Caravan and Outdoor Life’s publisher, Godfrey Castle, had this to say:: That is correct. Vacuum brakes (otherwise known as the PBR system) require having a vacuum hose fitted from the engine compartment to the towbar area on the car, while the caravan retains its normal overrun system, but gets an additional slave cylinder to apply the caravan brakes.
When vacuum is applied, i.e. when you apply the car’s brakes, the vacuum generated is also applied to the caravan’s brakes, as the slave cylinder pulls on the caravan’s brake rod mechanism. However, there can be a short delay on the caravan’s brakes.
You need to keep the system’s air connection between car and caravan clean when not in use, and the air hose on the car sealed off when you are not towing.
Which PBR unit will you fit? There is one version that allows you to apply the caravan’s brakes independently by pulling a hand lever mounted under your dashboard. It’s a wonderful feature, in that if the caravan starts to become unsettled at speed, you can pull the lever and gently apply the caravan’s brakes, pulling it back into straight-line towing.
Just remember, for a towing combination to be legal, the Tare of your vehicle must be equal to, or higher than, the caravan’s GVM.