Excessive brake pedal effort required

[Vanwill]

Well, guys, I have finally finished all four wheels of my project. Although I had new front drums and shoes and new camshaft bushings/seals before I finished, I decided to do NOTHING but clean up the pivot points on the shoes, S-cam rollers, and anchor pins. I did this to try to evaluate how much improvement might be available by a simple "clean and lubricate" maintenance. The "lubrication" consists of very sparingly applying anti-seize compound to the anchor pins and S-cam roller axles.

The rear brakes were a bit more difficult just due to their size and weight. The wheel/tire assembly weighs no more than the front (although there are two of them), but the rear drums are much larger, weighing 132# each. Even the rear drum came off without significant difficulty, after backing off the slacks.

The front brakes were 16.5" diameter X 5" wide. The rears were same diameter, but 7" wide. That surprised me, but should not have. The rear axle has approximately double the weight of the front and double the tire contact patch. The rear drums were in excellent condition with only minor heat-check cracks. The linings were considerably more worn than the front, but there was still nearly 1/2" of lining left. There was a TON of brake dust inside the drums and on the skeletonized backing plates (proper name--"spider"). The S-cam rollers seemed to have been rotating (as opposed to the front ones which were "skidding" on the S-cams) and nothing seemed seriously amiss. I did find an inner rear hub seal in its earliest beginnings of "weeping", but no significant gear oil had escaped--only enough to warn of its impending demise. I've ordered the seals and will replace them both when I pull things down again--might wait for cooler weather, but definitely want to get it all done before the Alaska trip in the spring.

Road test--Although the improvement from cleaning and lubrication of the rear brakes was not as great as the fronts, it was unquestionably significant. There is MUCH less pedal effort required to produce even severe braking. And the total amount of "braking power" is MUCH increased, although I have (purposely) not locked up a wheel as Roy did. After my second time of severe braking, I got a "Low Oil Pressure" alarm (oil sloshing to the front of the oil pan and uncovering the pickup). That surprised me, since the oil level is not down even two quarts (approximately 22 quart capacity). OTOH, the braking I was able to achieve without even hitting the "hard stop" in the treadle valve was much more than "bottoming out" had provided previously--even with a manual adjustment of the slacks down to 1". But after getting that same alarm again when I applied severe braking, I decided to wait until after my almost-due oil change before trying it again. However, I have achieved what I set out to do--HUGE reduction in pedal pressure to achieve severe braking.

This will surely mean that I will add the "clean/lubricate brakes" to my maintenance schedule. I'm not sure how often this should be checked. I will probably do it again in two years, OR at any indication of increased pedal pressure required to achieve reasonable braking. And if you ever hear a loud "pop" or "creak" when you brake hard, that almost surely means you are hearing your S-cams skidding on rollers that are not rotating. This apparently decreases braking efficiency tremendously.

This is a job I would like to have left to a heavy truck shop. But I don't think there is any chance they would have done it as diligently as I did--polishing all the rotating points, checking air pressure at the individual air chambers, and examining everything for possible problems. BTW, the new shoes/linings have roller and anchor pins surfaces MUCH rougher than the surfaces I polished on my existing shoes. I don't think any truck shop is going to polish those surfaces before installing them. Without doing that, the life of the anti-seize seems like it would be seriously reduced.

I'm now satisfied the braking is at least as good as it was when new. Although it may be a few weeks before I try to skid a wheel as Roy did, I feel certain just from the "seat of the pants" feel that it will do so. I will also install the new drums and shoes/linings on the front when I pull it all down to replace the S-camshaft bushings and seals, and let you know if it made any significant difference.

[Vanwill]

Originally Posted by twinboat

That statement is not true. As the slack moves past 90 degrees, of the rod, the force decreases, due to the change in leverage.

This is why, at rest, the slack should be slightly less then parallel to the flat face of the chamber, giving the max leverage at 90 degrees. ( The reason for the 1 inch travel requirement )

If you investigate, replacing brake chambers, they explain how to set the rod length. Many chambers come with extra long rods, that need to be cut.

Twinboat,

The source you cited gives an explanation that is THEORETICALLY correct. It is a common explanation on many sites on the Internet. Here is the reason I make the statement that WITHIN THE SAFE OPERATIONAL LIMITS of a brake chamber, unusually short air chamber strokes have no PRACTICAL effect. Like you, I have read that explanation on various sites. But my personal experience with two Dynasty's--a 1993 with manual slacks and my current 2000 with auto-slacks--has been that no amount of shortening the air chamber piston travel will have any effect on braking power whatsoever, AS LONG AS YOU ARE WITHIN THE SAFE OPERATING LIMITS OF THE AIR CHAMBER.

