[VAC] Re: Hydraulic brake controllers

[Dr. Gerald N. Johnson, electrical engineer]

Hydraulic trailer brake controllers have been discouraged by vehicle
manufacturers since they began shipping vehicles with dual brake
systems.

When those dual brake systems involved four wheel drum brakes, the dual
split was one front and one rear brake (opposite corners for straight
stopping on only one brake system working) and the expected oil flow was
exactly the same for either system. To detect a single brake system
failure they included a differential pressure switch to detect if the
pressure was different between the two brake systems. The master
cylinder had identical sized bores so the volume and pressure was
exactly the same. When we tap a hydraulic brake controller into just ONE
of those dual systems, we require more oil to flow because of the added
cylinder in the brake controller. So that unbalances the pressure with
the balanced master cylinder, reducing the pressure on the brakes on
that part of the dual brake system and can trigger the brake fault
light. To minimize that problem, the factory has specified that any
auxiliaries such as a hydraulic brake controller must have a very small
hydraulic volume. Far smaller than that originally used in hydraulic
trailer brake controllers.

For a while Tekonsa did make a low volume brake controller but I don't
think it met most vehicle makers limits, though it came close.

With the modern vehicle using disk brakes on the front and drum brakes
on the rear, the balance question is different since it takes a
different volume for disks and for drums and the dual system is split
front on one part, rear on the other. Adding the volume of a hydraulic
trailer brake controller to one or the other can upset the front to rear
braking balance and again trip the brake failure switch.

That unbalancing of braking action between the two brake systems is why
makers of hydraulic brake controllers are very reluctant to claim their
controller is compatible with the modern (since 1966 or so) vehicle with
dual brake systems.

So the electric brake controller companies have avoided the hydraulic
brake system with the pendulum or time applied work arounds that are
generally less controllable but are not invasive.

Seems to me that a hydraulic trailer brake controller with TWO
cylinders, one tapped into each of the dual brake lines, could avoid the
unbalanced hydraulic load problems and if the dual cylinders in the
controller ran a single rheostat through a balance lever (like the
equalizing bar in rear parking brakes) it would produce trailer braking
exactly proportional to tow vehicle braking. It would cost twice what
the last single hydraulic control cost, I think I paid about $49 for the
last tiny Tekonsa ten or 12 years ago.

In some quarters there is fear of litigation based on touching brake
systems. In a hydraulic brake system, any leak anywhere from any cause
can eliminate brakes. A vehicle modification that included a tap of the
brake lines, might lead to litigation if there was a brake failure or
collision that could be blamed on brake failure. Hence there might not
be a LEGAL statute that makes brake modification illegal, there is
definitely some potential liability.

Its conceivable that some states have enacted brake safety regulations
that require no changes to the original design, including no tapping for
a hydraulic trailer brake controller.

In the mean time, I'm getting along with my Kelsey-Hayes Micropower
controller mounted a couple inches from the steering wheel and adjusted
for gentle stops. If I need more trailer brake, I can handily apply an
index finger and get as much as the trailer tires will handle, and then
some.

Gerald J.



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