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Bendix Treadle Vac, Myths, Fiction and Facts
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Since the start of the internet and the formation of forums there has been a spike in talk and discussion of the BTV and how dangerous this unit can be. There has been posts stating that if one continues to use a car that is equipped with a BTV you are not only putting the lives of other at risk but your own as you will never know when it will fail. But what is interesting is the amount of vehicles that could have been equipped with the BTV during the 1950's and early 1960's. Conservatively speaking 10,000,000 vehicles:

Buick 1957
Cadillac 1956 &1958
Chevrolet 1954-58
Edsel 1958-59
Hudson 1954-57
Lincoln 1953-60
Mercedes 1952-62
Mercury 1953-58
Nash 1954-59
Oldsmobile 1953-56 and early 57
Packard 1952-56
Pontiac 1954-58

We decided to perform a study on the failure of the Bendix Treadle Vac not only to discover the problems that owners have, but in hopes to educate ourselves and others on the unit itself and put to rest some of the myths that have been put out on the internet. We offered a FREE rebuild to any of the members of the Packard forum with a BTV that had recently failed to analyze the causes of failure.

The study that we performed was done across five different units. Three came from customers for analysis and two from cars in Fred and Dan's private collection that had been in long term storage( 40 years). All units that were received where either working at the time of removal with the pedal not returning/sticking or non operational due to non use. Prior to disassembly the units were tested. All held vacuum and produced hydraulic pressure.

All units experienced similar problems. Firstly the brake fluid had crystallized within the hydraulic section in and around the bypass port. Secondly the hydraulic plunger seal had failed allowing fluid into the vacuum section, which then was later sucked into the running engine not showing a physical loss of fluid. In the units that experienced stuck or sticking pedals. DOT3 brake fluid is hygroscopic meaning it absorbs water from the atmosphere. The brake fluid which is prone to water absorption had been allowed to lie in the vacuum section creating rust build up. The friction between the leather seal of the vacuum piston and the vacuum can lead to the stuck pedal. The vacuum sections were in good working order, only needing cleaning and a new leather piston seal.
The crystallized brake fluid:

<a href="http://s671.photobucket.com/user/JamesRFrom/media/Packard%20400/13173834_1147400715302450_7164387240836746932_n.jpg.html" target="_blank"><img src="http://i671.photobucket.com/albums/vv72/JamesRFrom/Packard%20400/13173834_1147400715302450_7164387240836746932_n.jpg" border="0" alt=" photo 13173834_1147400715302450_7164387240836746932_n.jpg"/></a>

The buildup of rust that causes the sticking pedal:

<a href="http://s671.photobucket.com/user/JamesRFrom/media/BTV%20Unit%20Rebuilds/KIMG1457.jpg.html" target="_blank"><img src="http://i671.photobucket.com/albums/vv72/JamesRFrom/BTV%20Unit%20Rebuilds/KIMG1457.jpg" border="0" alt=" photo KIMG1457.jpg"/></a>

The major contributing factors to the failure of the BTV is not its design but two factors that plague all classic cars whether they have a BTV or manual master cylinder. First is moisture and second is time. The moisture is absorbed into the brake fluid which plays havoc on any brake systems including the BTV. This causes the fluid to crystallizes blocking passages as well as corroding the aluminum of the master cylinder and the steel of the piston. This causes the seal to rip and leak fluid into the vacuum section. Time: many of these vehicles no longer see the daily use that they once saw back in the 50's and 60's.

A photo of the rust pitted piston:
<a href="http://s671.photobucket.com/user/JamesRFrom/media/BTV%20Unit%20Rebuilds/KIMG1503.jpg.html" target="_blank"><img src="http://i671.photobucket.com/albums/vv72/JamesRFrom/BTV%20Unit%20Rebuilds/KIMG1503.jpg" border="0" alt=" photo KIMG1503.jpg"/></a>

These vehicles sit idle for months at a time especially the farther north you go or in some cases years or decades. In time, seals dry out and become stiff which creates opportunities for leakage.

What can be done?

Annual maintenance is key. Get familiar with your vehicle and its various systems. Following the factory service manual service intervals is not possible under today's conditions as you will not be racking up the miles as quickly as the factory engineers figured back in the 50's. Pertaining to the brake system the brake fluid should be checked at regular intervals. We recommend every three months for safety sake. Take a sample and check for cloudiness or discoloration. This is a sign of moisture which at that point would require flushing and refilling. At a minimum we recommend flushing and refilling every 2 years.


Prevention

You should consider flushing your brake system and changing over to silicone based DOT 5 brake fluid. It carries better performance characteristics than the regular DOT 3 as it will not absorb water.
At the time of rebuilding, use a reputable builder of BTVs. One item to ask the rebuilder prior sending your unit is if they plug or block off the bypass port as we have found this done on units that have failed. By blocking off that port it negates an important safety design feature and can cause sudden failure. Make sure to upgrade to a stainless steel hydraulic piston. This will eliminate the possibility of rust building on the piston and ripping the seal. Most importantly make sure that the bypass port is clean and clear. If fluid were to get past the piston seal it would be re-routed back to the reservoir instead of into the hydraulic section.