It is true that as you approach the limits of the chamber's travel, the force available from the chamber diminishes. But that is only when you are PAST the DOT-compliant 2" of travel (on a Type 30 chamber). If you are within the 2" limit of travel, there is full chamber force available. So, the diminished force available from the chamber only applies if your slacks are VERY out of adjustment and your air chamber is beyond its 2" rated safe travel.

The reason I've always seen for adjusting the slacks to less than 1" of travel is the one you cited--that leverage is lost due to the angularity. That is indeed true. But look at the attached sketch. For a Type 30 cylinder connected to a Haldex auto-slack, even when the chamber stroke is nearly out of DOT-compliance, the reduction in leverage is less than 2%. I don't think even the most sensitive among us can detect a decrease in braking force that small.

That being said, it is possible that SOME OTHER FACTOR is involved that might indeed reduce braking power when the slacks allow travel beyond 1". Since the Holy Grail of 1" is cited so often, I've tried to come up with some explanation for why it would give more effective braking, but I've not been able to think of anything. Even if one were to assume that the air chamber's output diminished with angularity (it does not because it is a flexible diaphragm), the most it could possibly be would be the same reduction that comes from the slack's angularity. So, if one doubled the less-than-2%, you still have less than 4% reduction. And possibly the S-cam ramps could be non-linear, but that would seem unlikely for a variety of reasons.

I never ignore the experiences of folks whose judgment I know to be sound, even when it does not agree with what I've experimentally verified on my own, or what SEEMS to be supported by irrefutable Newtonian mechanics. So, if others of you have found that adjusting slacks to less than 1" travel causes an unquestionable increase in braking power, please share your experiences--with all relevant details, of course.

[Vanwill]

Because the LF brake drum of my 2000 Dynasty showed a very slight uneven, ridged wear pattern, I ordered two new drums and new shoes/linings. When the drums arrived, I was sure I had been sent the wrong ones. Although all dimensions of the drums looked OK, the (10) holes for the studs were over 1.25 diameter. My studs were 22MM, and the holes in my original drums were .937 dia.

When I called my supplier, he sent me info from Webb (possibly the largest USA manufacturer of heavy truck brake drums). Long story short--Webb says that they have consolidated three part numbers into one, by putting the largest hole required by any of the three individual part numbers into the drum and saying that it now fits all three. Webb supplied their engineering analysis of why the clamping force of the studs locked the drum from slipping on the hub assembly, and it is sound. They claim they have not had even one problem with that change in several years. Full disclosure--I'm STILL going to machine a stainless steel bushing for one stud on each wheel, but Webb is surely correct--it is not necessary.

[Vanwill]

I have heard the term "glazing" applied to brake linings since I was a teenage mechanic. I'm not sure I have ever actually witnessed a case of it. But it has occurred to me that I had some type of "condition" on my brake linings when I performed my "lube and clean only" procedure and it might have been "glazing".

I reinstalled all my original shoes/linings, but in addition to polishing the roller axle contact surfaces and anchor pin surfaces, I cleaned the surface of the linings. Although they did not look unusual compared to dozens of other brake linings (automotive, not MH) I've seen in my lifetime, they DID have brake dust imbedded in their surface. I think I might have been able to remove that dust completely with a 150 PSI air nozzle with a small, pointed opening. But I also "scrubbed" them with a stiff wire brush, then cleaned every square inch of them with that high-pressure air hose. When done, they had a noticeably different appearance.

I know that Harry has posted that he occasionally "de-glazes" his linings by hard braking, and that it UNQUESTIONABLY stops his brakes from squealing. Harry is a sharp guy, and I trust anything he posts implicitly, just as I do from Roy and Bob and others with technical and mechanical expertise. But looking at the surface of my brake linings, I can't see how hard braking would "clean" the surface of the linings.

Have any of you guys ever read any explanation of how brake linings can be "de-glazed" other than by scrubbing and/or sanding their surface?

The reason I ask is that I went to considerable lengths to keep my brake experiment true to the "scientific method"--change only ONE thing at a time. It has occurred to me that cleaning the linings as thoroughly as I did might have had an effect on braking power, and that my great results might have been partially because of that cleaning of the lining, and not just due to the cleaning/lubing. BTW, I did NOT sand or abrade the linings. I only brushed them with a stiff wire brush and blew them clean with high-pressure air.

What say you?

[Papa_Jim]

We just got back from a short trip. I paid attention to my brakes. My front brakes 'clunk' every time I use them. Probably the S cam rollers are seized. I guess it's time to take the front wheels off and look things over.

