A Brake Diaphram

The front brakes on ALL prevo are standard non maxi single diaphragm brake chamber. Under normal braking they may see 10 or 15 pounds of air pressure, minimal. If it should leak it will only leak when you are using the brakes.

I have never looked here and had no idea if these were original or not so I opened them up. Good thing I did.

Everything is pretty crusty on the surface but very clean on the inside and will easily provide many more years of service with simply a new pancake and rubber brake line.

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You have to check the return spring behind the plunger to make sure it is not overly corroded. This one was fine.
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This is not new but the inside of the original chamber.
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Great photos and post Joe!

Great post Joe. This points out the fact that when updating chassis components there is no real need to install brand new brake chambers. For the hands on folks, they come apart easily and are easily overhauled. One must be very careful however when disassembling the emergency dual brake chambers.

Great post, Joe.

But I gotta ask.....

Which job is dirtier.....tearin' em apart to re-build?..... or replacin' em? :D

Paul, Paul, Paul.

I can't believe you even asked that question.

I could not have been more surprised to crawl under what I defined as one of the best looking and well kept Liberty's in the club (yours), only to emerge dirtier than I have ever gotten on anyone's yet,and 2 days in a row.

Then to add insult to injury you give me hand cleaner with enough grit to rip the first 2 layers of skin off prepping me for the burn of the gasoline you then handed me, and we hear complaints about water-boarding, sheesh.

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I could not get this dirty working on our bus if I tried and it is 15 years older than yours.

In answer to your question I guess it depends on whos I'm working on:o

I hear ya, Joe....

Was pretty surprized myself to see how much grime one (apparently clean) bus can deposit on a guy. I think the Liberty just enjoyed your touch so much she insisted on leaving her mark :p

It was fun though.......:D

If you do decide to just replace a diaphragm NEVER NEVER remove the clamp on a chamber with 2 sets of them.

NEVER .

Putting it another way if the chamber has only 1 air line going to it you are safe, If it has 2 air lines going to the chamber LEAVE IT TO SOMEONE WHO IS SURE WHAT THEY ARE DOING. If you pull the wrong clamps on a chamber with 2 sets of clamps it could kill you.

Here is all it takes to relieve the tension from the spring on that SERVICE BRAKE diaphragm so you can switch it.

I have finished up with the steer axle and I neglected showing some important steps that I can demonstrate with the tag.

Simultaneously take a large screwdriver shove it in the cleaves of the slack adjuster and pull back with one hand (just like you were checking slack) and with your other hand clamp the push rod with some vice grips right at the housing so it can't spring back
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Now when the 2 clamps are removed the tension is gone.

The 2 diaphragms on the steer were dated 1981 so that means they were not only original but sat on a shelf for 4 years before they were used. The new ones I'm installing were manufactured in 07. No good Mr. Parts guy, no good. I expect my parts deli fresh, thank you.

Joe has spoken the gospel with respect to monkeying around with combo brake chambers. The spring brake portion of those brakes are under considerable pressure when not caged. However, only the older combo brakes can be serviced anymore. The spring brake portion (the emergency brake part of the combo) is permanently assembled on newer models. I don't know when the change was made but they cannot be disassembled.

I have had a brake diaphragm failure and it was not from an aging rubber that cracked and failed, but instead was the result of a poor finish on the head of the plunger that the diaphragm pushes against. Apparently from driving in the snow belt the corrosion caused by all the salt created a rough surface that wore through the rubber creating a weak spot that failed. The failure was on the emergency brake diaphragm, and at the time I was able to rebuild the brake as Joe showed, but the spring had been caged using the T bolt supplied with the brake.

It is not the age of the diaphragm that is likely to be the problem, but the quality of the inside of the chamber. If there is any corrosion, change the chamber, not the diaphragm.

Back in about 1962 I was pulling a set of doubles, hauling tomatoes out of the Chico Bean Growers in Northern, CA. I was a real green truck driver and when I found a brake pot on the rear axle of my rear trailer leaking I called my boss, the owner of the truck. He instructed me, by phone, to replace the diaphragm. His instructions were not that clear and did not have the warning that you guys are rightly giving readers of this forum.

I released the air pressure to the trailer brake and replaced the rubber diaphragm. The cap was held on with a two-bolt clamp. After I charged the lines by moving the trailer brake lever full on. (that was close to 120psi) I checked the device for leaks. There was just a whisper of air so instead of draining the air lines I proceeded to use a wrench to tighten the charged pot clamp. You guessed it! I found myself 20-25 ft from the trailer in dust about 6 inches deep with my forehead split wide open and blood everywhere. When I put my hand to the wound all I could feel is a big pulpy mass which I actually thought was my brains. Some people put me in an old pickup and drove me, while I was holding my 'brains' inside my head, to the Chico Hospital. It turned out that my "brains" were just a pulpy mass of flesh where the clamp hit and and ripped everything off to my skull. After a lot of cleaning my forehead skin was reattached and stitched back in place. An xray revealed no fracture and, what the doctor said, one of the thickest skulls he had ever seen.

After a couple of weeks of rehab I was released back to work. Because the stitches were put in properly I never needed plastic surgery.

