Yes, my Dad had one of these tap-into-the-hydraulics brake controllers in the 60's and 70's, and it did work great. Instant response, and exactly proportional to brake pedal pressure. However, I do remember that adjusting the sensitivity was a matter of moving and reclamping a slider on a great big coiled-wire resistor under the hood.
There are a couple reasons why you can't tie into the hydraulics on a modern car. My understanding is that modern cars move a much smaller volume of brake fluid to activate the brakes than old cars did, and if you "steal" enough volume to move a trailer brake controller, it would disable the tow vehicle brakes.
The other reason seems to be that when the ABS is activated, the very high pressure pulses in the brake line would destroy any common brake controller.
Bill
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