Need help identifying an air valve.

peterbylt · January 22, 2017, 02:55:59 PM

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?

Peter

RichardEntrekin · January 22, 2017, 04:53:48 PM

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.

Busn-Gramps · January 23, 2017, 06:16:32 AM

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.

Jim Eh. · January 23, 2017, 06:49:34 AM

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.

buswarrior · 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

peterbylt · 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

Zephod · January 23, 2017, 08:09:32 AM

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.

Zephod · January 23, 2017, 08:18:20 AM

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.

buswarrior · January 23, 2017, 10:05:50 AM

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

eagle19952 · January 23, 2017, 12:05:58 PM

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

peterbylt · January 23, 2017, 04:52:29 PM

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.

Peter

buswarrior · January 23, 2017, 11:45:32 PM

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

RichardEntrekin · January 24, 2017, 05:05:10 AM

"

RichardEntrekin · 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.

daddysgirl · 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.