AS Jerry said there were vacuum-assisted mechanical brakes in the 30s. I have seen one like that and it had a valve to port vacuum to vacuum trailer brakes. I drove old gas-pot semis with vacuum-over-hydraulic brakes. The vacuum cans were huge compared to air cans. Bear in mind that excluding the air can, air brakes are mechanical (think about the S-cam linkage or, heaven forbid the wedge type. Air cans come in various sizes. One might be able to convert the system to air brakes rather than hydraulic. That would require an air compressor, tanks, a treadle valve, some sort of spring brake release valve....
Naa that's getting way too complicated. I don't see why vacuum-assisted mechanical wouldn't work well if you could find the parts. The Model A crowd argue that mechanical brakes in good shape are as good as hydraulic brakes. Oh, and a lot less trouble on a rig that sits most of the time. The brakes on my old non-hydraulic motorcycles never give trouble associated with sitting around un-ridden. The hydraulic ones in a damp climate are a constant pain in the pittuti unless you ride them all the time.
