3 Air Valves

[Steve Frazier]

I recently saw a photo on Facebook where a tractor had 3 air valves on the dash, the traditional yellow and red but with a third blue one. I never drove a truck set up like this. When I asked what the third valve was for the answers weren't making sense to me. Can anyone explain what the blue valve is for? Thanks!

 

[doug dirt]

blue air valve is parking brake for tractor/truck only

 

[Truck Shop]

Yellow button controls both tractor & trailer, red is trailer charge, blue is as said tractor parking.
But that system was simplified with the MV3 two button yellow/red in mid eighties. The first
MV3's were a bolt together white plastic valve that had several issues such as warpage between
the two halves, incessive leaking was a common issue, doubt there are any left on the road that
still have the white plastic valve. The MV3 can have either button pushed in while one is out,
but pulling the yellow automatically trips the red also, but yellow can be pulled while holding
red button in and vis/verse. The blue was liked for one reason for fact tractor brakes could be
released while the red button was pulled setting trailer brakes only, allowing brakes on tractor
to not freeze in winter parking. Below is a Bendix Test that describes what MV3 does and doesn't
do, plus the often confused trip pressures that are or aren't regulated by Federal DOT Motor Carrier
rules.
*
https://www.truckpartsandservice.co...endix-tech-tips-on-dash-valves-trip-pressures

 

[Mr. Wrench]

I have never seen a truck with the Blue air valve. I read about them in a Heavy Duty Truck repair manual but it wasn't real clear to me. So it sounds like, pull the red knob sets the trailer brakes, pull the yellow knob, that sets the trailer and tractor brakes, pull the blue knob and that sets just the tractor brakes. Is that right? They must not use that setup anymore. What time frame did they use it or was it just an option?

 

[Truck Shop]

I have never seen a truck with the Blue air valve.

Depending on your age probably not. The three button went out totally in 1984. Used from roughly
1968 to 84. Note-it was also used as a emergency release for tractor brakes on some brands by
holding in. Not all were plumbed the same or not all had a tank designated as emergency air, some
did some didn't. Air brake diagrams are generic when related to that.

 

[cuttin edge]

Yellow button controls both tractor & trailer, red is trailer charge, blue is as said tractor parking.
But that system was simplified with the MV3 two button yellow/red in mid eighties. The first
MV3's were a bolt together white plastic valve that had several issues such as warpage between
the two halves, incessive leaking was a common issue, doubt there are any left on the road that
still have the white plastic valve. The MV3 can have either button pushed in while one is out,
but pulling the yellow automatically trips the red also, but yellow can be pulled while holding
red button in and vis/verse. The blue was liked for one reason for fact tractor brakes could be
released while the red button was pulled setting trailer brakes only, allowing brakes on tractor
to not freeze in winter parking. Below is a Bendix Test that describes what MV3 does and doesn't
do, plus the often confused trip pressures that are or aren't regulated by Federal DOT Motor Carrier
rules.
*
https://www.truckpartsandservice.co...endix-tech-tips-on-dash-valves-trip-pressures

So does the air for your trailer service line go from the treadle valve go through red button valve, or does pulling the red button trigger another valve like tractor protection

 

[Shimmy1]

So does the air for your trailer service line go from the treadle valve go through red button valve, or does pulling the red button trigger another valve like tractor protection

Kind of a combination on ones I've worked on. The tractor protection valve is the beginning. It has it's own air supply. When you press the red button in, that sends air to the TPV, that signal tells it to open and send air to the emergency line to release brakes on trailer. That signal also allows the service side to function. The treadle valve (and Johnson bar, if equipped) send air to the TPV, which in turn tells the TPV to send air to the service brakes.

 

[masterwelder]

My 2000 RD Mack tractor had the blue. I used it all the time with the lowboy trailer.

You do need both yellow and blue charged to bobtail and obviously all three when hooked to a trailer.

Pulling the blue only will not cause the yellow, red or both to automatically pop out.

 

[cuttin edge]

My 2000 RD Mack tractor had the blue. I used it all the time with the lowboy trailer.

You do need both yellow and blue charged to bobtail and obviously all three when hooked to a trailer.

Pulling the blue only will not cause the yellow, red or both to automatically pop out.



