This topic has been in the back of my mind for YEARS. All Subarus since 1990 (and most before that) had front disc brakes, but there were no Imprezas ever sold in the US with no ABS and rear disc brakes. They either had no ABS + rear drums, ABS + rear drums, or ABS + rear discs. On the surface you might think “who cares? just swap on some rear discs and you’re good”. I used to think like that, in fact I did that conversion at least once.

Story time: my first actual Subaru project was back in… 2007? I spent a whole summer working a desk job and really spending all my time scouring the internet to learn about Subarus. Then I found a 1993 Impreza L, FWD, 5mt, rear drums, and no ABS. Of course in the process of converting to AWD (thats another post for another day), I swapped on rear disc brakes, then later a v3 STi engine and transmission, etc. Never once had I come across the words “brake bias” or heard of a “proportioning valve”.

Over the next couple years I drove the shit out of the car, did a few AutoX days, and eventually rode the thing into a ditch. Sold it, moved on. But what I didn’t do what ever actually think about mechanics and physics. I didn’t realize until later that the time I spun out at AutoX was because my rear brakes locked up in a corner, the same as that time I almost slid into the other lane while braking in a corner because my rear brakes locked up, and the same as the time when I wrecked sliding out on a corner because my rear brakes locked up (then overcorrected and nosed myself into the ditch).



It should be no huge surprise that drum brakes take more line pressure to get similar performance as disc brakes. After all, disc brakes are a superior technology and better performing. Also, it should be no surprise that you wouldn’t want to put equal pressure to the front and rear brakes in some situations, like under hard braking; all the weight is on the front brakes! If equal pressure was at the rears in that moment, the rears will lock up and assuredly put the car’s stability in jeopardy. Conversely, if you’re doing light braking, you’d probably want the fronts and rears to operate at a more equal rate since there’s much less weight transfer, and you wouldn’t want the car diving forward. Same reason why on a motorcycle the “proper” way to stop is always with both front+rear brakes, except in this case, YOU are the proportioning valve. The takeaway is: ideally you’d have a different split of F/R bias depending on how much brake line pressure is in effect. Here’s a neat chart that shows what this could look like (via stoptech):

So, it turns out there ARE features of the brake system in a car that determine the bias of brake line pressure at the front and rear brakes, and it could depend on a variety of design factors (weight and size of car, type and size of front/rear brakes, desired performance, intended use of vehicle, etc). The proportioning valve manages brake bias, and is also calibrated for a certain line pressure “split point” where the proportion changes. In the above example table, the F/R split is even until the split point of about 600psi, where the prop valve reduces the proportion of rear bias as line pressure increases.



Back to my story. Time passed and I stumbled onto some forum discussions about running rear discs on a non-ABS Impreza, mentions of prop valves and brake bias, and saw this catalog chart for 1995 Impreza models (thanks “JayGold”!):

Well, there it is. All models with rear drums have a prop valve split point much higher compared to rear discs (427psi vs 284psi). So if you swap rear discs to an Impreza that had rear drums, and do medium-hard braking, you’ll be maintaining even F/R split until a much higher line pressure.

The solution? Swap the prop valve from a car with rear discs! Wait. Not so easy. The Impreza is the entry level Subaru of its time. They weren’t about to make 100 different feature variations. If you got 2wd, you got rear drums and no ABS. If you wanted ABS and rear discs, you had AWD. ABS lines route totally differently through the engine bay compared to non-ABS, but more importantly the prop valve is complex on ABS cars. With non-ABS, there is typically 1 inlet and 2 outlets: 1 to fronts and 1 to rears, which split off to each wheel later for simplicity. With ABS, it will have 1 or 2 inlets (probably single or dual-stage booster) with 4 outlets each running to the ABS unit, then each to the wheels.

Now what? With this newfound information, I kept researching. Subarus are like Legos, and many use similar parts and layouts - people knows this. How hard could it be to find a non-ABS Impreza with rear discs? Turns out… difficult. Think back to the 90′s. AWD is a luxury. Rear discs are a luxury. ABS is a luxury. Subaru limited them to only existing together on the Impreza. Going back to that forum thread from earlier, 2 promising leads surfaced: the JDM WRX/STi RA, which was AWD, came without ABS, and definitely had 4-wheel disc brakes. Problem is, those were ONLY sold in Japan, so the part number was expensive, only sold in Japan, and hard to buy anyways. Grumble. Lead #2: Rumor has it some models of SVX came without ABS but with rear discs, unfortunately there was no good source for part number because Subaru doesn’t publish robust parts diagrams for these early 90s models. Plus I certainly wasn’t going to drive all over the state and rummage through junkyards just to see what options existed. I thought this was all fascinating but didn’t have a project Impreza anymore, and I eventually lost interest.



This brings us to 2016. I recently picked up a cheap 1992 Legacy wagon and threw it up on the lift the check it out. Wait a second, rear discs? Those are kinda rare on these early 90s Subarus. Wait another second, non-ABS? I looked up the definitive source on the Subaru build configurations, cars101.com. Sure enough, in 1992 on the Legacy, the L trim had 4-wheel disc brakes, but ABS was optional. I looked some more, and this was also optional in 1991, and standard in 1990 on manual transmission models.

There you have it. Research shows it could be part# 26873AA020, which is discontinued. At least you can hit the junkyards and hopefully set your project Impreza up with a less dangerous brake setup now

