor and see what it pumps too, so you know the problem is not else where in the system?
Maybe also use an external air source and detect when the governor releases pressure?
Quote from: JT4SC on February 14, 2026, 01:37:55 PM
OK, and can I do that while it's running or only when it's off and then turn it on and see what happens?
I do it with running try airing it up at the rear door passengers side with a air compressor if you have one to be sure it not a pressure leak
Quick update - the bus air system continued to do the shut off at ~85psi, turn back on at 70psi, and on and on.
I hit the brakes a bunch of times, the cycle continued, so it didn't seem to be the brakes. I bought the governor and was about to replace that when I tried one more thing - I actuated the door air system (pushed the red knob up so the door would start to work on air). The door air made some weird/different noises, so I opened and closed that red knob a few times, and opened and closed the door a few times. then left the red knob in the off position and was just about to turn off the bus and change the governor. Next thing you know the bus air went up to 120!!!
Could this whole issue be because of the door air system???
Also I was doing some research, and isn't there supposed to be a check valve on this line from the air compressor (see line with red arrow on attached pic).
If there is supposed to be a check valve could it be in a different location?
Thanks again everyone for all your help, so appreciated!!!
I would change the governor anyways on that one ,the PSI drop range is right from 85 psi to 70 before it kicks in, at 125 psi they drop about the same psi before pumping, my guess is that you have a unloader valve sticking, Owners never change those ,they change the governor if that doesn't fix it most people change the compressor for $500 for a set of $10.00 unloader valves. I sort of understand it, you have to drain coolant system and the compressor on MCI's are not friendly to do it and the head gasket for the compressor cost $25.00 now. I keep the unloader valves and head gasket here for the people that don't want buy another compressor, most that stop here I just tell those what needs to be done and they can do it a couple of hours the 1st time. Most of the time when I change a governor the unloader valves get changed also, not much goes wrong with a compressor if you have a lot of oil in your system then it needs replacing
If that's a Tu-Flo 550/750, the unloaders are under the plate on top of the head. The head only has to come off for the intake check valves. If there's nothing in the way, it shouldn't take more than 30 mins.
Quote from: azdieselman on February 22, 2026, 10:27:45 AM
If that's a Tu-Flo 550/750, the unloaders are under the plate on top of the head. The head only has to come off for the intake check valves. If there's nothing in the way, it shouldn't take more than 30 mins.
Kevin I think you are talking about the B models with the oval plate and 2 bolts with a single under loader valve that cost a 100 bucks ,he won't have that style on his C,I have a box full of the unloader valves for his older Bendix, I notice now they are 13 bucks .here is the Bendix # for the older 550-750 tuflo 279615
Quote from: azdieselman on February 22, 2026, 10:27:45 AM
If that's a Tu-Flo 550/750, the unloaders are under the plate on top of the head. The head only has to come off for the intake check valves. If there's nothing in the way, it shouldn't take more than 30 mins.
Mine is a 750.
Quote from: luvrbus on February 22, 2026, 12:54:49 PM
Kevin I think you are talking about the B models with the oval plate and 2 bolts with a single under loader valve that cost a 100 bucks ,he won't have that style on his C,I have a box full of the unloader valves for his older Bendix, I notice now they are 13 bucks .here is the Bendix # for the older 550-750 tuflo 279615
I can try to get a pic of the top just to confirm. Thanks so much guys!!!
Quote from: JT4SC on February 22, 2026, 04:48:47 PM
I can try to get a pic of the top just to confirm. Thanks so much guys!!!
Look up Bendix K109119 and you can see the one Kevin is thinking of ,yours will use the other number I posted. Kevin and I have been friends for long time he has the pleasure of working on the newer stuff, he is one of the best at working on the electronic controlled engines
Quote from: luvrbus on February 23, 2026, 04:02:04 AM
Look up Bendix K109119 and you can see the one Kevin is thinking of ,yours will use the other number I posted. Kevin and I have been friends for long time he has the pleasure of working on the newer stuff, he is one of the best at working on the electronic controlled engines
Done. I ordered the part # you gave me and will be here in a few days. Here's what I'm going to do either this weekend or next:
- Replace the governor
- Replace the unloader valve
- Change out the air dryer (is there just 1 air dryer, or is there a separate one for the brakes and a larger one for the overall air system?). I can see the huge one in the front of the bus, but not sure if there's another one somewhere else?