View attachment 354875

I ran a 1990 mack. Got it brand new. It was just a straight truck but it came plumbed as a tractor. The air lines were plugged at the tpv. The blue button did nothing, I don't think it was hooked up. You could push the red button and it had air, but obviously did nothing. But it was the only straight trucks I have ever driven where the spike was hooked to the truck brakes. Obviously it had been hooked up that way, red button had no effect on it. It was great for dumping, or pulling out on a hill

 

[Truck Shop]

So does the air for your trailer service line go from the treadle valve go through red button valve, or does pulling the red button trigger another valve like tractor protection

Note the diagram below, which shows three button & two button valves. Red is supply
through TP valve, Unless it also happens to have a TP valve with BTP for a BTP R14
service brake valve on tractor.
*

 

[Truck Shop]

My 2000 RD Mack tractor had the blue

And I forgot Mack continued the use of PP1 valves long after the rest quit using.
A friend has a 93 Super Liner with three PP1's.

 

[Truck Shop]

This diagram below shows a better image with type valves listed, in it shows a shunt line between
the PP7 trailer charge valve and PP1 tractor park valve. When trailer PP7 valve is pushed it puts
air through shunt to render tractor park valve useless, when trailer park valve is out trailer brakes
set or no trailer at all then the tractor park valve is effective. The System Park operates no matter.
The reason for a two button MV3 valve was three valve system was redundant because it is arranged
so both can be operated independent of one another. This diagram below is the standard for use
of a three button system although depending on how factory plumbed it could be different from
brand to brand.
*

 

[Steve Frazier]

Was it a regional thing too? I've been in NY all my life and was driving from the late 70s to 2000 and never ran into it here. I drove trucks from the early 60s up through new.

 

[Truck Shop]

Was it a regional thing too? I've been in NY all my life and was driving from the late 70s to 2000 and never ran into it here. I drove trucks from the early 60s up through new.

No it wasn't regional, most all brands switched to the MV3 simply because it became a integral
one piece valve that took up less space and less plumbing. Early designs were the common
flipper type TW-1, TW-4 or 11, with nothing more than a tractor/trailer charge valve using a PP1.
TW-1 below.
*

 

[Truck Shop]

One of the reasons for brands to stop using the 3 dash valve yellow, red, blue design
was the problem of drivers setting brakes with only the blue tractor park brake valve
while both red & yellow were still pushed in keeping system charged. As described
above the red valve trailer charge needs to be pulled in order for blue tractor park to
work, because in the bottom of PP7 valve is a one way shuttle. Brakes were to always
be set using the yellow system charge valve, which set parking brakes no matter what.
There had been instances of new/rookie drivers only pulling the blue button, which
depending on how plumbed never set any brakes with red valve still pushed in charging.
Red had to be pulled in order for blue to work properly, if study diagram above it will
be obvious.

 

[masterwelder]

Was it a regional thing too? I've been in NY all my life and was driving from the late 70s to 2000 and never ran into it here. I drove trucks from the early 60s up through new.

Im not sure about other Mfg's but all the Mack's set up as tractors or straight trucks specd with trailer supply, R's F's DM's U's etc had the blue valve. 70's through 2000's or discontinued the model.

 

[525isx]

i'll throw in some more confusion- I ordered my either my '93 or '96 pete with a black push valve "push to charge trailer while parked" in addition to yellow & red

 

[Truck Shop]

i'll throw in some more confusion- I ordered my either my '93 or '96 pete with a black push valve "push to charge trailer while parked" in addition to yellow & red

Totally not surprised, in those days special items could still be incorporated without dealer fuss.
*
Mack used the three button dash air brake control valves up until 1-15-2008. Federal DOT put
a stop to as of 1-15-2008. Did not meet regulations they finally decided 24 years after MV3 was
designed.
*
Below is probably the only current braking system incorporated in a 1960 B Model Mack.
Two button MV3 dash valves, aluminum air manifold in bulkhead, E6 dual control treadle
valve, with R12 relay valve using two QR1 release valves for quick exhaust and anti-compound-
ing with park brake applied. Anything can be updated.
*

 

[masterwelder]

I would like to see the finished overall pic of that B. Looks outstanding.

 

[Truck Shop]

I would like to see the finished overall pic of that B. Looks outstanding.

My Last One-Finished

After two and a half years it's finally done. Many hours of work and aggravation went into this Mack. But it was something I had to do-My Last One. Would I do it over again? I can't answer that right now. Was it worth it? Yeah it was worth it to me. Did I learn anything along the way? Of coarse...

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