Hard brake pedal.

[CHILI442]

Hello, I have a '68 Olds 442 with 13" C5 brakes on tall ATS spindlles (stock C5 calipers,/ Baer rotors). On the rear I have an 11" Ford explorer setup (single piston). The brakes were fine years ago when i was running the stock A body calipers on 12" B-body spindles with a '79 Trans Am master cylinder (1-1/8" I think) and stock rebuilt power booster. I went down to a 1" MC and the brakes got better, so I went down to a 15/16" MC and it was better yet, but I was stopping with the rear. The rear wheels were getting hot, but the fronts weren't. I took out the rear dampener I had in place and put in a proportioning valve. I set it up in the middle but adjusted it all the way to the front. The brakes seem pretty balanced, and they work fine in normal traffic, but if I have to get on the brakes hard, the pedal gets hard as a rock. I really have to stand on the brakes. They also don't seem to have that bite when I tap them. I understand the two piston calipers on the front have more area, and thus need more pressure, and that's why I dropped down in MC size. Could I be deadheading the system due to the smaller rear brake setup and the proportioning valve cranked all the way forward? Do I need a larger rear brake package? Someone told me that the corvette calipers are designed for a lot higher pressure and have a restricted orifice, and that corvettes have some kind of booster pump, but I can't find anything to support this. Anyone have any ideas? I would hate to lay down big coin on a Wilwood setup just to fix something that really isn't broken. Thanks in advance.

[Apogee]

We have plenty of customers running the C5 or C6 2-piston front brakes with the Explorer rears with good peformance, so there isn't anything inherently wrong with that setup from a front/rear bias perspective. That said, piston count and piston area are two different things, so I wouldn't assume that more pistons means more volume is required, as that's not the case. Your factory D52 calipers had 6.7 square inches of piston area and the C5/C6 calipers have 4.0 square inches, 40% less area, and that is why the C5/C6 calipers require more pressure to function properly. You achieve higher pressures (and lower volumetric flow rates) with a smaller bore master cylinder, larger booster(gain factor), higher pedal ratio, higher pedal effort, etc. Your solution to run a smaller bore MC is correct for the caliper change, but the logic behind it is a bit flawed.

Corvette calipers do not have a restricted orifice of any kind...I don't know of any that do for that matter. C5 and C6 Corvettes run dual-9" diaphragm brake boosters...I'm not sure as to what a "booster pump" may be, but I'm pretty sure there's nothing like that on a Corvette. Aftermarket auxiliary vacuum pumps are available for applications with low vacuum, is that to what you are referring? Are you running a factory single-11" diaphragm booster? If so, this will generate about 35% less boost than the C5/C6 boosters.

What is a "rear dampener" within the context of brakes? When you replaced it with a proportioning valve, you can only adjust the pressure curve to the rear brakes assuming you installed it in the rear brake circuit as intended. There is no way to increase or decrease front brake pressure, which should always be 100%, but rather only the rear pressure relative to the front.

If you want to increase the front brake torque, I would suggest that you first make sure everything is functioning as it should. A brake pressure gauge would come in handy if you have access to one for troubleshooting purposes. Assuming everything checks out, I would look at increasing the pad coefficient of friction (CoF) up front as that's the easiest way to increase your front brake torque and shift the bias forward.

Tobin
KORE3