I have taken no offense to the good comments and observations made here. Point taken. If it's not right, make it right.

Van, you have been a source of motivation and inspiration to me. Thank you.

Jim

[Vanwill]

Originally Posted by Papa_Jim

We just got back from a short trip. I paid attention to my brakes. My front brakes 'clunk' every time I use them. Probably the S cam rollers are seized. I guess it's time to take the front wheels off and look things over.

I have taken no offense to the good comments and observations made here. Point taken. If it's not right, make it right.

Van, you have been a source of motivation and inspiration to me. Thank you.

Jim

Yer a gem, Jim! Hope we meet in person some day soon! I'm getting old!

Yeah, the coach is always a learning experience, huh? How is your residential fridge doing?

I know you are satisfied with your coach after replacing all the bushings and adding that big cross-brace in the back. But I can tell you that the Watts link I added to my coach (when I thought it would make no difference) was a HUGE improvement. Not only does it reduce wandering even further, but the "feeling" is very akin to an anti-sway bar.

Take care, my friend.

[twinboat]

Originally Posted by Vanwill

Because the LF brake drum of my 2000 Dynasty showed a very slight uneven, ridged wear pattern, I ordered two new drums and new shoes/linings. When the drums arrived, I was sure I had been sent the wrong ones. Although all dimensions of the drums looked OK, the (10) holes for the studs were over 1.25 diameter. My studs were 22MM, and the holes in my original drums were .937 dia.

When I called my supplier, he sent me info from Webb (possibly the largest USA manufacturer of heavy truck brake drums). Long story short--Webb says that they have consolidated three part numbers into one, by putting the largest hole required by any of the three individual part numbers into the drum and saying that it now fits all three. Webb supplied their engineering analysis of why the clamping force of the studs locked the drum from slipping on the hub assembly, and it is sound. They claim they have not had even one problem with that change in several years. Full disclosure--I'm STILL going to machine a stainless steel bushing for one stud on each wheel, but Webb is surely correct--it is not necessary.

I would feel the same way as you with the larger holes in the drums, if I hadn't worked on E & F series Ford's.

They use center hole locating, on their wheels and have large, loose fitting, holes around the studs. The clamping force, is all what keeps the wheels from shifting.

I do agree, a close fit, makes more sense.

[twinboat]

I'm not going to argue you statement about travel having " any effect on braking power whatsoever" although you admit, up to a 4% loss, from a change in leverage.

Another factor, is time to application. The further the slack needs to move, the longer, it is going to take, for air, to pressurise the chamber, before building to the pressure needed, for that particular stop.

That will not lower the force to the shoes, but will cause a delay in the application of the brakes.

If you test that effect, you may feel the differance, I have.

[YC1]

Van, your comment on cleaning the pads has me trying to find a possible analogy. While using sandpaper or a file eventually becomes much less effective as they fill with removed material. The sandpaper or file once cleaned instantly revert back to functioning tools. Perhaps brake pads have some of the same issues. Although not as porous as sandpaper or a file those shoes must have some microscopic holes. Maybe that is the glazing issue and cleaning them resolved it.

My brakes work well but a good shot of air around the brake linings while parked with the brakes off and tires chocked may be a good measure.

[nodine]

Originally Posted by twinboat

I'm not going to argue you statement about travel having " any effect on braking power whatsoever" although you admit, up to a 4% loss, from a change in leverage.

Another factor, is time to application. The further the slack needs to move, the longer, it is going to take, for air, to pressurise the chamber, before building to the pressure needed, for that particular stop.

That will not lower the force to the shoes, but will cause a delay in the application of the brakes.

If you test that effect, you may feel the differance, I have.

That is a valid concern since at 60 MPH you will travel about 90 feet in one second. Could mean the difference in getting stopped in time to avoid a collision.

Bob

[nodine]

Originally Posted by Vanwill

I have heard the term "glazing" applied to brake linings since I was a teenage mechanic. I'm not sure I have ever actually witnessed a case of it. But it has occurred to me that I had some type of "condition" on my brake linings when I performed my "lube and clean only" procedure and it might have been "glazing".

I reinstalled all my original shoes/linings, but in addition to polishing the roller axle contact surfaces and anchor pin surfaces, I cleaned the surface of the linings. Although they did not look unusual compared to dozens of other brake linings (automotive, not MH) I've seen in my lifetime, they DID have brake dust imbedded in their surface. I think I might have been able to remove that dust completely with a 150 PSI air nozzle with a small, pointed opening. But I also "scrubbed" them with a stiff wire brush, then cleaned every square inch of them with that high-pressure air hose. When done, they had a noticeably different appearance.

