Mechanics

ABS CONVERSION

1991 Lincoln Mark VII • V8 Automatic • 115,000 miles

1991 lincoln mark vii, ABS is not working. Too expensive to replace parts. What Booster, Master cylinder, Adjustable proportioning valve are needed? Your help would be greatly appreciated. Thanks.
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Seajay1
June 21, 2011.



Wow, I see what you mean about cost. The best recommendation I can make is to find one or two cars in the salvage yard that have the standard brake system and use them for parts and reference. Your current hydraulic assembly has the push rod included that attaches to the brake pedal. That means the power assist comes from pressurized brake fluid, (similar to hydro-boost systems that use power steering fluid), and you will have to switch to a vacuum booster. Those are very low failure items so you should be able to get one from a salvage yard very inexpensively. You might get the master cylinder with it, but don't get one that has had the cover removed or any other part of the hydraulic system opened to outside air. Brake fluid loves to absorb moisture from the air and that can lead to corrosion of metal parts. I looked up the master cylinder on rockauto. Com and see they cost about double what I expected. I don't typically go the cheap route with master cylinders, but if you find a good one, and you're comfortable with brake work, you might want to pull the piston assembly out and clean out the bore, then reassemble it. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Running the brake pedal all the way to the floor, as many people do when pedal-bleeding the brakes, can rip those seals. If you don't clean it up first, be careful to never push the pedal more than halfway to the floor. If it's made of steel, you can use a cylinder hone or sandpaper wrapped around a long cotter pin, with a drill, to clean it up. If it's made of aluminum, don't use anything that can scratch the anodized coating. No sandpaper, no picks or other sharp objects either. Just rags and brake parts cleaner. Anything that scratches that coating will lead to rapid corrosion.

When it comes to calipers, I can't find any reference to "with" or "without ABS" so that tells me they are the same, which is typical. What you need to watch though is to get a master cylinder from a rear disc brake car if yours has rear disc, and get one from a drum brake car if yours has drums in the rear. The port for the rear brakes will have a residual check valve in it for drum brakes. That keeps about 10 psi on that hydraulic circuit so the lip seals in the wheel cylinders won't fall down and so air won't sneak in when the barometric pressure goes up overnight. If you have that residual check valve with rear disc brakes, the pressure that is maintained will keep those calipers from fully releasing.

Most people don't even know their car has a combination valve / proportioning valve, let alone know what is does. They fail so seldom that no parts stores stock them. Get a used one from the salvage yard but be sure it's from the same model car as yours. The pressure-differential valve is built in and is calibrated to the weight distribution of the car. It limits fluid pressure to the rear wheels as you push harder and harder on the pedal to prevent rear wheel lockup. Many cars with ABS don't use a separate combination valve. They either rely on the system to stop wheel lockup or they build those valves into the hydraulic controller. I can't tell from the photos, but I suspect there are four steel brake lines coming out of the master cylinder / controller assembly on the driver's side. The standard master cylinder appears to have only two lines coming out of the passenger side, which is typical. To address that difference, get the power booster, master cylinder, two lines, and combination valve as a complete package and leave them all bolted together. That will make the transplant easier. When you have the old system off the car, set it beside the replacement to compare the length of the push rods. If they're different, you may need the pedal from the donor car. It would seem unlikely they would design two different pedals, but check it anyway while it's easy to do. The replacement master cylinder is listed as having a 1 1/8" bore diameter. That is important for proper braking. The wrong diameter will cause too much pedal travel or the need to push too hard to stop the car. That's why you should stick with parts from the exact same year and model as your car. Parts can look the same but be different between years and models. One thing you can do when in doubt is look up the parts on rockauto. Com and compare the original part numbers they list for different applications.

Don't forget the vacuum hose and check valve for the booster, and possibly the vacuum port in the intake manifold if it's different than yours. If you are anywhere near Ohio, Indianapolis, down to southern Georgia / Alabama, there is a real nice chain of about two dozen salvage yards called "Pull-A-Part" where you pay your dollar, then you can spend all day there. Throw your tool box into one of their wheel barrows and have fun. Employees and customers are very friendly and helpful, parts are inexpensive, and the yards are real clean and well-organized. You can do an internet search of their inventory, and they bring in two rows of new cars every few days. On their menu from a few years ago, they list a master cylinder, "with a warranty" for $11.03. They also list a computer at $31.89. I heard of one fellow who got a very high failure GM truck $800.00 ABS computer for that price. You might even find the ABS controller for your car.

I just did a search and found three '91 models out of 15 Mark VIIs; one in Indianapolis, one in Cleveland, and one in Knoxville, TN. The oldest one was brought in April 29.

The steel lines leaving the hydraulic controller will have to be connected to the combination valve. If your ABS system is a three-channel system, the two rear wheels will be controlled together and there will just be one line running to the rear axle. If you have a four-channel system, the two rear brakes were modulated independently and there will be two lines going to the rear. You should be able to use either one of them to get to the axle, then use the brass tee / rubber hose, and you'll need the two steel lines that go to each wheel. If the steel line in front is too long, you can use a tubing bender to coil it up, and if the fitting is the wrong size, the auto parts stores have a whole bunch of adapters just for brake line applications. The combination valve will have two ports for the two lines going to the two front brakes. There's a good chance the fittings will be the right size.

If you bench-bleed the new master cylinder, there is a trick you can do to prevent having to bleed at all of the wheels. If the steel lines are angled up while they're disconnected, the fluid can't run out. If fluid is dribbling down from the master cylinder and combination valve, air won't be in them either. When you make the connections, leave them just a little loose, then have a helper push the brake pedal very slowly so it takes perhaps 20 seconds to get halfway down. You'll see the air bubbles come out at those connections. Tighten the fittings before he lets the pedal back up to prevent drawing air back in. Do that a second time, and even a third time if you're still seeing bubbles. Finally, push the pedal slowly again with the fittings tightened, hold it there for a few seconds, then let the pedal spring back rapidly. By pushing slowly, the fluid will go to the brakes but leave the air bubbles sit where they are if any are left in the system. When the pedal snaps back quickly, the fluid rushing back into the reservoir will wash any air bubbles back with it. Sometimes that takes a long time to occur after other similar service, but every time you're sitting at a red light, a little more air works its way back up and out, and the pedal starts to get better. This trick works real well right at the master cylinder when there are only two lines coming out. It is very seldom I have to bleed at the wheels after replacing a master cylinder. The only disadvantage to that is the old moisture-laden fluid doesn't get flushed out, but up here in Wisconsin, the road salt capital of the world, we're more likely to run into bleeder screws that are rusted tight so knowing tricks like this makes our lives easier.

The last thing has to do with that combination valve. There will be a switch on top in the middle that turns the red warning light on when there's a leak in one half of the system. I don't know where that wire is on your old unit but it's usually the same wire that goes to the parking brake switch on that pedal assembly and if there is a switch in the reservoir to warn of low fluid level, that's the same wire too. More importantly, Ford is the only manufacturer that uses a switch that is not spring-loaded to center itself. That makes it REAL frustrating to get the light to go off after over-zealous pedal bleeding. To prevent that valve from tripping the switch, don't push the pedal more than halfway to the floor. A better way to avoid that misery is to unscrew the switch, (fluid will not leak out), and screw in a special tool made just for Fords. It will hold that valve centered until you're ready to reinstall the switch when the pedal feels good.

Caradiodoc
Jun 22, 2011.


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