WondrousBread

I recently had to replace my brake booster since my factory booster failed. Looking for a replacement I came across the Subaru Legacy swap discussed online. Most of the info was provided by Clokker, but as far as which years of Legacy had the correct booster & cylinder the only information I could find was “late 1990s”. But there were a few obvious advantages to the swap:

- Availability: The Rx7 parts are getting harder to find. I couldn’t find the booster at all online so it may be NLA at this point. 90’s Legacy parts aren’t exactly common near me but it would take me awhile to find someone parting an Rx7 within driving distance.

- Cost: To have the stock booster rebuilt by the cheapest vendor on Rockauto would be about $140 at time of writing, not including shipping. The Subaru booster and master cylinder combo cost me a total of $60, plus $3 for one brake line I needed.

- Master Cylinder Size: The stock booster is 7/8”. The Subaru cylinder is 1-1/6”, meaning that’s a 3/16” gain. Now, a bigger bore doesn’t automatically equal “better” brakes, but it does mean you can drive larger brakes if you are looking to upgrade calipers later.

- Dual-Diaphragm Booster: This is tougher to quantify since I’m comparing it to a non-functional Rx7 booster, but I find the pedal effort and feel more predictable with the Subaru setup as compared to how I remember the Rx7 setup feeling. I understand dual-diaphragm boosters are considered an upgrade, but I’m just taking that at face value since I can’t be sure about this. The stock Rx7 setup is also designed to feel a bit softer and have a longer throw.

So what is the goal with this post? To swap to the Subaru Booster and Master Cylinder while retaining as many OEM parts as possible and keeping the setup clean and safe. Also to provide more clear direction in case someone with less experience wants or needs to do this swap, and to try and shed light on which years of Legacy have the necessary parts.

I should also note that this guide does not apply for ABS equipped Rx7s. I don’t know enough about ABS to advise anyone on changing the sizing of brake components or the line arrangement. It may be feasible but I don’t know how to do it, so I advise against it.

Disclaimer: I am not a professional mechanic. Whenever you follow a guide on the internet, you are choosing to take responsibility for your actions (whether or not you followed the guide properly). This guide is no exception. Brake work is considered advanced as far as DIY jobs go because the consequences of failure can be life threatening.

If you choose to follow this guide you are assuming responsibility for the outcome, and you are modifying a part of your car that is an important safety device. Make sure to double and triple check your work as you go, and also make sure to read this guide thoroughly and make sure you are capable of finishing the job properly once you start. I will be explaining things as clearly as possible but as with all custom work the final result is highly dependent on your skill. Also remember at all times that brake fluid is hazardous to your health and eats paint. Follow this guide at your own risk.

Now that the dour safety discussion is over, we can talk about what we need for this swap:

- The Subaru Legacy brake booster and master cylinder (details below). Try to get them from the same car as a unit so the pushrod clearance is good.

- One 8” section of prefabricated metric brake line. M10 fittings, 3/16” hose, double flared.

- Access to an angle grinder, dremel, or other tool with cutting wheel attachment

- One metric brake line tee (sometimes called a splitter). Some cars have this from the factory so you may find it at the junkyard, otherwise they are available on Amazon.

- 10mm brake line wrench (don’t trust the open ended wrench, it will round off the fittings)

- Assorted wrenches and sockets.

- One brake booster check valve (again, junkyard or Amazon).

- Replacement hose for the vacuum line with clamps (optional, as required by your setup).

- Brake line bending tool (optional).

- A way to cut and join wires (soldering iron, crimping tool, anything except twist and tape. Optional).

- Appropriate PPE (nitrile gloves, safety glasses).

- DOT3 or DOT4 brake fluid.

Which years of Subaru Legacy am I looking for?

I don’t have a firm answer to that question, but I have enough to get you most of the way there. The booster you are looking for looks like this:



This particular booster and MC are from a 1996 Legacy AWD with ABS. It's a dual diaphragm booster (two steps in the body of the booster) and the MC has two outputs on the right hand side (as viewed from the front of the car). This is the reason you need to tee the brake lines, as the stock MC has three outlets. This MC is a 1-1/16" bore as mentioned above.

I had an opportunity to inspect a booster from a 1998 non-ABS Legacy (not sure what drive-train layout) and it was a single diaphragm booster with four outputs on the MC. This may work, but dual diaphragm boosters are apparently considered superior and the extra output will need to be dealt with.

From some reading I did online, apparently years 1995-2000 are compatible.