I know all of the jokes this little story will generate but I can tell you the best outcome. I have a tremendous respect for anything air-powered and assure you that there is NEVER a short cut when working on this equipment. Next time you see me check out the scar on my forehead that runs from the top of my nose to the hairline. I am truly lucky to be alive.
Mind all the good advice given in this forum about safety. We don't need any more statistics.

Darl,


When we were parked next to you at POG III in Kerrville Martha told me that you had a hard head. This story just proves that she knows you like a book.

Just kidding; I couldn't resist!:D

Thanks for the post: it just may save someone's life!

I don't think anybody can paint a clearer picture than Darl just did. Our coaches have to be respected and I can think of a number of things as owners who work on them we have to be aware of.

Obviously spring brakes must be respected. But if you are contemplating getting under the coach make absolutely certain it is properly supported under the chassis support points. It doesn't matter if you are merely going to be under it for a second or two. Don't do it without the coach being supported.

Stay away from exposed belts when the engine is running. You will not stand a chance if you wear loose clothes and get caught in a belt.

Don't under any circumstances work on an inverter unless you are 100% sure you have removed not only 120V power, but the 12 or 24 volt circuit to the batteries has been disconnected.

I'm sure others can suggest more things to be aware of, but those leap out at me as biggies.

I just stumbled across this thread looking for an oldy to revisit. He is lucky he didn't kill himself.

Let's go deeper.

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A modern maxi looks like this. 1 clamp that's removable for the service diaphragm and 1 that's not for the bigger emergency brake spring I'm pointing.

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Here is an old style where both chambers and diaphragms can be opened and repaired.

The main spring can kill u back when there where bad consiquenses for stupid decisions.

I've seen this type on Prevo and I'm quite confident there r still a few out there on late 80s stuff.

I will rebuild these, both ends, when I see them, because u can for less than 20 bucks instead of replacement.

I'll open one up and uncage it open far enough so u can see the difference between the main spring and the service brake spring.

Let's have a caging a chamber and removing a diaphragm 101

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Here is the caging tool and where it goes. It gets twisted after inserted then tightened all the way down.

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2 good tips here. First if u release the emergency brake first air pressure does most of the work u only have to crank down the last few turns manually. Also the threads go deeper than a deep well will go and this rachet with hollow sockets works great here.

Only half way there.

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When u first start backing the brake off by depressing the lock ring on the 9/16 adjuster nut the slack does not move. The shaft turns only.

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When the cams, not visible, bottom out on the brake shoe rollers, not visible, u will feel that for sure. Then keep going and the slack adjuster will move outward.

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The slack pulls the pushrod that squeezes the smaller service brake spring and when u install the new diaphragm there is no struggle.



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Another thing. Only between the clamps is there 2 seporate pressurized chambers. The emergency half that is energized only when u release the parking brake and the service side that is only energized when u r using the brakes for stopping. Outside the 2 diaphragms and clamps is open to atmosphere. If your unfamiliar with chambers and u want to verify what line is service and what is emergency the ports r not lined up and clearly visible what line is going to what half the chamber. In the photo the left line is feeding the serice brake side and the right one is emergency air.

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Now I've removed the service brake clamp and with a tap the 2 pieces seporate and the diaphragm comes out.

In the photo I've turned the slack adjuster back to release the service brake sprint so u can see it.

Nother good tip. If u don't hold the spring down by backing the slack out first u r faced with trying to push it back during diaphragm instalation and it's much more difficult. When u push on this plunger u r not just compressing IT'S spring u r also having to overcome the springs that hold the brake shoes to the rollers. It's doable but unnessessary, do it my way.

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Now here is the emergency half opened up, again these r the old style very dangerous I'm showing this because I thought u might want to see and I know what I'm doing.

If u zoom in the middle u can see how the cage tool locks in from the back side.

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Here is the emergency half open with the diaphragm no longer present and I backed the cage bolt out as far as I could so that main spring is exposed.

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Before u loosen these clamps note where the nuts r so u know how tight is tight. I take only 1 turn from 1 nut and the other completely off and it comes free. This makes reassembly better.

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So y put the diaphragm up and rest the clamps like in the photo before lifting the chamber back up.

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Then put the chamber up with 1 hand and bring the clamp into place with your other hand. Mild blows with a hammer while squeezing the open end of the clamp brings the 2 half's close enough togeather to get the nut and bolt started.

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2 last tips first before u go for the slack adjuster and release its return spring and adjust the brake, it's a good time to grease the s cam shaft. U got it almost 180 from where it normally sits when the slack is backed out. I'm pointing to that grease fitting for the S cam shaft.

Now go back to the adjuster on the slack and let the slack walk back, when it stops moving continue on the adjuster until u then begin to push the shoes outward and tighten down till the adjuster nut stops. If tight u cant turn the brake drum at that point u r bottomwd out the brake is tight . Then about a 1/4 turn back from tight is where I put the 9/16 bolt for the slack. pull the slack to check it. Make slight adjustments to feel if nessessary.

The less slack the better, however if u want them all equal and if 1 hub requires additional slack to release the drum sufficiently the rest need to be loosened to match its slack.

And that's it. How to cage a brake replace a diaphragm and a look at a old style chamber and it's main spring exposed.

Where else do u get this stuff?

Joe knows his stuff.

Holy Moly! I was just doing some research and I am really happy I checked out this thread! I had no idea of the danger and certainly won’t attempt this on my own.... so, I guess this really does make my phone call to Joe Camper all the more valid!