The main thing that we all need to bear in mind is not the miles we put on between service but time.For most of us time moves faster than our odometers. When something sits that holds fluid whether it be an automobile, boat or plane, seals dry out and the environment has an effect. So inspection is imperative prior to operating.
The premise of this study is to debunk the myths that the BTV is and was dangerous. If that was the case, all the manufacturers would have halted use immediately instead of using it for an almost 10 year run. There were also no TSB's (Technical Service Bulletin) during the 50's as well. To that, the naysayers will say "Why did they stop using it?" Also, why did GM stop producing the 350? Simply put, technology improved, but that doesn't mean that there was anything wrong. It's just natural progression.

From our own research online through the various forums and pages of Google, we have not found any direct accounts of any BTVs that suddenly stopped working due to catastrophic failure where a lack of maintenance was not present at the time of disassembly. The same can be said for the single line manual master cylinders of the time. We have sold numerous manual master cylinders over our 50 years in business and many were purchased to replace ones that had failed. Failure of master cylinders is not just limited to the BTV as all are serviceable items.

Thanks
James From
Kanter Auto

Thanks for the very informative study.
I use DOT4, checks the level every second month during the summer and after the car has been in storage during the winter I test all the brake fluid while bleeding the system thoroughly (Pen type tester)to check if there has been any moisture present, i.e. the brake fluid is replaced every year.
I have not had any problems during the five years following the above procedure.

Tom

The BTV was a very early attempt at one of the FIRST power brake units offerd in MASS production. Which means that there was virtualy no power brake designs PRIOR to the BTV.

Therefore WHY was that design (specifically using the COMPENSATOR VALVE) ever concocted by engineers to begin with???

I know the answer. Lets see if anyone else can answer that question.

I'm sure the compensator valve design was not "concocted" by Bendix engineers, as a graduate engineer with 2 degrees we were never taught to concoct anything nor did I encounter this in my employment. To be sure Bendix, one of the outstanding engineering firms responsible for much of the early aerospace work was well experienced in safety. ( Factoid, our 734 Speedster roadster was owned by the president of Bendix from 1947-1952). No engineer or engineering/manufacturing firm guesses at the most important safety system in an automobile.

The compensating valve gets 100% of the braking hydraulic pressure applied to it which, by design, tightly closes the valve. The face of it is rubber which will allow for minor surface irregularities or particulate matter. The "crud" which forms over many years from fluid which is not changed will be squeezed and will not cause a loss of seal.

Our investigation has shown a reason for failure, the compensator theory is just that, a theory with no facts to support it. If that were the problem how come failures were not rampant in the 50's??

Just the facts, ma'am...please

There have been several in this forum who have reported a failure - some permanent and for obvious reasons, and some intermittent. Aside from the obvious and generally permanent failures caused by a leak in a cylinder, or line and even a hard pedal caused by a vacuum section problem, what component other than the compensator valve would allow the pedal to go partially or worst case all the way to the floor at one stop and everything be perfectly fine the next time brakes are applied.

If we can rule out everything but the compensator valve that would allow this to happen, what would keep the valve from closing other than some kind of crud preventing a seal. I think for the most part we have ruled out the spring and the reason for the crud has been proven without doubt -- neglect -- but as I see it a hard crystalized blob of crud is the only thing that would cause an intermittent failure to close. In some cases the cause does not appear to be squeezed or swept away by a rush of fluid going past it.

In some cases I would concede the compensator valve could be one of those poor quality repro valves such as the valve with nothing but a layer of thin flat rubber you or someone else posted a photo of a few years ago but in other cases I see no other explanation other than hard crud. To get that crud out requires at a minimum total disassembly and cleaning of the hydraulic section and not just taking the lid off and cleaning the reservoir.

Fred and James thank you. Outstanding article and pictures all of which will be copied and added to my "forgotten memory" folder.

Thanks Fred and James for this study and your documentation. No doubt long term storage affects the BTV in the manner in which you describe.

In my personal case this was NOT the situation. To reiterate my experiences. I had 4 BTV failures in 4 years, all the BTVs were from different vendors except the first one which was in my 55 Pat when I bought it from the 2nd owner. That one failed after about 1 year of relatively frequent driving (2-3 times a week). The 3 replacements also failed after about 1 year. Not all failures were no-notice catastrophic pedal-to-the-floor-no-brakes ones. A couple were pedal-slowly-to-the-floor ones. Anyway, I'd finally had it with the BTV and engineered a solution using new, modern components. That was 13 years ago and it has operated flawlessly ever since.

FWIW & YMMV.

Craig

HH56,

The mode of failure could be low fluid caused by fluid leaking by the primary seal. Upon using the pedal again the fluid could reach the reservoir via the return passage.



This is the important passage that some rebuilders fill with silicone as they do not understand the system

The contour of the rubber on the compensating valve is important, allowing a good seal. As you note there have been some incorrectly reproduced valves used by some rebuilders which do not seal reliably. Thus is not the fault of Bendix design engineers or the design or age of the unit, it is the fault of whomever reproduced the part.

If someone reproduces engine pistons that have too much clearance and are made of the wrong alloy and the engine burns oil so much that it runs low and the engine seizes it is not the fauilt of Packard engineers.

Craig,

4 failures after rebuilds in a short period point to improper rebuilding. In the 50's when many more rebuilders were skilled in BTV work failures after rebuilds were rare. I believe I responded to your experience before and I wonder why others have not had your experience of failures.