Weird air system issue

[JT4SC]

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. 

[Jim Blackwood]

Strange problem. I'm really curious to see what the cause is. Sorry not to be more helpful though.

Jim

[luvrbus]

Air dryer needs servicing or a broken unloader valve on the compressor, but I would start by changing the governor first 

[JT4SC]

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?

[luvrbus]

Try adjusting the governor remember it turns left (up) to increase the pressure the opposite of most regulators

[JT4SC]

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?

[freds]

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?

[luvrbus]

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

[JT4SC]

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???

[JT4SC]

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!!!

[luvrbus]

 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   

[azdieselman]

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.

[luvrbus]

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

[JT4SC]

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.

[JT4SC]

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!!!