Altogether I can tell you that you are looking for the ABS model, but the drive train layout may or may not matter. Years are 1995-2000. That should get you in the ballpark, then you just need to see it in person before buying and ensure it has the stepped body on the booster and the two outputs on the right hand side.

The booster has the same firewall bolt pattern, pushrod thread size and pitch, and MC bolt pattern as the stock FC parts. I believe these dimensions are shared by all FC models, but I can only vouch for S4 NA.

To remove the stock brake booster and master cylinder, these are the steps you’ll need to follow (mind the order):

- Disconnect negative battery terminal

- Remove trailing ignition coils (three nuts, two spark plug wires, two connectors underneath)

- Remove the bracket for the coils that sits closest to the MC.

- Take pictures of all the factory lines on the MC that you can refer to later.

- Remove the factory clamp and vacuum line from the brake booster.

- Disconnect the wire for the level sensor.

- Crack loose (but don’t remove) the 10mm bolt holding the proportioning valve onto the booster.

- Place cardboard underneath the brake line fittings around the master cylinder and booster, and then place blue shop towels above that.

- Crack all of the brake line fittings loose until they start to drip. Don’t forget the two on top of the proportioning valve.

- One fitting at a time, loosen them slowly until they are all the way loose. Use shop towels to catch the fluid before it gets on your paint or wiring harnesses and change the towels frequently. The fluid in the reservoir shouldn’t leak down unless you press the brake pedal, so don’t worry about it for now.

- Once all the fittings are drained you can move to the inside of the car.

- Getting under the dash is a pain, but once you’re under there you need to use a set of needle nose pliers to carefully remove the clip from the clevis pin on the brake pedal. Then use a (14mm?) wrench to crack loose the locknut on the pushrod for the brake booster. Pop the clevis pin out and put it with the clip in a safe place. At no point should you press down on the pedal, or fluid will shoot out of the cylinder into your engine bay.

- Use a 12mm socket to loosen the four mount bolts for the booster.

Now you can take the booster and cylinder out from the engine bay. If you change the orientation of the booster fluid will want to drip so keep it flat if possible. Put it straight into a thick grocery bag or similar plastic bag that will prevent it leaking onto the garage floor.

Now turn your attention to the Subaru booster. There is exactly one modification required:



The red paint denotes the threads I had to cut from the pushrod. The Subaru pushrod is just slightly longer than the FC one and it collides with the pedal. In my case it was 5 threads, but I highly recommend cutting two threads at a time and test fitting (remembering to actually snug down the four 12mm nuts every time so the clearance is the same as once it’s installed). Cutting too much off would be a major problem as you can no longer adjust the pedal properly. If your booster included the Subaru clevis, you can remove it by putting a screwdriver through the clevis pin hole and holding it steady while cracking the locknut loose with a wrench. Then remove the FC clevis from the FC booster and install it in the place of the Subaru clevis. IF you ended up cutting the same number of threads as I did, you can thread the clevis on just enough to engage all of its threads and you will be within 1 thread of adjustment. Don’t forget to keep the locknut on the shaft, you’ll be kicking yourself if you install the booster only to find you forgot the important locknut and need to take it back out.



Once this is done you can put the booster into the car. Seriously, that’s the extent of the modifications required to the booster. After this it’s just lines and hoses.

Reinstall the booster in the same way you removed the FC booster:

- Install the four mount bolts, make sure they’re on tight.

- Spin the clevis out or in such that the clevis pin slides through the clevis and brake pedal with little friction. You are adjusting the pedal to the booster such that at the top of the pedal travel (no pressure on the pedal) there is no pressure on the booster.

- Install the clevis pin and clip.

- Firmly clamp down the locknut with a wrench.
Turn your attention to the FC booster and MC again for a moment. Remove the 10mm bolt that holds the prop valve to its bracket and set the valve aside. Then remove the bracket itself by removing the nuts holding the master cylinder to the booster. Take the little plastic level sensor out of the reservoir (no fluid will be lost). Take all of the lines you have removed here, the level sensor, the prop valve, and the prop valve bracket, and put them in a safe place. You can put the booster and MC themselves aside as you won’t be needing them again.


Don't be like me, wear gloves!

The way the stock system works is that the one output goes to the front right brake caliper. Then the other two outputs go to this prop valve. The output that goes to the leftmost fitting in this photo goes through the prop valve itself and then out the other end to the rear of the car. There, a splitter sends it to each rear caliper. The rightmost fitting is a pass-through. The fitting on top right sends the fluid out to the passenger side.

