What is the little valve under my brake master cylinder?

[Lof8]

I'm in the process of mounting up an ix booster for my 24 valve swap and my stock master cylinder has a valve mounted under it that has 2 lines going to the MC. What is it?

I've been told to remove it.
Any input?

[paulkeith]

not 100%, but it looks like a proportioning/metering valve. not something I'd recommend removing.

[Lof8]

not 100%, but it looks like a proportioning/metering valve. not something I'd recommend removing.

This was my line of thinking, however, none of the swap pictures I've seen have this device and an experienced swapper says he's removed them without problems. I'd like to find out what it is and make my own decision or get some other experienced people to chime in.

[Jean]

It's a stock brake bias valve..i would leave it alone personally.

[Lof8]

looks like I'll need to work on mounting it differently. Any ideas as to why some cars do not seem to have it?

[Jean]

Not sure, both of mine have it IIRC. Is it in a way?

[Lof8]

the ix MC mounting points are turned 45 degrees compared to stock and it puts this bias valve and the shock tower in the same place at the same time. I'll probably fab up a new mounting bracket to put the bias valve directly under the MC again

[Massive Lee]

If you increase front braking power by a substantial amount and stiffen up the suspension, it is not a bad idea to remove that valve and replace it with an adjustable valve if you race.

[Lof8]

Hey Lee, I don't do any racing, but I do autocross a bit and plan to eventually pick up one of your front street kits. what is your opinion on removing the bias valve? Why do some cars seem to not have it from the factory?
thanks

[Massive Lee]

I believe that all cars with front and rear discs have that valve. Maybe the 1984-85 318s don't have it as they have rear drums.

Be aware that slamming the brakes, and not having the valve, will make for some interesting rear end swaying... You need to replace it with an adjustable valve to tune the rear pressure.

If you autocross, you might want to increase rear pressure to also increase rear brake power for tight corners...

[Jean]

My 318 with drum brakes has the valve as well FWIW. When I converted it to 325i brakes front/rear I left the valve in place, autocrossed for a full season and a half with stiff suspension/rt615 tires and had no issues with brakes.

I am not sure if the valve is different on different cars / years...


just my 2 cents.

[RangerGress]

There's two different brake bias valves in the E30. The more common one is a separae little cylinder about the size of a 35mm camera film container which is fastened some inches below the MC. That's the single pipe design.

The less common design is what you and I have, where it's dual pipe and is fastened to the underside of the MC.

Don't pull the bias valve unless you plan to replace it with a different kind of bias valve.

[paulkeith]

I know that on many vehicles in addition to providing front/rear bias, the valve also includes a safety feature to plug either the front or rear system in the even of a leak on either end. Does this one have any wires going to it?

[RangerGress]

Neither E30 bias valve design has wires going to it. The dual pipe design won't close in the event of a leak. I've seen it's insides and there's nothing there that would function like that. The 4 pipe design I couldn't say. That's the one I have, but I've not seen it's guts.

[paulkeith]

ten four, I know i fought the one on my CJ-7 for days trying to bleed the brakes. if you open up the rear system and then try to bleed it afterward, it senses the drop in pressure in the rear as a leak so the valve closes off the rear, making it a little difficult to bleed said rear circuit.

just throwing it out there as something else to consider.
Paul