1989 MCI 96A3, this valve is leaking badly, it is in the center of the bus just in front of the drive axle.
I took this picture laying on the ground looking straight up at it.
This is my first time working on the MCI air systems and believe this must be the rear Brake relay Valve.
I would like to get a replacement before I get under there again.
Does anyone know the name /part number and a source for it?
(https://busconversionmagazine.com/forum/proxy.php?request=http%3A%2F%2Fwww.peterbylt.com%2FMCI96A3%2Fleakyairvalve.jpg&hash=e881339d827b21a09204d2a9127675e7cc99f691)
Peter
Looks like an air brake relay valve to me. I can't help with the part number cause my coach is some other brand. I can help with a trick though. When you get the new valve, don't go to the trouble to take out the old one and replace it with the new one. Just pull the guts from the new one and replace the guts in the old one. It's the plastic insert and o rings that are leaking. That trick will save you a couple of hours.
Make sure you bleed all air pressure before removing the circlip at the bottom. The valve is under pressure even when the brake pedal is not depressed.
Hi Peterbylt, when is it leaking ? If it is leaking when the parking brake is set it is more than likely a bad parking brake diaphragm. I would check before changing the valve ! Good luck.
Actually it looks like an R-6 Bendix service relay valve but kind of hard to tell from that picture.
Like previous post, if it is leaking only when the parking brake is released then it is probably a brake chamber issue. The problem with the brake chamber is that it may be leaking past the pushrod seals between the chambers. In any case, you would have to replace the brake chamber. The way to test this is to remove one of the service air lines at the brake chamber and see if their is air coming out of the service port on the brake chamber with the brakes released. This is a more common cause of a service valve exhaust leak.
It could also be related to a treadle valve issue but I would check for a leaking brake chamber first.
If it is leaking only upon a brake application it is most likely a defective service relay valve.
Hang on.
This coach has DD3 parking brakes.
Remove nothing. Block the wheels so it won't roll.
Air up the coach.
RELEASE the parking brake, and see if the relay valve is leaking differently from with the parking brake APPLIED.
If a DD3 chamber has a bad parking diaphragm, it will leak when parked, and won't leak when released.
Jim Eh, yours works for spring brake chambers.
happy coaching!
buswarrior
The Valve leaks all the time, as soon as air starts to build in the system it leaks.
The air comes out of the orifice that you see in the picture.
It comes out at a good enough rate that the bus can never reach full pressure.
It will get up to around 90 psi and I can release and use the brakes, but that's about it.
Peter
I drive air brakes every day at work.
You should drain your air tanks at least weekly. I drain my work tanks every time I park the work bus for more than 2 hours.
With no air in the system, the emergency brakes should hold the vehicle from moving. Chocks will make sure of this.
Not sure what that valve is. It's not a bleed valve nor a brake assembley.
Quote from: peterbylt on January 23, 2017, 08:05:43 AM
The Valve leaks all the time, as soon as air starts to build in the system it leaks.
The air comes out of the orifice that you see in the picture.
It comes out at a good enough rate that the bus can never reach full pressure.
It will get up to around 90 psi and I can release and use the brakes, but that's about it.
Peter
Pressures... below 35psi emergency brakes come on.
Below 65psi low air warning light and buzzer come on
Below 100psi do not move the vehicle.
At 125psi the governor cuts in to release pressure
At 150psi the emergency pressure release operates.
Oops... I left the lift switched on and that bleeds air if I try to move the bus. Had to switch it off quickly and started rolling with about 90psi (not good but I didn't think anybody would notice). By the time I was off the lot I was at 125 psi.
It sounds like you need a new valve. Check your air hoses too. They should not be cracked, frayed or excessively worn.
peterbylt, does the leak stop with the parking brake released?
No need to replace a valve that isn't broken, you need to diagnose whether it is the relay valve leaking, or that it is doing its job correctly and letting the leaked parking diaphragm air out via the service brake circuit.
Zephod, this is a DD3 system, not a spring brake system.
happy coaching!
buswarrior
Quote from: buswarrior on January 23, 2017, 10:05:50 AM
Zephod, this is a DD3 system, not a spring brake system.
yup old school, totally different animal ;)
Got over to the bus after work.
The Valve starts leaking as soon as there is air pressure in the system.
The valve leaks regardless of if the Parking Brake is released or not.
The leak stops when I step on the brake pedal and resumes when the pedal is released.
From the looks of it in the pictures and what I have read, I believe the Valve is Vertical R-12.
(https://busconversionmagazine.com/forum/proxy.php?request=http%3A%2F%2Fwww.peterbylt.com%2FMCI96A3%2Fleakyairvalve1.jpg&hash=4cbfaf35583c05617b4d628f8856d30ec6463afd)
Peter
Broken.
Change it out.
Be prepared to use the penetrating oil of your choice and leave it to soak in, both airline and bracket fasteners.
happy coaching!
buswarrior
"
"Broken.
Change it out.
Be prepared to use the penetrating oil of your choice and leave it to soak in, both airline and bracket fasteners."
That's what I am trying to say with my first post. Release all air pressure. Remove the circlip that you can see in the photo, and replace the plastic innards and o rings with ones from a new valve. Leave the old valve body in place. save yourself the trouble mentioned above. I got this trick from a long time bus mechanic who services hundreds of passenger coaches per year. Yeah, you end with a useless new valve body.
Could that valve effect the airbags and or possibly make the emergency brake pull lever have a red ring when it's engaged?
