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Old 08-06-2015, 01:15 AM   #1
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Parking Brake Leak?

Sorry, I don't even know what to call it...

The yellow diamond you push / pull to engage or disengage the parking brake is connected to... what? What do you call the thing that actually engages the parking brake? It's not just a switch; it's not just a lever It's called a...?

Anyways, whatever it's called, mine is leaking. When I push to disengage the PB it hisses for a long time like air is bleeding out of it. And the rear gauge looks like it's maybe 5#'s low until it stops hissing - eventually - and then it fills up.

Is this a part I can replace myself? Is this something I have to have a shop do? If so, what do I tell them needs to be done?

Thanks!
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Old 08-06-2015, 02:33 AM   #2
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Quote:
Originally Posted by Wryly Blithe View Post
Sorry, I don't even know what to call it...

The yellow diamond you push / pull to engage or disengage the parking brake is connected to... what? What do you call the thing that actually engages the parking brake? It's not just a switch; it's not just a lever It's called a...?

Anyways, whatever it's called, mine is leaking. When I push to disengage the PB it hisses for a long time like air is bleeding out of it. And the rear gauge looks like it's maybe 5#'s low until it stops hissing - eventually - and then it fills up.

Is this a part I can replace myself? Is this something I have to have a shop do? If so, what do I tell them needs to be done?

Thanks!
The yellow diamond is the parking brake control valve plunger, you could probably replace the valve yourself, it's not complicated.
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Old 08-06-2015, 05:44 PM   #3
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Yes you should be able to replace yourself. Take lots of pictures and mark the wires and air lines as to location. There should be a manufacturer and part number on the valve. Do an internet search and you should be able to find. You might also try taking it to a local truck shop, might get lucky as most of these are pretty common.
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Old 08-06-2015, 08:22 PM   #4
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CONTROL VALVE PLUNGER! That's exactly what I was looking for rivraton, thank you! I didn't even know what to search for, and I sure as hell wasn't calling up my local shop and asking them if they had any parking brake thinga-ma-jiggies laying around

Thanks to both of you for all the information!

Now, let's just hope it was the plunger and not one of the lines feeding it!
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Old 08-06-2015, 09:18 PM   #5
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Sorry, I don't even know what to call it...

The yellow diamond you push / pull to engage or disengage the parking brake is connected to... what? What do you call the thing that actually engages the parking brake? It's not just a switch; it's not just a lever It's called a...?

Anyways, whatever it's called, mine is leaking. When I push to disengage the PB it hisses for a long time like air is bleeding out of it. And the rear gauge looks like it's maybe 5#'s low until it stops hissing - eventually - and then it fills up.

Is this a part I can replace myself? Is this something I have to have a shop do? If so, what do I tell them needs to be done?

Thanks!
Sir,
You've been given good information and, even a picture of the mechanism you MAY or may not have to replace. That valve is basically a very simply valve mechanism. If you don't know how the parking brake system works, I'll give you the ultra short version.

Obviously, your rear brakes are your parking brakes. They are the same exact brakes you use when you step on the brake pedal. The main difference in operation between the two is, the service brake system uses AIR PRESSURE from the air system, to apply the brakes. The parking brake systems, uses SPRINGS and LACK OF AIR PRESSURE to apply the parking brakes.

When you pull that yellow button, you're simply releasing the "Stored" air pressure in those rear brake cans, and effectively, letting those rather large and seriously STRONG SPRINGS apply those rear brakes. If you knew all this, sorry for the non-needed explanation.
Scott
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Old 08-06-2015, 11:47 PM   #6
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Sir,
You've been given good information and, even a picture of the mechanism you MAY or may not have to replace. That valve is basically a very simply valve mechanism. If you don't know how the parking brake system works, I'll give you the ultra short version.

Obviously, your rear brakes are your parking brakes. They are the same exact brakes you use when you step on the brake pedal. The main difference in operation between the two is, the service brake system uses AIR PRESSURE from the air system, to apply the brakes. The parking brake systems, uses SPRINGS and LACK OF AIR PRESSURE to apply the parking brakes.

When you pull that yellow button, you're simply releasing the "Stored" air pressure in those rear brake cans, and effectively, letting those rather large and seriously STRONG SPRINGS apply those rear brakes. If you knew all this, sorry for the non-needed explanation.
Scott
@Fire Up Thank you - I actually didn't know that. I mean, I knew some of it, like the rears are the parking brakes / service brakes, etc., but I didn't know that it was a release of pressure that allowed the springs to engage the brakes. That makes a lot of sense, and I really appreciate the info.

But now I'm having a little trouble wrapping my brain around the symptoms. When I push in the plunger, what used to be a short blast of air (which I now know is a pressure release) it's a much longer hissing sound. So, the pressure is still being released, I suppose, just not as quickly? So... instead of a leak, there's a blockage maybe? That doesn't make sense.

