Brake Valve/Booster thingy

A mysterious part found on 250 Series Ferraris is a brake valve that is often mounted below the radiator and is plumbed into the front brake system. In a discussion about brakes on the Vintage Ferrari Forum, one very good description and explanation came from Brian Brown who is a mechanic that works for Patrick Otis in Berkeley, CA. Parker Hall, of Kilimanjaro Designs, send me a personal e-mail with his description of the valve, and I decided to record these two notes permanently on my website, so we can all be enlightened. Thanks guys for the information!


The unit incorporated into the front brake circuit is not a valve, nor is it a safety braker. It is a proportioning cylinder. It is designed to increase the pressure to the front brakes.

It is very simple, has only two parts: a piston and a return spring. The way it works is that brake fluid pressure from the power brake booster is applied to the backside of the piston. This moves the piston forward. The front side of the piston has the same outer diameter as the rear, but the center of the front of the piston extends through a seal to the outside of the unit. so the effective area of the front side of the piston is the outer circle minus the inner circle (diameter of the part that sticks out from the unit).

Let's say that there is 100 psi coming from the brake booster and the back side of the piston is 1 square inch, now there is 100 pound of force pushing on the piston. Let's say that the front side of the piston is 1/2 a square inch (due to the center missing from the doughnut). 100 pounds of force divided by 1/2" creates 200PSI coming out of the unit.

The unit can cause the front brakes to stick on. If the piston sticks, then the front brakes will stay on. Also if the unit is improperly assembled, then the front brakes will stick on. There is a flat spot on the top of the piston, it must line up with the compensation ports on the housing, if it doesn't then then pressure in the unit cannot be released when the brakes are released.

The hardest thing about rebuilding the unit is finding seals that will work.
Otherwise it is about the simplest part on the whole car.

Later cars used a proportioning valve in the rear circuit to limit pressure to the rear brakes, rather than boosting pressure to the front.

Brian

The booster equalizer on the front crossmember has two functions. The easy one is to split the flow equally to the front wheels. The more tricky one is to boost the pressure to the front wheels. This was an early effort to equalize the pressure to all four wheels. The booster/equalizer has a piston on the incoming side and a tube or rod on the outgoing side. The larger surface area on the intake side is reduced by the cross sectional area of the rod on the outgoing side thereby increasing the pressure and reducing the volume to the front wheels. The outgoing side has a rubber boot(which is often damaged allowing the piston to stick in one position). You can tell if the assembly is working by pinching the boot and having someone apply the brakes and feeling the rod protrude out of the cylinder, inside the boot. The 250 series cars had 2.125' front cylinders and 1.750" rears whereas the 275's had 2.125" and only 1.625" rear cylinders, as Ferrari learned to balance the brakes without using the booster/ equalizer. Note no booster/ equalizers on the 275/330's. There are two or three different set ups of the booster/equalizers so it might be a little difficult to find the correct lathe cut "O"rings.The nut on the big end is often very difficult to remove. Get the right wrench and use a little heat and plenty of penetrating oil. Start with Rutland's and ask for Serio or Bryan and have all the measurements in hand when you call. You can also remove all the guts, weld up the small end and turn the booster/ equalizer into an "equalizer" with no apparent loss of balance or breaking under normal driving . Don't ask me how I learned all of this, it was too painful to repeat. Best Wishes...Parker Hall