I have a different coach, but I have learned that, for me...It's all connected. If you have to change it anyway, change everything around it made of rubber more than 5 years old.
There is a problem with only changing the insides, and getting short cut advice from fleet mechanics.
Our buses are old, and often found in horrible conditions that a fleet mechanic has never seen.
If there is any corrosion inside that valve body, replacing the moving bits may not last long, if at all.
Compatibility between generations of valves can cause trouble too.
Some busnuts cannot afford $$$ to experiment, spending twice for the same problem.
Nothing is easy on an old, worn out, discarded commercial vehicle...!
happy coaching!
buswarrior
Quote from: RichardEntrekin on January 24, 2017, 05:07:06 AM
"Broken.
Change it out.
Be prepared to use the penetrating oil of your choice and leave it to soak in, both airline and bracket fasteners."
That's what I am trying to say with my first post. Release all air pressure. Remove the circlip that you can see in the photo, and replace the plastic innards and o rings with ones from a new valve. Leave the old valve body in place. save yourself the trouble mentioned above. I got this trick from a long time bus mechanic who services hundreds of passenger coaches per year. Yeah, you end with a useless new valve body.
Wow I just learned a new trick.
And yes Buswarrior has a good point too. However this time I am going to side with the Richard & fleet mechanic as IF while changing out the internal parts of the valve you carefully clean and inspect the the valve "body" itself all should be OK.
However if it does not work when done it's easy to go back remove the internals and replace the valve "body" with the new one with the internals re-installed and only a slight loss in time & labor.
Quote from: daddysgirl on January 24, 2017, 06:10:35 AM
Could that valve effect the airbags and or possibly make the emergency brake pull lever have a red ring when it's engaged?
I have a different coach, but I have learned that, for me...It's all connected. If you have to change it anyway, change everything around it made of rubber more than 5 years old.
I don't think it would have a DIRECT effect on the airbags. However the fact that it leaks to the point where he's not getting over 90 psi which is usually when the accessory systems start filling on most coaches that could in fact have an effect on the airbags and other systems since they are on the accessory side of the air system.
(clear as mud?)
;D BK ;D
BK: Clear as spring water...or possibly 40W ;D
But because this is an air valve, and given its location, wouldn't it be easier to replace it as a unit? Old one out...new one in?
I want to thank everyone for the Replies, advice and knowledge.
I will be replacing the entire valve.
I am not under any time constraint or in a compromising or emergency situation, would hate to drop and lose an o-ring or circle clip trying to save a few minutes.
Might as well take advantage of a completely new valve.
I just got off the phone with Luke from US Coach, a new R-12 relay valve has been ordered and is on its way.
Luke agreed with our assessment that the Relay valve is the issue.
Once I have this air leak repaired, I can start chasing down the air leaks that I can't hear.... :D
Peter
All good points on the merits of either approach. And thanks to everyone for keeping the differences in thought civil.
Quote from: RichardEntrekin on January 24, 2017, 05:07:06 AM
"Broken.
That's what I am trying to say with my first post. Release all air pressure. Remove the circlip that you can see in the photo, and replace the plastic innards and o rings with ones from a new valve. Leave the old valve body in place. save yourself the trouble mentioned above. I got this trick from a long time bus mechanic who services hundreds of passenger coaches per year. Yeah, you end with a useless new valve body.
uhm...more often than not the seats and valve body are worn too...
Good choice to replace the entire valve. The snap ring at the bottom will not get you to all of the moving parts anyway. The top cover must also be removed as that is where the large piston (with o ring around the outside) is located. That piston is what the control air pressure is applied on top of to push the piston down and open the supply air pressure port to the outlet ports. Essentially the same thing you do when you step on the treadle / brake valve with your foot. You step on the brake valve at the front and send control air to the relay valve, that in turn sends air to the brake chambers. :)
Quote from: daddysgirl on January 24, 2017, 10:33:05 AM
Clear as spring water...or possibly 40W ;D
But because this is an air valve, and given its location, wouldn't it be easier to replace it as a unit? Old one out...new one in?
the pain arises when one does not clearly note/know which fitting went where, what angle they need to be at, if the threads are deeper and the last 1/3 turn you need to clock the fitting is going to strip...or which ports are plugged.. then the doubts arise about sealant and someone saying Teflon tape is better than dope... the list goes on.... and on ... i am a doper with a bent for complete replacement including new fittings ( this leaves no doubt as to what went where...then there is the matter of replacing the hoses while i'm there.... :) ...using a minimum 40 lb. vice properly mounted to a 4x8 3/4 inch steel plate bench... :)
Quote from: buswarrior on January 23, 2017, 07:33:14 AM
Hang on.
This coach has DD3 parking brakes.
Remove nothing. Block the wheels so it won't roll.
Air up the coach.
RELEASE the parking brake, and see if the relay valve is leaking differently from with the parking brake APPLIED.
If a DD3 chamber has a bad parking diaphragm, it will leak when parked, and won't leak when released.
Jim Eh, yours works for spring brake chambers.
happy coaching!
buswarrior
Regardless if it is a spring brake chamber or a DD3 if the pushrod seals are leaking it will exhaust air out the service relay exhaust port! I said nothing about a leaking diaphram. If the diaphram is leaking it will exhaust air from the small holes in the pot housing and in the case of a spring brake, also out the caging bolt hole.
The test was if the air leaked only when the parking brake was released. I did however forget to pass on the obvious information about safety.