Absolutely no. A mechanic caught doing that will be fired. Compression fittings are fine for water lines for your sink, but pressures there top out at around 50 psi. You won't find compression fittings anywhere on a car, especially for brakes and power steering systems. With normal pedal effort and normal, but hard braking, line pressures can easily exceed 2000 psi, even higher when you make the tires skid. Typical high-side pressure for power steering systems reaches around 1200 psi.
Where some people think they're getting away with a compression fitting is for the rear brake system. Pressures there are kept lower to prevent rear wheel lockup under hard braking. A compression fitting might not blow apart, but you don't want to be the person sitting in a courtroom trying to convince a jury you made a legitimate repair.
You didn't say what you're trying to repair, but I'm guessing it has to do with replacing a rusted-out steel line. There's two legitimate ways to do that. One way is to cut out the rusted or damaged section, then add in a replacement section. This is the preferred way on some models where the engineers ran the original lines through holes in the frame or body, and replacing the entire piece is just about impossible. The line has to be cut in areas where the metal is still rust free, clean and solid. Slide on a soft metal line nut on each end, then make double flares. Every professional has done this many times, and every one of them will have at least one story of forgetting to put the line nut on first. No harm done other than you need to cut that flare off to put the nut on, then make another double flare. Use a pair of brass unions, then connect them with a new line of the right length. That new section can be made the same way, but it's far easier and faster to buy a premade piece of the right length.
The easier way to do this is to follow the rusted line to each end, remove the entire piece, then buy a new line of the right length. Those are made in standard replacement lengths. Chances are the one that will be long enough will be too long, usually by only a few inches. Just put a couple of bends in it to take up the extra length. These lines are also available with an anodized coating that resists rust better than original lines. You must also get lines with the right type of flares on each end. For most vehicles in the U.S. that will be the common double flare. They look like very tiny funnels. Most imports use metric "iso" flares, often called "bubble flares". If you aren't sure, just take the old line with you to the parts store to match it up.
If you're making your own flares, there are some new tools available that do a really nice job, but they're expensive. One tool set makes multiple types of flares in various sizes. Only buy that tool if you'll have more use of it later. Inexpensive double flaring kits are nice to have on hand, but I have not always had the best luck with them. Most of my problem comes from using bulk steel line that comes coiled up. Coiling weakens one side of the tube, then during the flaring process, the flare tends to end up off center. The tool works better when used on a piece of line that was never bent.
Long lines usually need to have bends put in them to match the original line. There are very inexpensive tools, or jigs, for doing that to prevent kinking a line, but I've always had good luck using my thumbs and gently massaging the line into a bend. It takes a little patience. You may be able to borrow all of these tools from an auto parts store that rents or borrows tools. Most often they make you buy the tool, but you get a full refund when you take it back. If you decide to keep a tool, you still return that used one, then they order or give you a brand new one. Beats having to buy a tool you may never use again.
When a line runs through the frame or over places where there's no chance you can run the new one, you don't necessarily have to follow the same route. Those original lines were installed on the assembly line long before the drivetrain and exhaust systems were installed. Sometimes it's not practical to follow the same route, and sometimes it's just impossible. The two biggest things to stay away from are the exhaust system and places where the tires can throw rocks up that hit the line. Hot exhaust parts can make the brake fluid boil, resulting in vapor that leads to one type of brake fade. Over time, brake fluid absorbs humidity from the air. That moisture lowers the brake fluid's boiling point from well over 400 degrees Fahrenheit to near 212 degrees. Rocks can chip away the line, promoting rust and a repeat problem in the same area. If you have to run a line in a questionable area, consider sliding a section of rubber hose over it. That hose should fit very loosely so water can evaporate rather than collect in there.
I like to leave the old line in place to give me something to attach the new line to. Use nylon tie straps to hold them together. Make those straps tight enough to prevent the new line from vibrating or sliding back and forth over the old one. That sliding will also grind away on the new line. Long stretches should be held up in multiple places. I prefer tie straps no more than about a foot apart. If a section can sag, it can flex, and flexing leads to "work-hardening". That turns metal brittle, and it will crack.
Do not use any type of grease or lubricant on the fittings or near the flares. All rubber parts that contact brake fluid are not compatible with petroleum products. Brake fluid contaminated with even one little drop of oil or grease is a story for another day, but it will lead to an extremely expensive repair. Professionals even work to avoid introducing fingerprint grease to brake parts.
When you have to reroute a line or put bends in it to use up extra inches, try to do that without introducing extra high spots where air can get trapped. Usually it's easy to bleed out, but why invite more work? I only use gravity-bleeding. Loosen the cover on the master cylinder's reservoir to prevent vacuum from building and impeding fluid flow. If you resort to the less-preferred pedal-bleeding with a helper, instruct them to never push the brake pedal more than halfway to the floor. Doing so can cause damage to the rubber lip seals as they run over crud and corrosion that develop over time in the areas where the pistons don't normally travel.
Also be careful to not let the reservoir run empty, particularly if the vehicle has anti-lock brakes. On many models, a scanner is needed to bleed the trapped air out of the hydraulic controller. That frustration can be avoided if the reservoir is refilled before air reaches that controller. Gravity is going to want to make the brake fluid run out where you cut the original line. A simple way to prevent that is to place a stick between the brake pedal and the driver's seat to hold the pedal down an inch or two. Gravity won't be strong enough to pull brake fluid past the rubber lip seals.
Other than when gravity-bleeding, it's a good idea to keep the cover snapped on tightly on the brake fluid reservoir. That reduces the moisture getting sucked out of the air by the fluid, and any fluid that does run out will create a vacuum that slows or stops that fluid draining.
This article may present you with some hints you aren't already aware of:
https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system
Also check out this video to see if I overlooked anything:
https://youtu.be/sO43p4ZFUiM
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Wednesday, May 15th, 2024 AT 4:52 PM