I know that Harry has posted that he occasionally "de-glazes" his linings by hard braking, and that it UNQUESTIONABLY stops his brakes from squealing. Harry is a sharp guy, and I trust anything he posts implicitly, just as I do from Roy and Bob and others with technical and mechanical expertise. But looking at the surface of my brake linings, I can't see how hard braking would "clean" the surface of the linings.

Have any of you guys ever read any explanation of how brake linings can be "de-glazed" other than by scrubbing and/or sanding their surface?

The reason I ask is that I went to considerable lengths to keep my brake experiment true to the "scientific method"--change only ONE thing at a time. It has occurred to me that cleaning the linings as thoroughly as I did might have had an effect on braking power, and that my great results might have been partially because of that cleaning of the lining, and not just due to the cleaning/lubing. BTW, I did NOT sand or abrade the linings. I only brushed them with a stiff wire brush and blew them clean with high-pressure air.

What say you?

Van,

I don't have an answer to your question but I can tell you that almost every time we leave the driveway with our coach the brakes will squeal until we have applied them a few times. I always assumed it was rust built up from sitting in our humid weather and it quickly wears off. Just my guess.

We know that humidity is not a problem were Harry lives so maybe my guess is all wrong.

Bob

[twinboat]

After looking at your slack adjuster chart, another point came to mind.

Brake chambers are linial, having a fixed mount. When moving beyond 90 degrees, the force become non-linial, causing the chamber end of the push rod, to distort the dighpram, in the chamber and pulling the slack adjuster, away from the S cam shaft, instead of turning it.

Wouldn't that use up some of the force created?

Not an engineer.

[rmcb]

Van,
After I performed Harry's "deglazing" maneuver several times, I sure thought my brakes worked better. But since I didn't take the time to set up a stopping distance test, I cannot be absolutely sure how much of this was perception. (On a side note, the dash-dash-dash skid marks are still on the street. At least one neighbor thought the garbage truck left them. I had to come clean that it was me!)

I also cannot be sure whether any improvement was a result of "conditioning" the linings or some sort of "freeing up" of the S cam rollers/pivot points. Like Bob, I often notice my brakes squealing when I first drive the coach after it has sit for a while. I also thought that was due to surface rust on the drums but have never pulled one to see. After a few brake applications, it seems to go away. It's pretty arid here, so I am not sure whether the drums are actually rusting. our car and truck have disc brakes so I don't notice anything like that happening on them. I suppose this squeal could also be glazing from slow driving through town on the way home.

I hope to get the time this week to take one set of my front shoes down to Sac CL &B to talk to them about heat, fading, glazing, and options for different shoes.

I'll report on what I find out.

Roy

[Vanwill]

Originally Posted by twinboat

After looking at your slack adjuster chart, another point came to mind.

Brake chambers are linial, having a fixed mount. When moving beyond 90 degrees, the force become non-linial, causing the chamber end of the push rod, to distort the dighpram, in the chamber and pulling the slack adjuster, away from the S cam shaft, instead of turning it.

Wouldn't that use up some of the force created?

Not an engineer.

Twinboat,

That is a good observation. However, the amount that the end of the brake chamber rod deviates from straight outward is very small, so the amount that the diaphragm "distorts" is also very small. But even then, since this is a diaphragm-type cylinder, its cross-sectional area remains the same. Cross-sectional area X pressure yields the force. The reason the chambers are not de-rated until they reach 2" of stroke is that the diaphragm remains "infinitely flexible" (in the engineering sense) until it passes the 2" point. Only when its "piston" approaches its limit does part of the air pressure have to be used up in "stretching" the diaphragm, thereby reducing its available pressure.

But I did account for that in the 4% figure. The loss of S-camshaft torque is less than 2% due only to the loss of leverage. I was conceding that even if the same angularity concerns could be applied to the chamber's rod, you would only double that 2%. So the total of those two concerns is still less than 4%, which I don't think the average person can detect without instruments.

Still, I think it makes good common sense to have the slack's centerline and the chamber's rod make approximately a 90* angle at actuation. I don't think you are going to get that from auto-slacks, though. No matter what I do, I cannot get these auto-slacks to adjust themselves to less than 1-1/4" to 1-3/8" on all four wheels. From what I've read, that is normal.

Thanks for your thoughts. As I said, my experience with adjusting slacks to less than "normal" travel yielded no PERCEPTIBLE change in braking power. Other folks who are more "sensitive" might be able to tell a difference. I would like to hear from anyone who has experienced a noticeable difference in braking by adjust from 1.5" down to 1.0". There could be something in play there that I've missed.