First, we will install the prop valve bracket. Remove one nut from the MC and put one side of the bracket on. Then thread the nut some of the way back on. Move to the other side and do the same thing, before threading down both nuts and tightening them firmly.





Install the prop valve, put the bolt back in, and reinstall the top lines. Now you can see the beauty of this swap; two of the three factory hardlines remain 100% stock.

This is the ideal arrangement that I recommend:

- Take the small brake line that used to be the leftmost on the prop valve and connect it to the front output on the MC.

- Bend it to meet the splitter at a location under the ignition coils.

- Unwind the loop in the front left line and connect it to one splitter output.

- Connect the other splitter output to the rightmost (pass through to the front right) on the prop valve.

- Connect the rear output on the MC directly to the leftmost fitting on the prop valve to send fluid to the rear.

At this point you can take the level sensor from the Rx7 MC and snip the wires. Do the same with the Subaru level sensor. Then solder or crimp the wires from the Subaru sensor onto the connector from the Rx7 sensor, and insulate the joints. Plug it into the stock location on the harness.
The last thing to connect is the vacuum line. The stock line integrates the necessary one-way valve in the hose, but we can’t use the stock hose because the nipple is on the opposite side from the factory. So run the hose out to the one-way valve, then from the one way valve to the factory hardline. Don’t forget the hose clamps, and mind the orientation of the one-way valve. The air should go from the booster towards the engine, but NOT the other way around.

Ignore the line arrangement in this photograph, it isn’t correct. It was redone later to match the above diagram.

Now we are at the final step; bleeding the MC and then the lines and calipers. Bleeding brakes can be tricky and this is another point where I’m going to harp on the fact that this is an advanced project you should tackle only if you are experienced in this area. I will tell you how I did it, but the responsibility is yours when doing this.

For every bit of air trapped in your brake system, braking power decreases. Air is compressible and therefore the enemy of a firm and effective brake pedal. Bleeding the brakes is the process of removing all of the air from the system. I will guide you (generally) through the process of bleeding the MC and then the rest of the lines and calipers. Keep the following in mind; If at any point the reservoir runs out of fluid, you will need to start the process over again. Check after every five pumps of the pedal to make sure the fluid level doesn’t drop too much.

Starting with the MC:

- Fill the reservoir with fresh fluid, almost but not all the way to the top. It is highly recommended to buy a new unopened bottle of brake fluid; Opened brake fluid has limited shelf life as it absorbs water from the air.

- Thread in all of the fittings on the MC and prop valve almost all the way down, minus one or two threads.

- Make sure you still have plenty of towel below all the fittings to catch fluid.

- Get into the car. With the car off, gently push the brake pedal down all the way to the floor. Do this while checking under the hood after every pump. Once a fitting is leaking fluid (without obvious bubbles), close it down firmly.

- Once all the fittings have been closed down, the MC is most of the way bench-bled. This means it is mostly clear of air and filled with fluid. In my experience at this point you are ready to move on to bleeding the calipers; If any air remains in the MC, bleeding the calipers should help to purge it.

The Calipers:

- Jack up the car one corner at a time and use a jack stand for safety. Start with the rear right.

- Crack the 8mm bleed screw on the caliper, and connect a small section of hose to the screw. Direct the hose into a jar or other receptacle. You can let it gravity bleed for awhile but then you will need to start pressing the pedal to really get it done.

At this point there are two ways to proceed.

If you have a helper:

- Have your helper open the bleed screw with the wrench. Then you pump the pedal to the floor and hold it. Have the helper close the bleed screw with the wrench. Then let off the pedal. Do this in batches of 8-10 pumps, making sure the fluid doesn’t drop to zero in the reservoir. This way in between pumps, the closed bleed screw forces the caliper to suck fluid from the reservoir instead of sucking back the air it just exhaled.

If you don’t have a helper:

- You could try directing the hose into a jar as above but keeping the jar half filled with fluid so it sucks back into the pedal. I don’t like this method because I find it fiddly and unreliable. I highly recommend grabbing one of those little “One-man brake bleeder” kits at a local auto parts store. It’s a small container with a hose and an integrated one-way valve to do the job of the helper in the above scenario. There are other vacuum tools and pumps that do similar jobs but all that matters is that you find one that works for you.

You need to remember to close down the bleed screw whenever you are not pumping the pedal or there is no hose on the end of the screw to keep it from inhaling air. You will know the brakes are bled when there are no more air bubbles coming from the bleed screws for several consistent pumps. If you see any more air bubbles, just keep pumping. It can take a long time but it will work eventually.