Excuse me just a moment... I'm going to go get a lot smarter and then come back later with some intelligent questions
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Old 08-07-2015, 12:10 AM   #7
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OK , thanks for the description of the change in the rate of exhausted pressure from the park brake system.
There may be an internal blockage in the valve, cause by a problem with the seals in the valve.
A question though. Has any work been done in the dash area around the valve? Radio work? Anything? Because something might have been placed in front of the valve's exhaust port , even the wiring harness in the wrong place could slow thing up. Even work under the coach that may have pinched an air supply line from the valve to the brake chambers, causing slow return to the valve from the chambers.
NOTE: Until you have things operating properly be cautious , parking because until all the air escape stops , the park brake isn't fully applied.
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Old 08-07-2015, 12:34 AM   #8
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NOTE: Until you have things operating properly be cautious , parking because until all the air escape stops , the park brake isn't fully applied.
Yep, very true!
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Old 08-07-2015, 08:21 AM   #9
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The valve is supposed to release air when you pull the knob and not release air when you push on it. Pushing the knob sends air to the parking brake, thereby releasing the brake. Replacing the value will probably fix your problem.
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Old 08-07-2015, 08:41 AM   #10
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Replacing the value will probably fix your problem.
I can tell you from personal experience that this may or may not fix his problem. Last year I had a problem that sounded just like the OP's, so I replaced the yellow button plunger valve thingy! Guess what, it didn't solve the problem! The root cause of my issue turned out to be a parking brake air actuator that was slightly out of alignment which meant that the "pushrod" that applied the brake was binding.

Air brake systems often consist of the yellow button valve, a relay valve and one or more actuators. Just because you hear a long hiss from the inside valve doesn't mean that there isn't a problem with a different component.
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Old 08-07-2015, 10:30 AM   #11
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@Skip426 - No work done recently (or ever, as far as I know) in, under or near the dash. There was extensive generator work done where it was removed and replaced at least twice, maybe the boned something up in the gen bay. I'll take a look, but that was weeks ago and this issue just popped up last week.

And not to worry - the issue isn't when applying the brakes; that part works fine. It's when I'm releasing the brakes that there's the longer than normal hiss.

@ChasA - I think you're right... I'm going to definitely start with that valve. Something Skip426 said triggered a memory for me: I can change the sound of the hiss by turning the yellow knob. I'm sorry I didn't mention this before, I only just remembered it just now. That's gotta mean a bad plunger valve... it's right there, it changes pitch when turned... yeah, that's it.

@Docj - I hear ya, and you may very well be right, but I gotta start somewhere and the plunger valve is the logical place. I hope it doesn't turn out to be what you went through, I got places to go!
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Old 08-07-2015, 10:51 AM   #12
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WB,

Before buying a new control valve, you have to verify your spring brake relay valve is not sticking.

You'll have to chock your wheels, and not be on a hill, then have an assistant release your parking brake with the engine off as you are under the coach, and in front of the that relay valve.

You will know right away what valve that is when it continues to leak for a few moments. Also to find it, follow the lines from the brake air chambers (emer) side to the valve. it should be in the center of the axle, mounted on the frame.

To clarify that, the (emer) side of the chamber is the parking brake side, the (serv) air line side is your service brakes.

This valve can be disassembled, cleaned, lubed, and reused. But since you are not experienced at this type of work, I would strongly suggest replacing the valve with a new one.

Use teflon tape on all the cleaned fittings when you put this back together. These are all pipe thread, you can use the liquid thread sealer if you prefered.

I can't load the diagram so I hope that helps.
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Old 08-07-2015, 10:54 AM   #13
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When you PUSH-IN the knob to release the parking brakes you will get a drop in air pressure because the valve is sending air to the rear spring brake to release the parking brake so the drop in pressure when releasing it is normal.
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Old 08-07-2015, 12:08 PM   #14
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WB,

Before buying a new control valve, you have to verify your spring brake relay valve is not sticking.

You'll have to chock your wheels, and not be on a hill, then have an assistant release your parking brake with the engine off as you are under the coach, and in front of the that relay valve.

You will know right away what valve that is when it continues to leak for a few moments. Also to find it, follow the lines from the brake air chambers (emer) side to the valve. it should be in the center of the axle, mounted on the frame.

To clarify that, the (emer) side of the chamber is the parking brake side, the (serv) air line side is your service brakes.

This valve can be disassembled, cleaned, lubed, and reused. But since you are not experienced at this type of work, I would strongly suggest replacing the valve with a new one.

Use teflon tape on all the cleaned fittings when you put this back together. These are all pipe thread, you can use the liquid thread sealer if you prefered.

I can't load the diagram so I hope that helps.
Hmmm... that's an interesting idea, and you're right - I should check it out. But let me ask: If this were the problem, would I hear the longer-than-normal hissing in the cab right behind plunger in the dash? That's definitely where the hissing sound is coming from, so just curious. But thanks for the info - I love learning new stuff everyday!
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