You Don't Need A Specialized Tool To Reverse-Bleed Your Brakes

So you've swapped out your old, weathered brake lines for some fresh stainless steel. Great! Now you just have to fill those fancy new lines with brake fluid, a tiresome and boring process that belongs somewhere on one of the lower levels of Hell. It makes your hands ache, it takes what seems like centuries, and it feels like you're going to go through about six gallons of brake fluid just to get every bubble out. It's bad. But there's a better way: Stop pushing your brake fluid through the system from the top, and instead put it in the bottom. 

Reverse-bleeding is a better method for flushing air from fully empty lines, because it works with physics rather than against it. The whole goal of brake bleeding is to flush out any air (a compressible gas) with brake fluid (a functionally incompressible liquid) to ensure all the pressure from your levers or pedals makes it out to your calipers, and as RevZilla's Ari Henning reminds us air is less dense than brake fluid — it's more effective to force fluid up from the caliper to the master cylinder, pushing air in the direction it naturally wants to go, than to force fluid down and allow bubbles within your lines. Conveniently, reverse-bleeding doesn't even require any specialized tools like a fancy pressure bleeding setup, provided you're working with a simple brake system like a motorcycle or older, pre-ABS car. It can be done with the garden-variety enormous syringes you probably already have around the house.

Reverse-bleeding is simply enough: Open your master cylinder, empty it of fluid, fill a syringe with fresh fluid, connect it to your caliper's bleed screw with a length of hose, and slowly but firmly press fluid through until your lever or pedal is firm. Once your lines are full, you can do a bit of traditional bleeding for the last few stubborn bubbles, but it won't require nearly the time or effort that traditionally bleeding the full system would need. Reverse-bleeding may only get you 90 percent of the way there, but it'll get you to that 90 percent mark a hell of a lot faster. 

Reverse-bleeding can be done on cars, but the process gets more complicated as brake systems do. It's best used on motorcycles with simpler, smaller systems, though it's not out of the question for bigger ABS-laden cars — it just might require some extra pressurization equipment, which costs it the simplicity of the syringe-based motorcycle process. On bikes or older, pre-ABS cars, though, the simplified process can save you plenty of time on an otherwise tiresome job — one you'll routinely have to do on cars of a certain age as rubber brake lines stiffen and rot with time, and eventually require replacement before they burst under the pressure of brake fluid.

Reverse bleeding isn't a one-size-fits-all solution for ending the banality of brake bleeding, but it's a speedrunning cheat when you've got completely empty brake lines. If you've replaced yours due to wear or age, or had a line burst on you, reverse-bleeding can massively simplify things. You may not need to do it all the time, but it'll be massively helpful on the rare occasions when you really need it.