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Common problems with Treadle Vac systems

Started by TJ Hopland, May 25, 2015, 04:00:11 PM

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TJ Hopland

I have some work to do on a 57 soon so I have been reading up on the Treadle vac systems.   I read a lot about sudden failures.   Like other power boosters if the booster portion fails you just loose the power but if you can press hard enough you still should be able to stop the car?  I assume that the sudden failures are not related to the booster part rather the master cylinder portion?   And the reason they are sudden complete failures is that they are just single output MC's? 

I have read about people replacing or trying to replace the whole system with a newer style vacuum booster.  Has anyone ever tried just replacing the master cylinder?  If its just the MC that is the weak point it would seem like replacing just that would be an option.  In the street rod world there are several dual MC's with the option for remote reservoir so the odd positions should not be an issue.  Or is it that the whole booster and MC are just so wacky that they really have to work together to work?   Looking at diagrams it does not really look like a typical MC but that could just be the lack of detail in the drawings. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

m-mman

No, Treadle-vacs are really a one piece unit. There really isn't a divide between the master cylinder and the booster parts in the traditional sense.

One of the things that make these master cylinders unique is that when the pedal is pushed the system isn't really compressing the fluid in the typical sense

In a normal MC the cup moves forward (sealing the vent and compensating ports) and then squeezes the fluid directly.

In the Treadle vac the 'rod' moves forward and it's very presence in the fluid chamber displaces fluid in all directions to develop the pressure. A problem/fail point is that to be able to create the pressure a little spring has to pull the check valve into position so that it will seal. If there is dirt under the seal/seat (or spring failure) it might not hold pressure and the displaced/compressed fluid could simply escape back into the reservoir, resulting it a no brake situation.

By modern standards it is not a good design but one has to remember that the many power features that are standard/mandatory today were very experimental back then.

Treadle Vacs are functional and reliable BUT they need a quality cleaning and rebuild by someone who knows how to do it correctly. There should be no quick, short cut fixes of a Treadle Vac.
Actually there should be no short cut repairs to anything in the brake system. An old car needs a COMPLETE rebuild/replacement of virtually every brake part including the oft forgotten hoses. 

As happens sometimes, modern designs are still functional even when they are haphazardly fixed by an amateur with poor quality parts.

The person/shop who rebuilt my Treadle vac told me he has more customer reporting failures from poor quality imported dual master conversion parts than from his rebuilt originals.
1929 341B Town Sedan
1971 Miller-Meteor Lifeliner ambulance
Other non-Cadillac cars
Near Los Angeles, California

CLC #29634

TJ Hopland

Interesting description of how the MC part works.   Looking at the diagram that kinda makes sense.   It is pretty simple in a lot of ways.  I could imagine that it would also be different than typical systems as far as the stroke vs fluid displacement.   Its neat but there must have been a reason that anyone who was anyone used em for about 3 years then they disappeared.   I read that in the later 50's Lincoln, Packard used them as well as most of the GM's.   
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

cadman56

m-mman exactly describes the operation of the Bendix treadle vac.  I rebuilt two different units on 56 caddys with kits from one of the popular vendors & the units worked just fine.  Only once in a very great while I would not have much brake at initial brake.  A reapply did the trick.
White Sunday shirt clean is the rule on these.  m-mman is correct about that little valve.
I don't know about the 57 though.
Good luck.
Larry
1956 Cadillac Coupe deVille (sold)
1956 Cadillac Convertible (sold)
1956 Cadillac Eldorado Seville (sold)
1967 Cadillac Eldorado (sold)
1968 Cadillac Convertible (Sold)
1991 Cadillac Fleetwood Brougham dElegance
Larry Blanchard CLC #5820

TJ Hopland

From what I am reading at least about 57's is they are all the same except for design changes to the MC part required to accommodate the mounting position for various makes and models.    Pontiac and Packard had em directly under the pedal and steering column.   Chev had em more or less in the 'modern' horizontal ish position high on the firewall.   Cadillac had em vertical.

The not much brake must be that valve not seating so some of the pressure bleeds back to the reservoir?   
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Roger Zimmermann

There is some confusion here: the Treadle vac system was used for the '56 and '58 models (except Brougham in 1958). 1957 cars have a system called Hydrovac which is completely different. That Hydrovac system was used also in 1954 and 55. Here, the master cylinder is completely separated from the booster, it is down on the frame rail.
1956 Sedan de Ville (sold)
1956 Eldorado Biarritz
1957 Eldorado Brougham (sold)
1972 Coupe de Ville
2011 DTS
CLCMRC benefactor #101

m-mman

Opps! yup I started to tell the treadle vac story such that I forgot that 57s have a hydra vac.

Interesting that Cad would bounce between the two very different treadle and hydra vacs units. I have always wondered about why they did that. Reliable supply? Unit cost? Two engineers fighting about which was best? I guess we will never know.

And to answer TJ's other questin. Yes, lots of people used a BENDIX treadle vac (or the hydra vac). They did this because developing a brand new technology is expensive. It needs design, testing, and production. It is always easier to just buy one off the shelf.

Independent manufacturers (Packard, Nash) and low R&D manufacturers (early 50s Ford) were especially apt to buy from a supplier.
GM? they had a huge engineering department but I guess solving the power brake problem was not high on the list. I easily excuse them because they did solve the automatic transmission problem. (and became the supplier to most everybody else)
1929 341B Town Sedan
1971 Miller-Meteor Lifeliner ambulance
Other non-Cadillac cars
Near Los Angeles, California

CLC #29634

TJ Hopland

Did GM ever own Bendix?   At one point the owned Moraine?   Seems like in reading about these things they call the non Bendix one a Delco-Moraine?
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

TJ Hopland

I got to drive the car over the weekend.   What an unusual pedal feel.   I drive a wide variety of vehicles and equipment in various states of repair so I can usually adjust fairly well to different feels even with that it was a little weird.   Its like there is almost no pedal travel yet 90% of the stop requires very little effort.  Its just that last actually come to a stop that it feels more like manual brakes were it does take some effort.     
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason