Last Words On Master Cylinder Bore Diameter
Joey Carvalho; Santa Rosa, CA: Excellent job on the magazine. I felt I needed to write after reading about the 7/8-inch-bore Wilwood master cylinder in the Sept. '10 tech column. This master is an excellent product and I continue to use it today in my Torino with front and rear manual disc brakes. However, I hate to say it, but it seems to have longer travel than other master cylinders of the same size. I was using a Ford OEM 7/8-inch master until it started to leak, and I went crazy trying to bleed out what seemed like air after I installed the Wilwood. Yes, my Torino has long pedal travel. Yes, my pedal ratio is correct. And yes, I get 1,250 psi front and 975 psi in the rear. I did try a 1-inch unit but that didn't produce enough pressure. I believe the reason lies in the design of the Wilwood. Bench-bleed it and you will discover that approximately the first 25 percent of the stroke sends fluid to the front circuit only. Push a little more and front and rear fluid is pumped. Installed in the car, you will have more travel than normal until the rear circuit contributes volume. The prop valve is doing its job properly, balancing the pressure to front and rear still. At 200 bucks, I will live with it. Thanks again for the great magazine.
Jeff Smith: We shared your letter with Dustin Burr at Wilwood, and this was the response from one of Wilwood's engineers:
"There is a flaw in the reader's observations. He is basing them on what he sees during bench bleeding. There is no pressure developed during bench bleeding because the system is open. There is a differential in return spring preloads between the front and rear circuits, with the rear being higher by perhaps 5 pounds of force. I don't know the exact figure off hand. That is so the front piston is held in the correct location upon full release of the stroke. Because of this differential, during a bench-bleed stroke, the front spring is collapsing (and pushing out fluid) until its spring load matches the rear, then the rear starts pushing fluid. That may well take 25 percent of the stroke (I have not measured it). That is what the reader is seeing."
Wilwood's Dustin Burr added to this explanation: "Essentially what our engineer is saying is that what you see when bench-bleeding the master cylinder is not the same as what happens when the system is closed and under pressure. Under pressure, the return spring is overcome much quicker and fluid is pressurized at both ports at the same time.
"My two cents on brake pedal travel is that much of this is entirely subjective. Two guys can drive the same car and one will tell you the travel is too long, and the other will tell you the travel is perfect. Both of their opinions are based on the brake lever travel they are accustomed to feeling. So, if the pedal feels like the car you drive every day, you'll probably say the pedal feels good. If it doesn't feel like the car you drive every day, you'll say the pedal doesn't feel good. Drag racers typically use large-bore master cylinders because a very high and hard pedal at the end of the 1,320 is comforting, even though a smaller bore that requires less effort is more effective at stopping the car. Road racers typically go the other direction, with smaller-bore master cylinders that require less effort and are easier to modulate. Neither of those two opinions matters at all to brake function. The purpose of a brake pedal is to create and modulate pressure in the brake system. If the pedal allows you to create enough pressure to operate the brakes and can be manipulated for different stopping levels, then it's doing its job correctly.
"One of the things that plagues many hot rodders is this: Nearly everyone has a late-model car as a daily driver, and nearly all late-model cars are built with vacuum boosters, large-bore master cylinders, and short pedal ratios. This makes for a pedal with little travel but plenty of system pressure due to the vacuum booster. Everyone has become accustomed to that pedal feel and by default assumes it is the "correct" pedal feel of a good brake system. Of course, the problem is that many hot rods and muscle cars don't have enough vacuum to support a booster and rarely enough room for a booster of sufficient size. So they go with a manual brake system, which functions well but feels different from their daily driver. It doesn't feel "correct" to them because their daily driver has been telling them how it should feel for 30 years or so. But isn't that the point of having a hot rod, that everything feels different from your daily driver? Different doesn't mean wrong. No one thinks uncontrollable amounts of power are wrong, they're different—and that's why we build them, right?"
Wilwood; Camarillo, CA; 805/388-1188; Wilwood.com