I have BMW 320i brake calipers on the front and stock calipers on the rear. Just replaced the front pads, but the rears still have plenty of pad left. I want to replace the rear brake pressure valve with a T fitting to get more braking in the rear. I doubt the rears will ever lock up before the fronts with the larger 320i brakes. It looks like I'll have to extend a few lines and maybe fab a bracket. If anyone has done this, do you have any tips or pictures? Thanks

The general consensus is don't. I'm sure @eric_shea will have some insight for you...

According to the guys at PMB, the pressure regulator works like a T fitting until the system pressure reaches 525psi (panic braking) and they don't recommend removing it on a street car.

Besides, if the fronts are working better with the bigger 320i calipers, wouldn't that make it easier to transfer weight forward under hard braking and cause the rears to lock up even easier?

Bad idea.

As a engineer that designed and released brake systems you have it backwards.

Brake size does not determine when the rears lock up as a single variable. Tire grip, vehicle load condition, and dynamic weight transfer determine when the tires skid.

Because you have more brake on the front, you are actually likely to increase the weight transfer from rear to front which will encourage the rear brakes to lock up earlier.

Rear skid is inherently an unstable vehicle condition. It tends to be even more unstable in a mid engine vehicle with a low moment of inertia (i.e. the 914).

I could write you a whole white paper on this topic if you really care about the details. I'll leave it as this -- going to a T fitting is the worst possible thing you could do for a street driven vehicle.

Others will chime in and/or go take a look at PMB's web site if you don't believe me.

I would listen to Superhawk996 - but that's just me.

What gives you that idea?

Also, why do you think the 320 calipers will stop you any better than the stock calipers if you are running them on the same solid rotor?

I have never had an issue locking up stock front calipers at will even with 225 width autox rubber. If you want to regain balance by swapping parts, swap the stock calipers back on. Swapping the rear anti lock valve wont change the balance but it will help you spin out of control under improper breaking. All it takes is the rear end getting a little light...


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Thanks for the vote of confidence. But no one should listen blindly without questioning.

Heres a good example for doubters w.r.t. loading.

If you had the trunk loaded with gold bars, you would have more grip at the rear and the rear brakes could exert more force without locking up. However, if you had the gold bars in the frunk, there would be very little grip at the rear and the rears will lock up sooner.

Modern ABS and stability control systems use electronics to vary the quantity of rear brake line pressures in response to loading. We now try to maximize the amount of work the rear brakes do early in the stop before weight transfer to the front occurs. By doing this stopping distance can be shortend significantly. However, it also has the effect of tending to wear the rear brake pads much faster than what people were historically used to.

It used to be very common to go though multiple sets of front pads to a single set of rear pads. New cars, often go though two sets of rears to each set of fronts. Go figure. No free lunch!

WoW! That is an amazing photo! You need some gold bars back there!

"Stoppies" are very common on Sportbike motorcycles. I've never seen a 914 do one!

GO to the Classic Section and READ everything you want to know about brakes along with the URBAN MYTHS.

Wow, glad I asked before I did anything! Went tp PMB web sight and read about the pressure valve only functions during a panic stop. Thanks to all for the advice. I think I will send PMB my pressure valve for a rebuild just for piece of mind.

Extra weight that comes with an engine swap would change this equation, I'm assuming. (Which I shouldn't, because we all know how THAT can go...) Would the addition of an adjustable proportioning valve that's properly dialed in be a satisfactory alternative to the factory pressure limiter?

Depends on what you are looking for in a "satisfactory alternative". The factory valve is a safety device, the function of which a proportioning valve doesn't perform.

Done it. The common VW tee fits with no effort.

But the balance is so far off that you would brake better with the stock front calipers which let the rears actually do something.

I'd post the math stuff but it's complicated.

Well that solves a mystery! I thought there might be something wrong with the brakes on the DD because of this. Everything seemed to be in good shape other than faster wear on the rear pads. Now I know.

BTW, feel free to put those gold bars in either my trunk or frunk. I'm not picky.