Conrad & Tricia Holsomback wrote:

> Help again, please.
> We have no experience with hydraulic vac brakes. What do we needto know?
>
> Conrad & Tricia Holsomback
> mailto:triconholsomback@greenwood.net

Hydra Vac brakes were first offered as an option in the mid 70's. Theiruse
continued for about ten years. When the 34' triple axle was first introduced
the Hydra Vac brakes were only necessary on two of the three axlesdue to
their tremendous stopping power. It was one of the very few trailerbrake
systems that could stop trailer and tow vehicle.
The system was a vacuum over hydraulic disk brake system controlledby a
electrical solenoid that was controlled by a Midland dash mounted brake
controller. The master cylinder, brake lines, and disk brakes werea typical
hydraulic brake system. The master cylinder was mounted either underthe
front coach or on the front A frame behind the LP tanks. It was mounted
vertically on a vacuum diaphragm much like a power brake booster usedon cars
and trucks. The vacuum source was a dedicated line tee'd into the towvehicle
vacuum, typically at the power brake booster. No diesels need apply.The rear
vacuum fitting had a check valve to prevent loose of tow vehicle vacuum.To
date this arrangement has not had a problem passing tow vehicle manufacturers
approval. The vacuum operated the master cylinder under the controlof an
electrical solenoid. This solenoid was in turn was controlled by aMidland
brake controller. The reason for the Midland controller was it's current
carrying capacity. Today's controllers are more than up to the task.When the
system lost vacuum from the tow vehicle the system acted to hold thebrakes
locked on. In order to release it, it was necessary to pull the releaseknob
on the roadside of the Hydra Vac unit. Unfortunately the parts werenot
commonly available. Inland RV in Corona, Ca is one of the only sourcesleft.

Charlie