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I used to build a lot of grain trucks in the 70's with air ride/spring lift pushers and tag suspensions on hydravac equipped chassis using an electric compressor. These had 1/2 HP 12VDC motors that ran units much like a small gagrage compressor. I used standard 1750 CID trailer brake reservoirs and leaf spring lifts. They worked perfectly and I never had to service them. A typical 7-9K front steering axle was used but the tie rods were welded so they couldn't steer. Some modern setups now use a spring loaded self centering device on long-arm tags like concrete mixers, etc. where severe tire scuffing is a big problem. We built pup trailers too but with all new chassis and suspension parts.

There are commercially available vacuum hydraulic brake complete setups for those wanting to build larger trailers. Also I would mention that 12VDC hydraulic pump/valve/reservoir units are available in nearly any combination of valve configurations so having one 3 way and one 4 way valve is no problem at all. This is often easier for home built applications than having a true wet kit and jumping from the truck.