Now that you have the rear right caliper bled, you can move to the others. I use this order: rear right, rear left, front right, front left. You want to bleed the farthest caliper first and then move closer.

For the rear calipers, you will need to bleed two screws. One is further down underneath the caliper pointing downward, one is on top pointing upwards. I bleed the bottom screw first for 20 pumps, and then the top screw for the last 10 pumps. Every time I have done this, this arrangement led to a complete bleed with no bubbles.

You will feel the pedal getting stiffer as you bleed the brakes further. Once you have no more bubbles and you tighten the last bleed screw down, you’ll find the pedal is very firm and doesn’t go to the floor anymore. This is good, it means the brakes are well bled. However, keep in mind for the first few drives that you have just done major surgery on the brake system and to take it easy while you shake out any possible issues.

Once all four wheels are done, you can start reassembly.

- Reinstall the ignition coil mounting bracket..

- Reinstall the spark plug wires onto the ignition coils (you can install them with the cylinder in place, but the Subaru reservoir makes it a tight squeeze).

- Connect the two connectors for the ignition coils and drop the coils into place.

- Tighten the three mount nuts for the ignition coils.

Reinstall the negative battery terminal. No smoke? Good.

Wipe up any brake fluid below the master cylinder. Spray down the area with some soapy water (avoiding the wiring harnesses) and wipe it up, just so you don’t leave any residue. I also recommend having a helper watch the cylinder while you press the pedal a few times and make sure no fluid leaks. Have your helper walk around and eye the calipers as well looking for puddles.

Sit in the car and pump the pedal a few times with the car off. It should be very firm and the pedal should not sink to the floor under pressure. If it sinks down or is spongy, you have a leak or have failed to bleed all the air from the system. Check your work and try again, this is not something to take a chance on.

If the pedal is firm and doesn’t sink, then hold pressure on the pedal. Turn the key and start the car, and you should feel the pedal sink down slightly as vacuum applied on the booster reduces the pedal effort. It should NOT become spongy; only easier to apply the same firm pressure you were applying before.

Then take your foot off the brake pedal and shut off the car. Wait 5 minutes and come back, then press the pedal again. The first press should still have the reduced pedal effort you noted above, before subsequent presses get tougher. If the first press does not have the reduced pedal effort above, your one-way valve is broken. It is important that you get this right so that if you have a sudden engine failure you will still have enough brake assist to make it to the side of the road.

Does it feel right? Congratulations, you’ve done it! As mentioned before you need to be careful for the first few drives and make sure no problems crop up. Check for fluid when parking the car and under the cylinder as well.

In my case not only are the brakes far easier to press (no surprise since my previous booster had failed), but much firmer and the bite point is up higher. This is much more pleasant to drive and easier to modulate.
That’s about it. If anyone thinks of improvements or spots any errors in my write-up, please comment below. Thanks for reading Good luck.

diabolical1

iTrader: (3)

this is phenomenal. i have always been interested in this mod (props to Clokker - 'cause he originated it and gave me the inspiration), but i have also been afraid of screwing it up. you have single-handedly installed some confidence in me now.

thank you!!!

WondrousBread

Thank you! That was actually the whole reason for the write-up. I was a bit reluctant to tackle it too, especially since there wasn't a lot of info out there on the specifics. Hopefully this helps anyone who is in the same position.

JerryLH3

iTrader: (11)

Fantastic write up for those interested in this swap. I only have one thing to add. The brake level sensor connector is a two position Tyco Amp Econoseal (at least for an S5). Can pick one of those up and crimp that in if you cut the Subaru wires as close as possible to the connector.

Sash91

iTrader: (1)

Great and detailed write-up! I might get a swing at doing this soon with the JDL MFG bracket kit. It uses Acura TL Type-S Brembos with an Evo X rear disk. Coupled with a fresh booster, I'd imagine it would feel stout!

WondrousBread

Thanks, hopefully it helps you out Looking at the website I don't think the TL caliper is that much larger, but the extra fluid displacement should help either way. Remember to get an adjustable prop valve since you're changing the fronts and will now need to adjust against the stock rears.

The big difference with the Subaru parts is feel, they feel much more like a modern car. The FC parts aren't bad or anything, just unusual. Weirdly long throw.

Thanks for putting JDL manufacturing on my radar too, they make some really cool stuff (REW intake adapters for 6 port irons and the full-range TPS adapter in particular interest me). Bookmarked it for later