A brake pipe bender is sized by the outside diameter of the tube, and that groove is the whole tool. The degree markings and the handle length are convenience. The groove is what holds the wall round while the tube travels around the former, and a bender whose groove doesn’t match your line will flatten it on the first pull.
Size the bender to the tube, not to the fitting
Brake and clutch hard line runs in a narrow set of sizes. 3/16″ (4.75mm) is what almost every passenger car uses for the bulk of the system. 1/4″ turns up on older trucks and on some rear circuits. 5/16″ and 3/8″ appear between the master cylinder and a proportioning valve, and on heavy clutch lines.
Benders are cut for those outside diameters and they don’t cross over. Run 3/16″ tube through a 1/4″ groove and the wall is unsupported on both sides, so it ovals instead of bending. The triple-head benders sold for automotive work carry 3/16″, 1/4″ and 5/16″ grooves on one former, which covers most jobs from a single tool. 3/8″ is usually a separate, larger bender.
Measure the tube with calipers rather than reading a size off the nut. A 3/16″ line takes a 3/8-24 inverted flare nut, and the hex on that nut measures nothing like 3/16″. Sizing a tool off a wrench or a fitting is how people end up with the wrong groove.
What kind of bender the job actually needs
A lever bender is the one with two arms and a rotating former marked 0 to 180 degrees. Clamp the tube into the groove, swing the arm, read the angle off the scale. It gives repeatable bends at a known radius, and it’s what you want on a bench when you’re making a line to a pattern.
A bending spring is a tight coil that slides over the tube (or inside it, for the internal type). It supports the wall while you pull the bend by hand. It costs a few dollars and it does work, but the radius is whatever your hands produce and it’s hard to repeat twice. Fine for a gentle sweep in copper-nickel, poor for a tight bend in steel.
Plier-style and clamshell benders are short-bodied tools for bending a line that’s already on the car. They cover the case where a run has to leave a bracket at an angle you didn’t predict, and they’ll pull 90 degrees in a spot where a lever bender’s arms have nowhere to go. You give up some control over the radius in exchange for reaching.
Ratcheting benders belong mostly to larger tube in hydraulic and refrigeration work. On 3/16″ brake line they’re more tool than the job asks for.
Bend radius is where a line gets scrapped
Every bender has a fixed centerline radius built into its former, and that number is the tightest bend the tool can make. Automotive benders for 3/16″ tube typically sit somewhere between 9/16″ and 3/4″ centerline. The working rule for steel hard line is a centerline radius of at least three times the tube’s outside diameter, which lands at about 9/16″ on 3/16″ line.
Go tighter than that, either with a former that’s too small or by pulling the tube around something by hand, and the inside wall of the bend buckles. That’s a kink. It shows up as a ripple or a flat spot on the inner radius and it permanently narrows the bore. A kinked line can’t be straightened and bent again. The metal at that spot is already work-hardened, and flattening it back out puts a crack exactly where the pressure peaks. You cut that section off and start it over, and if the far end is already flared to length, that means the whole run.
Copper-nickel takes a tighter radius than mild steel and forgives a lot, which is most of why it’s popular for hand-formed lines. Stainless forgives the least.
Springback, and why stainless fights back
Tube doesn’t stay where you put it. Release the lever and the bend opens up a few degrees as the elastic part of the deformation recovers. On mild steel line, figure 2 to 5 degrees. Copper-nickel barely moves. Stainless springs back further and work-hardens as it goes, so the last part of a bend takes more force than the first part did.
The fix is to overbend past your target and let it relax back to where you wanted it. Check the angle after it settles rather than while the lever’s still loaded. And on stainless in particular, don’t correct an overbend by bending it the other way. Reversing a bend through a spot that’s already hardened is how a line cracks two months later, in the driveway, after the job looked finished.
Where not to put a bend
This is the one that costs the most remade lines. Leave roughly an inch of straight tube behind every flare before a bend starts.
Two separate things need that inch. The tube nut has to spin freely along the line to reach its port, and a bend sitting too close to the flare catches the nut at an angle where it won’t turn and won’t pull the flare in square. Separately, a flaring tool’s die has to clamp a straight length of tube. Bend inside where the jaws close and you can’t flare that end at all. So the flare comes off, the line gets shorter, and now it doesn’t reach.
The related one: slide the nuts onto the line before you flare it, and before you bend it if any of the bends are tight enough that a nut won’t travel through them. A nut will not pass over a finished flare. Cutting a good flare off because a nut got left in the parts tray is the most common redo in brake line work.
An order of work that avoids most of this
Cut the tube a little long. Deburr both ends inside and out, because a burr on the inside diameter gets folded into the flare and becomes a leak path. Slide both nuts on and check which way round they face. Flare one end. Bend to the pattern of the old line if you still have it, which beats measuring a run from scratch. Trial fit, trim, then flare the last end once you know the line reaches.
Bending is the easier half
A bend that’s a few degrees off can usually be persuaded into place. A bad flare weeps under pedal pressure and the wheel comes back off the car to find out why. That’s where the real time on a brake line job goes, and it’s why the flaring tool deserves more thought than the bender.
The BrakeQuip BQ351 Brake Flare Tool Kit handles 3/16″, 1/4″, 5/16″ and 3/8″ in double, single and bubble flare at 45 degree SAE, with a 3/16″ DIN die for European fittings, and it forms stainless with a drop of oil on the punch. Those are the same four sizes a brake line bender needs to cover, so the two tools work across the same range of tube.