Treadle Vac - compensating valve?

[nellis003]

Hi all,

I just installed a Treadle-Vac power brake system into a '58 Edsel Citation that previously had manual brakes. The brakes are working - too well. You barely touch the pedal at low speeds and the car noses down badly, like the front brakes are grabbing way too hard.

Is there a check valve that the power brake system is supposed to have, or are the shoes maybe too soft for power brakes? The only components I changed were the master cylinder and the pedal setup. Should I have done anything else?

Thanks for any guidance.

Nick

[Bill Steiner]

I cant remember a ck valve on a single master cyl set up. But it's been a long time. There was one on the duel cyl. set up i think. I would ck and see if rear brakes are working correctly. Bill

[alcan]

I have run into this problem a few times years ago. The cause was the master cylinder had too large a bore size for the design of the booster unit. Usually it was because a master cyl for a manual brake car was installed. They look identical and bolt right on so it was easy to do. Ford ( and others ) masters normally used a 1 inch bore but in 1958 some models of power brakes had a 21/32 inch bore. The smaller bore reduces the hydraulic line pressure to the wheel cylinders; in this case by about 30%.

[estout81]

You need to get an old motor manual and study the Bendix Treadle Vac system as it is nuique. Ford and GM both used this unit in the late 50's and early 60's. I think your problem might be a broken or stuck spring in the rod that goes into the m/cyl. There is a rubber thing on the end that goes into the cyl and on the other end is a spring that controls a valve that gives feedback to the pedal. This info is all from "old memory" so you better get an old book and study.

[Motorhead6]

Au Contrere. The larger size bore reduces the pressure. You want to size up to reduce the braking pressure.

[nellis003]

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I ended up installing a proportioning valve. They're normally used on cars which have had front brake conversions in order to keep the rear brakes from locking up. I put one on the front brakes and dialed it down, and now it doesn't nosedive when I hit the brakes.

Unfortunately, I was driving it today and the power assist disappeared on me halfway through the trip. I started another post on that problem, but if anyone has an idea on why that would be happening I'm all ears.

Thanks
Nick