- Delete the AC compressor belts and cap the air lines that go to the air piston belt tensioner (it's also leaking air bad). I'm going to eventually remove the OTR AC system and go with a mini split anyways, so this will start that project.
You guys see any issues with any of these plans? I'm assuming it's ok to simply cap the air lines to the belt tensioner?
It just has the 1 air dryer up front by the axle, if you are careful you can reuse the head gasket using the "right stuff" sealant, they make several different head gaskets for that compressor, when you buy the kit they always seem to have the wrong firggin gasket 8),if you do need a gasket you can match it on E Bay for under 20 bucks compared to Fleetprides 35 bucks
Quote from: luvrbus on February 25, 2026, 06:20:25 AM
It just has the 1 air dryer up front by the axle, if you are careful you can reuse the head gasket using the "right stuff" sealant, they make several different head gaskets for that compressor, when you buy the kit they always seem to have the wrong firggin gasket 8),if you do need a gasket you can match it on E Bay for under 20 bucks compared to Fleetprides 35 bucks
Thanks Cliff!!!
What about the check valve - is there supposed to be a check valve on this steel braided line in the picture attached?
Quote from: JT4SC on February 25, 2026, 07:46:14 AM
Thanks Cliff!!!
What about the check valve - is there supposed to be a check valve on this steel braided line in the picture attached?
The C in the shop doesn't have a check valve on the discharge line ,it does have a small drip tank where it connects to drain tied to valve to use shop air to air the bus up inside the rear door ,the check valve is at the front tank,that braided gets hot, real hot so they don't restrict flow with a check valve. I had to replace that hose on a MCI man it cost almost $400.00 ($397.80)
Quote from: luvrbus on February 25, 2026, 08:11:25 AM
The C in the shop doesn't have a check valve on the discharge line ,it does have a small drip tank where it connects to drain tied to valve to use shop air to air the bus up inside the rear door ,the check valve is at the front tank,that braided gets hot, real hot so they don't restrict flow with a check valve. I had to replace that hose on a MCI man it cost almost $400.00 ($397.80)
Ok cool I'll look for the check valve at the front of the bus. Thanks!!
Good morning guys,
Still chasing gremlins on the air system, I've located what I believe may be the issue - this relay valve (pic attached). Only problem is I don't know if this is a Bendix R12 or R14 relay valve? Do you guys know? Is there an easy way to tell while it's still in place, or does it have to come out to know for sure?
Either will work for you the only thing different is the R-14 has a anti compound feature to work with the spring brakes it is a upgrade from the R-12 ,there will be a small metal tag on the relay on one of the bolts that identifies the relay, the prices on the relays are all over the map,NAPA quoted me over 400 bucks for new Bendix R-14 I went with Midland I not much into the rebuild stuff, but the kits are cheap and they are easy to rebuild unless the bolts strips or break off. They will have different cracking pressure that is why you need the tag from it ,the front relays have a higher cracking pressure so the rear axles apply first most if the time the front will have a 3 to 4 psi cracking pressure
Let me tell you guys, I'm having the time of my life with this bus (heavy sarcasm)! While chasing the air pressure issue we noticed the brake chamber on the passenger side drive axle wasn't disengaging, even when releasing the parking brake and/or hitting the brake pedal.
So I bought a new one and installed it today. The brake changer came with the push rod in, we installed it that way, and when we aired up the bus the rod came out and engaged the brake, which I believe is supposed to happen since the parking brake was engaged. But when I disengaged the parking brake the push rod stayed out, which is the exact same problem we had with the original brake chamber.
So I have to assume the issue is inside the drum somewhere? Any advice on what could be keeping the rod from disengaging? I've attached a picture showing the position the push rod is stuck in for your reference (you'll see an air line is not attached, that's because we were in the middle of troubleshooting).
If disconnected and will release with air pressure applied, then it is inside the drum, if you have automatic slack adjusters those things are terrible about binding and not letting the parking brakes release on the D-3 system, check your air pressure it should be 90% of showed PSI to release the D-3 and should read 0 when released on the port.The older buses love to throw you curve balls and most of the time it is simple problem. A friend did a brake job on his Eagle and one side was not working as it should we found that the supplier sold him 2 right handed S-cams that he installed after a day of searching for the problem
Is there a parts supplier that is good at dealing with these older buses? While working on the brake/air issue I found there has been an oil leak I believe where the bearings are located, which got oil all over the brake drum, pads, etc.
I'm looking to replace the brake shoes and the axle seal/gasket, however when I plug the part #'s in to Google there's zero as far as places to buy these things. I called NFI and they said the parts have been discontinued and they can't help.
Luke at 856-794-3104,Coach Bus Parts 800-724-8464 any bearing supply can match the seals or even Fleetpride or Napa and any friction shop can reline the shoes, Las Vegas is close to me, so I buy from P&O
702-384-2530 there is another supplier in Vegas but I don't remember the name but I buy studs there
Quote from: luvrbus on March 18, 2026, 01:26:07 PMLuke at 856-794-3104,Coach Bus Parts 800-724-8464 any bearing supply can match the seals or even Fleetpride or Napa and any friction shop can reline the shoes, Las Vegas is close to me, so I buy from P&O
702-384-2530 there is another supplier in Vegas but I don't remember the name but I buy studs there
God bless you Cliff!!
When were the wheel ends disaasembled on that coach last?
Stuck brakes and leaking axle seals are part of that. Others will follow?
S-cam tubes taken apart, hard grease all chipped out and fresh lube, NEW axles seals all the way around.
One thing leads to another...
happy coaching!
buswarrior
Quote from: JT4SC on March 15, 2026, 06:47:26 PMSo I have to assume the issue is inside the drum somewhere? Any advice on what could be keeping the rod from disengaging?
Did you get to the bottom of this? The DD3 air chamber needs air supplied to what they call the "locking port" in order to
unlock the brakes. This comes from the inversion valve, which is commanded by your push/pull parking valve.
The locking mechanism inside the air chamber is a sort of ratchet, so sometimes after applying "lock" air to unlock the lock, you have to pulse the main brake application air, to push the rod out briefly and then allow it to retract.
While working on mine, on the bench, I would test by applying air from a compressor/tank to the lock port, and every time the rod would just retract using the internal spring. But when installed, sometimes I have to pulse the brake pedal.
Hope that helps.
Quote from: KF6-BusFan on May 10, 2026, 11:03:42 AMDid you get to the bottom of this? The DD3 air chamber needs air supplied to what they call the "locking port" in order to unlock the brakes. This comes from the inversion valve, which is commanded by your push/pull parking valve.
The locking mechanism inside the air chamber is a sort of ratchet, so sometimes after applying "lock" air to unlock the lock, you have to pulse the main brake application air, to push the rod out briefly and then allow it to retract.
While working on mine, on the bench, I would test by applying air from a compressor/tank to the lock port, and every time the rod would just retract using the internal spring. But when installed, sometimes I have to pulse the brake pedal.
Hope that helps.
Hey guys,
Well I've kind of fixed it, the bus is now building air and not stopping at ~100psi, everything is doing what it's supposed to do. The only issue is I don't know exactly what fixed it. What I do know is I had a bunch of deferred maintenance that all lead to this, and I'm slowly (and expensively) fixing it.
What I THINK was most likely the cause of the issue was actually the air dryer. I'm embarrassed to say this but in almost 20 years of owning this coach I have never changed the air dryer cartridge. Why, you ask, would I be dumb enough not to do this? Honestly because I didn't know I was supposed to. I don't come from a bus mechanic/driver/enthusiast background, I bought this bus as a 27 year old kid who had spent exactly 1 night RV'ing prior to buying the coach. So, to say I've learned a lot about busses over the years would be an understatement, but I just had not yet learned about the air system and maintenance. When I had the oil changed I would ask them to check the brakes and air system, and they would always say they're good, but I didn't take the time to learn it myself until now.
So, all that to say I've done a lot of work:
- I've (so far) gone thru 3 axles, brake and air systems (passenger drive axle, driver and passenger front axles), changed seals, brake pads, drum and brake chamber on the passenger drive axle, front hub caps, air bags and shocks, air relay and air pressure regulator on the drive axle, check valve that I think had failed on the rear air tank, deleted the air system on the AC belt tensioner in the engine bay (it had been leaking air - going to eventually get rid of the entire OTR air system and go to a mini split), replaced the governor and unloader valve on the air compressor, and changed the air dryer.
Still need to through the driver side drive axle and both tag axles. VERY costly and very time consuming, but it needs to be done.
Thank you all again for all your help, I would have been defeated in the bus game a long time ago if it wasn't for you.