Air leaking from front when brakes applied

[Don Bradner]

I think you would find that, if you blocked that hose, you couldn't set the
parking brake at all.

I know just enough about this to be dangerous, so take this as a general thought
rather than expert opinion. It is based on having replaced my parking valve and
looking at the drawiings of how it works.

There are 3 ports on the parking valve. Air source, Air delivery, and Air
exhaust. Its "normal" at rest position (pushed by a spring) would have air
running from source to delivery and spring brake applied.

When you push in and overcome that spring, air pressure keeps it closed with air
running from source to delivery, which holds the spring brake off. If air
pressure drops below the spring value, the switch pops to its resting position.

When the air is allowed to exhaust, another valve in the system opens, called a
Quick Release valve. The line running to the dash is not large enough to truly
exhaust the air quickly from the chambers - that happens at the QR valve.

When you step on the brakes, I'm guessing the QR-valve expels additional air and
you get expelling air out to the exhaust.

When you block the exhaust, the air now coming in opens the valve allowing air
to pass to the parking brake, thus releasing it.

On my coach, the exhaust line runs out through the dash, and I did not trace it,
but it makes sense that it would go down somewhere in front. This line is not
required, but otherwise you get more air noise in the cab.

On 11/3/2009 at 4:26 PM P & D wrote:

>That is what it is doing, parking brake on and full of air, when I step
>on brake pedal, I hear air expelling from hose. I had never heard that
>before (it probably was doing it)
>
>
>
>I did not run air down to see if it stopped, but if I turn park brake off
>(push knob in) then there is no air expelled from hose.
>
>
>
>I am starting to think that is normal?