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

Posted by BH On 2009/5/16 21:41:19

I think it was 1967 that dual-chambered (tandem) master cylinders became a requirement - but only for passenger cars.

In fact, the 1973 Chevy C60 wrecker used by the Chevy-Buick dealership where my dad worked came from the factory with a single chambered master cylinder, and they ran that wrecker for nearly 20 years - until the dealer retired and sold the business - without incident. Dad even towed coach-type buses with that wrecker (powered only by a 366-cid engine) - not just for mechanical breakdowns, but out to NJ and back for collision repair.

That a tandem master cylinder is required for disc/drum combinations is NOT TRUE. Tandem master cylinders were actually in use before that 1967 federal mandate. Pop the hood on a 1963 Studebaker Lark with four-wheel drum brakes and you'll find a tandem master cylinder. Yet, pop the hood on an original, unrestored 1963 or 1964 Studebaker Avanti that came from the factory with front disc and rear drum brakes as standard equipment, and you'll find and a SINGLE-chambered master cylinder.

Technically, all vehicles require some sort of proportioning between front and rear brakes. On vehicles with drum brakes at all four wheels, this was originally accomplished simply by using different diameter wheel cylinder bores, front vs. rear. Using disc brakes in conjunction with drum brakes adds a new twist because disc are direct-acting while drum brakes have a self-energizing effect. IIRC, it was only about 1976 when genuine proportioning valves came into widespread use.

Since brake proportioning is based on vehicle weight distribution as well as disc vs. drum, I also don't believe that failure in a tandem master cylinder leaves you with 50% braking. Moreover, while tandem systems were initially spilt front /rear, they started splitting them diagonally by the 1980s - that is, left front tied to right rear and right front to left rear. That always puzzled me because the proportioning is usually biased toward the front brakes. If you blow a rear line in a diagonally-split system, you're effectively trying to stop the car with one front brake.

I've personally experienced rear brake line failure in cars with both types of split tandem systems, but would not have been able to bring the car to a complete safe stop had the parking brake not been in good working order. I also once stopped a VW Beetle that developed a hydraulic failure with a single-chambered master cylinder with nothing more than the parking brake AS A PASSENGER, when the driver didn't know how to react - thanks to the handle being located between the front bucket seats.