What is the Name of This Brake Part?

[Brookshire my]

It diverts the one brake line to the two going to the back brakes.

[John Rogers]

That is the brake proportioning valve and can be the giant-sized headache when new comers try to bleed their brakes. Most serious racers of any type remove it and put in a tee. The upsides are firmer brake pedal and ease of bleeding. The downside is the balance of the car's braking will very likely feel different so if you do remove it remember that 914s do not have ABS and to practice emergency stopping in a large empty area.
John Rogers the oldracer

[GregAmy]

I strongly recommend against removing this device from a street car.

This is called a "knee valve" where the input pressure is "kneed off" to a lower pressure level as the master cylinder pressure is increased. Its purpose is to reduce rear brake pressure to avoid rear brake lockup, which is very important especially in rainy conditions.

Even racers, when removing this valve, will install a manually-adjustable knee valve. It would be incomprehensible for racers to remove the valve entirely without it. Example attached below as a photo.

What street car owners can do is shim the spring inside the factory brake prop valve with a washer to change the knee point and allow more hydraulic pressure to the back. I've done this on my street car and it has minimized initial front brake lockup -- but that's my personal preference and it will get removed if/when I sell the car.

So I'll reiterate: don't remove that valve from your street car.

Source: I'm a racing driver, pro and amateur, 914s and many others, and a race car builder/preparer of some 40 years. The valve below is what I'm using in my 914 race car (as well as others). There's other versions that use a rotating knob but I prefer the lever since I can tell with a manual touch where the setting is without having to look at it.

[Dave at Pelican Parts]

Note that Eric from PMB dislikes calling it a "proportioning valve", for reasons that don't quite make sense to me. (We've had good-natured arguments about it in the past.)

It can be very annoying when air gets into it, as it is difficult to bleed and often makes a big mess when you bleed it.

Definitely not something you want to remove unless you are willing to re-engineer the brake system and brake balance, and do a whole lot of testing during that process.

--DD

[GregAmy]

Knowing how accurate I've seen Eric get, it may have something to do with the word "proportion", given that these valves don't actually "proportion" pressures (say, a 60/40 split) but instead "knee over" the set point when the valve kicks in.

Below is a chart of what the Tilton lever-type valve does. Note that at each lever setting the pressure at which it "knees over" increases, but from that point the rear pressure follows its own a straight line but at a different slope. So for dry conditions where you're not worried too much about rear lockup you use Position 7 to maximize rear brake pressure; in slick conditions to avoid rear lockup you might use P1 to minimize rear brake pressure...or somewhere in between.

Our stock "knee" valve works the same exact way; at some point as the input pressure is rising the valve knees and the rear pressure changes a different linear rate...but not proportionally. I've never tested a stock one to see where it cracks over, but for safety purposes I suggest it's probably on the lower end of this scale (locking rear brakes in the rain and spinning off the road backwards is far more dangerous than locking the fronts and understeering off the road straight).

The washer trick for the stock valve increases the resisting spring's force so that it takes a higher pressure to knee over, similar to moving the lever from say Position 2 to Position 3. This increases your brake bias forward (but don't do it too much, Piastri!)

I wouldn't go too crazy on it; too much might make for a "saucy" ride in the rain...

[callard]

Thanks Greg.