PHR Project Car
A 40-year-old Chevelle with a 600-plus-horsepower big-block is a prime candidate for a brake upgrade, unless you happen to have a perverse death wish. And if that Chevelle is PHR's project Street Sweeper, which had previously been converted to disc brakes without so much as a master cylinder upgrade or the addition of a proportioning valve, you'd have to be outright suicidal to let that condition persist. We have neither a death wish, nor are we suicidal, so action was called for.
And now, some background. Our '68 Chevelle project car was like a lot of hand-me-down, nine-owner muscle cars. A forensic investigation revealed that our Chevelle's mishmash of brake hardware consisted of a junkyard swap from stock drums to some questionable stock discs, which were relying on the original drum-brake master cylinder and no proportioning valve to speak of. Then, last month, we upgraded to Classic Performance Part's (CPP) disc brake kit with 13-inch rotors and dual 52mm PBR calipers ($799). (We already had late-model Ford truck drum brakes from our Currie Plus-9 rearend install.) During the CPP disc brake upgrade, we also added CPP's new aluminum master cylinder with a built-in proportioning valve (PN MCPV1, $199), which is the subject of our story this month.
CPP's engineered brake system showed a huge improvement in instrumented testing. As we reported last month, our best stopping distance with the mismatched old system was 137 feet from 60 mph-and that was with the benefit of ultra-sticky Nitto NT01 R-compound tires. Moreover, fade was so bad that it wasn't long before stops were clocking in at 160 feet from 60 mph. After CPP's 13-inch system was installed with a new CPP master cylinder (and the brake bias dialed in), we were rewarded with a best stop of 120 feet, and a max distance (with fade) of just 126 feet. All we can say is "wow!"
The MCPV1 master cylinder is available in both 1-inch and 1 1/8-inch bore diameters, to handle a variety of applications from manual to power assist. CPP's Danny Nix recommends you call their tech line for a specific recommendation, but says that the larger bore version is generally recommended for larger boosters, and for OEM-style caliper pistons, which were (and still are) the largest in terms of needing to feed a large volume of fluid.
Classic Performance Parts' new master cylinder (PN MCPV1, $199) comes in two flavors: 1-in
We also dig the fact that the CPP master has its fluid ports located on the bottom of the reservoir, instead of on the side. "The first thing you do when you install a master cylinder," says Nix, "is you bend the brake lines down. We figured, why not just design the ports to go straight down to start with? It cleans up the engine bay, and plumbs easier." You'll also note that CPP's master has dual ports for both front and rear circuits, a move they made to satisfy the custom and street rod market, which sometimes demands the extra plumbing to individual wheels for aesthetic reasons. In our installation, these auxiliary ports remained plugged. At the bottom of the CPP unit, there is also a handy port for a hydraulic brake light switch. Since Chevelles all had pedal-mounted brake light switches, this is not applicable here, but the CPP unit is offered for other applications which do use this. As you might expect, the CPP master was designed for a variety of vehicles, from muscle cars and street rods, to customs and trucks. This is why the MCPV1 comes with two reservoir covers-a flat cover (the one we used) and a remote fill version with threaded ports for remote lines. For custom show cars, this is a nice alternative that allows mounting down on the frame, thus cleaning up the engine bay.
The coolest feature of the CPP master cylinder, however, is the built-in proportioning valve. We know that several other manufacturers now offer masters with built-in prop valves, but the CPP unit ups the ante considerably. To understand why, we must first explain OEM prop valves. These perform two functions: to limit the max amount of brake pressure sent to the rear, and to deliver that brake pressure (up to that max threshold) at a predetermined ratio relative to the front brake pressure. But things get sticky with most aftermarket proportioning valves-which eliminate the max threshold and only perform a proportioning function. Imagine for a moment a max-effort car with really great brakes. Under hard braking, a much higher percent of the total braking is being done by the front brakes. In that scenario, a prop-only valve is going to let the rear pressure soar in proportion to the front pressure, which will result in rear lockup-unless the ratio is dialed back so severely that the rear brakes become nearly ineffective. What is needed is an additional threshold adjustment for the max rear pressure-so that an aggressive overall proportion ratio can be maintained for effective rear braking. The CPP master has both a max pressure adjustment (a small 1/8-inch Allen hex) inside the valve that houses the proportioning adjustment (a 1/4-inch Allen hex). These are concentric-one inside the other-for easy access.
Dual-Pot Versus Single-Pot
As some of you know, federal safety standards required OEs to begin building cars with dual-circuit brake systems starting in 1967. Prior to 1967, cars coming out of Detroit had single-pot master cylinders. Any hydraulic failure in the brake system-whether in the front or the rear-would cause a complete loss of braking. Dual-pot master cylinders came on the scene in 1967, and provided some measure of redundancy, allowing some limited brake power to be applied if one circuit failed. Since pre-'67 Chevelles-like most cars of that era-didn't have proportioning valves (or even the need for them), any upgrade to a dual-pot master in a pre-'67 car requires the addition of a prop valve. For those pre-'67 Chevelle owners out there, you'll appreciate CPP's built-in prop valve, because it greatly simplifies the plumbing operation.
Our '68 had manual brakes, and with a 496ci big-block making a meager 10 inches of engine
Our stock '68 Chevelle pushrod. The end closest is the one that seats in the master cylind
A previous owner of our '68 converted to disc brakes and didn't use a prop valve, so all w
The CPP master cylinder ports use a common parts' store brake line fitting, which is 3/8-2
This view of the bottom of CPP's master cylinder shows a few things: bottom-fed ports, the
Installing the MCPV1 is simple: Guide the pushrod into the back of the master cylinder, an
Tighten the brake lines, remove the reservoir lid with an Allen wrench, and add some brake
Bleeding the brakes with a fresh fill is a whole lot easier with a plastic syringe. Typica
This is the adjustable proportioning valve in CPP's master cylinder. Note the concentric A