Re: BRAKES: Easamatic Power Brakes aka Bendix Treadle-Vac--Change or Not??

Posted by PackardV8 On 2009/5/16 22:20:31

DRUM require somekind of residual check valve to keep a small amount of pressure against the wheel cylinder seals.

Disk brakes DO NOT require such a preload. If disk had slight pressure left in the lines after pedal released then the calipers would be dragging all of the time.

HENCE the TANDEM cylinder for rear drum/front disk.
However, this may have been only a very early-on engineering decision when disk(FRONT) was promoted.
I am not sure how various manufactures handled this issue of providing residual pressure to the REAR DRUMS while NO provision for such pressure to the front disk. At some point later on, about 1980 or there-abouts, some kind of a cup preloader was used INSIDE THE rear wheel cylinders.

I do not remember ever observing any check valve in the front most port of any dual cylinder i've worked on.

As for government mandateing, it only takes about $10k OF bribe money and we could probably have dragging our feet on the ground mandated as a brake system.