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Old 05-25-2015, 06:40 PM   #1
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Air/Hydraulic Brake Fluid Reservoirs

I have a 2002 Monaco Knight 36PST with Cummins ISC 315 eng. Air/hydraulic brakes. The master cylinders for the brakes look just like the one in my pickup except my pickup has a vacuum booster instead of an air booster. Question is, what is the hole in the top of the fluid reservoir cap/plug for? The hole is a standard 1/2-20 bolt thread. When I bought the mh the bottom reservoir cap had a plastic plug in it like you might see for shipping to keep the threads clean, the top reservoir had a paper towel stuck in the hole and was just wicking fluid out. I turned a couple plugs on the lathe, thread them and cut "O" ring grooves in the shoulders and screwed them in the holes, Is that going to hurt anything? I posted this same question on the Monaco forum last night at 9:30 but haven's seen any responses yet, lookers, but no reply. Any thoughts?
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Old 05-25-2015, 06:57 PM   #2
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Just a guess here. Maybe there's supposed to be a plug with a tiny hole in it to allow for the expansion/contraction of the fluid.
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Old 05-25-2015, 07:20 PM   #3
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On wheel loaders, there are lines on the caps, up to a remote reservoir. The reservoir cover, had a diaphragm under the cap, that followed the fluid level down, to keep moisture out.

If your reservoirs have diaphragms over the fluid, you should have vents in the caps.

In either case, if you seal the holes, the fluid won't be able to flow into the cylinder, as needed, just like a finger on top of a straw.

Sounds like something is amiss.
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Old 05-25-2015, 08:08 PM   #4
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There are no diaphragms in the caps. they look just like the cap on my boat trailer surge brake reservoir, and I'm pretty sure the fluid reservoir caps on the car dollies with surge brakes are the same, except no holes in the top of the caps. Solid cap with an "O" ring.
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Old 05-25-2015, 11:49 PM   #5
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Originally Posted by rv9apilot View Post
There are no diaphragms in the caps. they look just like the cap on my boat trailer surge brake reservoir, and I'm pretty sure the fluid reservoir caps on the car dollies with surge brakes are the same, except no holes in the top of the caps. Solid cap with an "O" ring.
rv9,
In my opinion, since you're messing with BRAKES, I'd definitely consult someone who's really familiar with this SPECIFIC SYSTEM. There's not many of those around and, the specifics of how they operate and, whether or not the master cylinders need to be vented or not, most likely can properly be answered by an expert that knows those specific systems. The main reason I emphasize contacting an expert that's qualified on YOUR SPECIFIC SYSTEM is because it's a very rare system, AIR OVER HYDRAULIC.

Many of us can guess on your theory on if or, if not those holes should be covered or not but, there may be some operational characteristics of that system, that is different then normal hydraulic brake systems. Good luck.

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Old 05-26-2015, 01:07 PM   #6
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Penkse diesel trucks also use air/hydraulic disc as you don't need an air brake endorsement to drive them, so they are around still. In fact they are now producing total air disc systems. My MH has the same system. Both cylinders are plumbed to the main reservoir which just has a lid on it to service fluid. I did not notice a separate plug or cap on the reservoir but will have another look. I have had to change one of the units out due to a leaking seal, a fairly easy job.


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Old 05-26-2015, 01:17 PM   #7
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Can you post a photograph?


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Old 05-26-2015, 04:21 PM   #8
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Fire Up, I drive part time for a charter bus company. Some of our shorter smaller buses have the air/hydraulic brakes while the larger busses (42' and 44') have air only. I was thinking about asking one of the mechanics we have about it. Fact, one of the mechanics came from a diesel shop that worked on motorhome.
MartinP, I'll see if I can figure out how to post photos.
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Old 05-26-2015, 05:22 PM   #9
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Photos of reservoir caps and homemade plugs.
Attached Thumbnails
Click image for larger version

Name:	Brake Reservoirs 001.jpg
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Size:	246.1 KB
ID:	95322   Click image for larger version

Name:	Brake Reservoirs 002.jpg
Views:	79
Size:	339.3 KB
ID:	95323  

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Old 05-26-2015, 07:44 PM   #10
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Fire Up, I drive part time for a charter bus company. Some of our shorter smaller buses have the air/hydraulic brakes while the larger busses (42' and 44') have air only. I was thinking about asking one of the mechanics we have about it. Fact, one of the mechanics came from a diesel shop that worked on motorhome.
MartinP, I'll see if I can figure out how to post photos.
Well Sir, as usual, I learn something every day. Can't remember what I learned yesterday but, no biggie. Since I only mess with motor homes, I was not aware of that type of system being used on other types of vehicles. Thanks for the heads up.
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Old 05-28-2015, 05:36 PM   #11
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rv9apilot,


Thanks for the photos, my setup is different due to the fact that where you have plugs on the top of your cylinders, I have a line that goes to a main reservoir from each unit. The reservoir has a cap that is removed for servicing fluid. I am guessing that those plugs are removed to service the fluid and are therefore vented. Hope this helps.


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