When I first started working on School bus brakes, oh, maybe 1960 or so, it was a two-person job and the rear bleeders were waaaaay back there so there was a certain amount of yelling back and forth to the "driver" for "UP!"......."Down!" on the pedal to sync with closing/opening of the bleeder. That's how I bled the brakes on Pearl the first few times and had a wife and kids to depend on for the legwork. The process worked well and seemed fool-proof enough for us but it was time-consuming and tedious. For a single person job on a school bus it would have required one helluva long broomstick with multiple bends to get from here to there. 
Then my Dad was talked into a metal tank, pressure bleeder from "Snap-On Eddie", our local Snap-On guy, that included reservoir adapter covers for just about anything. The GM/Ford/IH adapters were simple metal plates with rubber on one side that fit in place of the baled cover on the fluid reservoir, had some way of securing it to the reservoir (usually just the bale wire but sometimes screw-down attachments) and a quick disconnect for the fluid tube. That was a work-life changing moment. Now, we could bleed the entire system in less than 1/4 the time, with a single person and no brake pedal pumping. Eureka! The only issue was with the GM reservoir which had a gasket for the cover that was formed with pockets in it so sometimes when you put the cover back on after bleeding it would force a bunch of fluid out. THAT was cured with a Turkey Baster to suck some of the fluid out of the reservoir before installing the cover. No big deal.
Even with that system we sometimes got leaks with the adapter plates or, if it was a screw-on cap like on my brother's BMW or Mini with the rubber washer inside of the cap, but any of those were easily replaced so that system lasted for over 30 years. I had already made my home-built bug sprayer bleeder by the time I cleaned out my Dad's old shop getting it ready to sell, but I found that old Snap-On bleeder, with much of the paint peeling off from brake fluid spilled on it over the years, and a box of adapters, some used a lot and some never used at all, sitting high on a shelf in a shop closet. Couldn't find anyone interested in it (it did look a little rough, for sure) so it ended up at the scrap metal salvage yard, along with 6 other loads of thrown out stuff from the shop. Made me feel like the story of the brave little toaster.