Converting Turbo Coupe ABS
 Master Cylinder to Vacuum-assisted Brakes

By Garrett Hunt (RedLX)

Since these cars are getting to be 20 years old now, the ABS system tends to conk out. If a new relay doesn't fix it, you're looking at major repairs. Putting in a new master cylinder runs in the $1000+ range, and throwing in a used one is really a crapshoot. The cost effective (and upgradeable) alternative is to convert to regular vacuum-assisted brakes. I figured I'd post this up since I've had several people ask about doing the conversion, and I couldn't find much info on it before I did it myself.

I used a '89 5.0 Mustang brake booster (and by the way, I had almost no problems getting this booster in without removing anything. Biggest problem is that some of the wire bundles like to fall behind it when you're trying to seat it against the firewall). I cannot guarantee other boosters, such as one from another Thunderbird with vac-assisted brakes, will be as easy to fit, as they are larger in diameter.

I used a SVO master cylinder (1 1/4" bore) because of the fact that I also installed 73mm front calipers, and this gave me a good pedal feel. I originally used an '85 Lincoln Town Car MC (1" bore) but the pedal was mushier than I liked with the 73mm calipers. I think that for the four wheel disc on the Turbo Coupe, the SVO master cylinder is probably your best bet since it was also on a four-wheel-disc car. Of course, there are also options with the later model master cylinders with plastic tanks, such as the '93 Cobra (1" bore), and I've heard some SN95 V8 master cylinders had a 1 5/16" bore. These, however, have metric fittings on them, which I'll explain later. You can't beat the SVO master cylinder on cost, though (around $40 from the parts store including the core charge).

Getting the old master cylinder out and the new booster in is honestly the hardest part. You can tee the lines off the rear port of the MC so you don't have to buy a proportioning valve. However, you have to get a metric tee and a metric female-female union for the tee. I found most of my adapters but one at Napa, but I had to go to 2 different stores. The metric tee, I actually picked up part #7919, the metric fittings on this tee are the right size but it's only got a 3/16" inverted flare fitting on the side. I tried to find one with a 1/4" inverted flare fitting but apparently they don't make one like that. Advance Auto parts carries the 3x16"x10-1.0 union that you need.

Now this is where things get a little complicated. For starters, the Thunderbird's factory prop valve has a 3/16" inverted flare fitting on the rear and a 5/16" fitting on the front, so you'll need an adapter fitting for the front. Also, the early style MC's (ones without a plastic reservoir) have standard fittings on them, while the later model ones (with plastic reservoirs) have metric fittings. You can get metric-standard adapters (to adapt a male metric fitting to a standard port) but not the other way around, don't ask me why. Also you can get lines with metric "bubble" fittings on one end and standard inverted flare fittings on the other end now, which would make plumbing in one of the newer MC's a lot easier. Anyway, you'll have to connect the "third" brake line (the one that runs to the driver's side front caliper) to one side of the tee, and the other port on it will be connected to the rear port on the proportioning valve. I happened to buy a complete '89 5.0 booster & MC with all the lines which provided me with a line to run from the tee to the prop valve. If you can find a 5.0 Mustang in a junkyard, get the booster and both the lines that run from the master cylinder to the proportioning valve, as one of them (sorry but I can't remember which) is just right to run from the tee to the prop valve.  Otherwise, you'll have to get one of the aforementioned lines (metric on one end, standard on the other) and bend it up to fit here. The front port is easier, since you just run a line from the front port of the MC to the front port of the prop valve.

I'll add a note here, some people have said that on the TC proportioning valve, if you remove the sensor threaded into the back of the proportioning valve, it's open to the inside of the prop valve and you could theoretically run a line from there. I haven't tried this myself and I don't know exactly what that sensor does, so it may be worth a try if you want to go for a cleaner conversion. No tee would be required as the lines would tee from the prop valve itself.

On the Lincoln MC, the front port on it was 5/16" also so I ended up adapting it to 1/4" line and running that. I also ran a 1/4" line from the rear port to the tee. The SVO MC is slightly different. On it, the front port is 1/4", but the rear port is some oddball standard size. I had to go to AutoZone and dig through all their adapter fittings until I found the right one, and I can't for the life of me remember exactly what size it is.

Finally, for the vacuum source for the booster, I just teed off that black plastic line that runs from the upper intake to the vacuum tree. A short length of 3/8" rubber hose and a 3/8" vacuum tee from Advance are all you need.

Now you just need to bleed the brakes and check thoroughly for leaks. With all the adapters and such needed, you may have to chase down a couple.

If you want to remove the wiring and such for the ABS, you're on your own. I decided I didn't feel like going through all of the hassle required to pull out a couple pounds of wiring.