Partitions in brake reservoir

[Murray_Calavera]

Hi all,

So I was doing a major service on the Mrs Subi today, I went to flush the brake fluid and found something interesting. The brake reservoir has a partition in it. 

I drained the front accessible partition, no fluid drained back in from the rear partition (the levels didn't average out). Next thought, perhaps front/rear brakes use front/rear partition in the reservoir. Nope. Put a litre of fluid through it starting with bleeding the back, nothing drained from the rear partition in the reservoir. 

I checked the Subi workshop manual, nothing is mentioned about the rear partition that I could see. I read the sections on the brake lines, master cylinder, ABS Sequence Control and  VDC Sequence Control. 

The brakes bled up fine and work perfectly, but I really want to flush the rear partition that is filled with old fluid which is noticeably darker. I've uploaded a photo from the workshop manual to show the partitions. 

Edit: does anyone know what the rear partition is used for? If all the brake lines were completely flushed and it wasn't touched, when would the rear partition be used during normal operation? 

Edited May 4, 2020 by Murray_Calavera

[Rusty Nuts]

Dual master cylinders are typically a single unit that is divided inside. The master cylinder has a single brake fluid reservoir that has an interior partition as well as a single piston that is segmented to push fluid into two different brake lines.Dual master cylinders make power brakes less prone to failure. Each chamber of the master cylinder operates the brakes to one set of wheels. If the brake line going to one set of wheels is broken or the brake fluid is lost through other means, the other set of brakes will still work properly. In a single master cylinder system, a broken brake line would cause the power brake system to fail entirely. Many systems are split diagonally ie Front Right / Rear Left.

Edited May 4, 2020 by Rusty Nuts

[Murray_Calavera]

I thought this as well, however all 4 calipers have been bled and the rear partition hasn't been touched at all. 

As each caliper was bled, only the front partition drained. 

[Rusty Nuts]

Give Subaru a call and ask them.

[Murray_Calavera]

Yeah that's the plan at this stage, thought I'd pick everyone's brain here over night while waiting for tomorrow to arrive though. 

[Gareth]

That's interesting.....not something i have seen before so i can't shed any light on the subject. What type of subaru is it?

If you find out can you let us know.....

[Murray_Calavera]

It's a 4th gen Subaru Impreza. I'll eventually get to the bottom of it, when I do I'll post up the outcome.