The rule of thumb is that you need at least 15 inches of engine vacuum to operate a power brake booster safely. With a heads-up driver and some caution you might run some boosters with even less engine vacuum. It really depends on the car, the brakes, the master cylinder, the booster, and how adept you are at anticipating the traffic ahead of you. But there is always some risk: You might not get enough brake boost in some panic situations. Our experience has been mixed. On the 620hp solid-roller 427ci small-block Chevy in our '75 Laguna, we get almost 8 inches of vacuum, and our vacuum booster seems pretty happy with that. The hydraulic-roller 400ci small-block in our 1968 Nova, however, makes almost 100 hp less, but it's maxed out at just 4 inches of vacuum at idle. The brake booster in the Nova works only about half the time—and then mostly at highway cruising speed. In clogged traffic or city driving, however, that vacuum booster is nothing but dead weight.
We were one of the first to try CPP’s new Hydra Stop system, taking our 1968 Nova project
Today's radical street cams are great at making power, but let's face it—you are still on the hook for getting your street machine slowed down. If you need help in the braking department due to a big bumpstick (such as our Nova's), there are really only two solutions: convert to a manual brake booster (and lose all hope of easy footwork), or convert to some form of hydraulically assisted brake booster (and restore that light touch to your power brakes). We knew which fix we preferred.
Hydraulic assist is not a new idea—it's been around on production vehicles for decades, and they are the solution of choice in applications where engine vacuum is not an option for assist. Diesels and turbochargers are the most common OEM powertrain pairings with hydraulically assisted brakes, and companies like Bosch have built millions of hydraulic assist brake boosters for use in OEM production. Late in 2013, however, Classic Performance Products (CPP) tapped into the vast tier-one OEM supply line to source a hydraulic brake assist unit that could be used on classic Chevys—the result being a complete line of Hydra Stop brake kits.
Hydra Stop is available in a couple of versions, including the “Show Stopper” ($899, 6474H
Like all hydraulic assisted brakes, CPP's Hydra Stop uses fluid pressure from the power steering system to assist in applying pressure to the master cylinder. Here's how it works. Normally, in the case of a vacuum booster, the pressure differential between two sides of a diaphragm is used to apply pressure to the master cylinder. When engine vacuum is low, the pressure differential is low, so stopping power is limited or even nonexistent. With a hydraulic assist unit, however, pressure from the power steering system is stored in an accumulator, and momentarily discharged to the master cylinder when brake pressure is applied. The beauty of a hydro assist is that pedal effort is low and stopping power is consistent, irrespective of the engine's load.
CPP saw a broad potential market with Bosch's existing OEM hydraulic assist technology; the challenge was adapting it to a variety of classic cars (Chevys first) and getting the price into regular-guy territory. As we shall see with PHR's Project Nova, CPP has succeeded in doing both!
Recognizing the fact that hydro-assisted brakes are not a one-size-fits-all solution, CPP got busy designing kits and supporting hardware for a variety of needs and budgets. At the top of the heap is the Show Stopper system (PN 6474HBK-SB, $899) which features CPP's polished aluminum MCPV-1 master cylinder (a breakthrough product in its own right), the hydraulic booster Hydra Stop assembly with matching chrome accumulator cover, billet aluminum firewall mounting bracket, and a kit with all hoses and hardware. For a little less at $749, you can get the Street Beast kit (PN 6474HBK-BSB) that includes an aluminum Corvette master cylinder, side-mounted proportioning valve, and a hose/hardware kit. While not as dressy as the Show Stopper, the Street Beast is nearly as functional, but at a lower price point. It's worth noting, however, that the MCPV-1 master cylinder in the Show Stopper system includes an integral proportioning valve and brake light switch. (The prop valve on the MCPV-1 is a double-adjustable type, giving you not only control over rear brake bias, but allowing control over the rear brake's max pressure.) From there, you can buy individual pieces, such as the Street Beast hydraulic assist booster, for $389 (PN 6474BHK-BSB). It's possible to mix and match CPP's Hydra Stop components a la carte with CPP's line of steering boxes, power steering pumps, and various brake systems, and that's just what we did with our Nova.
CPP sells the Street Beast Hydra Stop booster as a stand-alone piece for $389 (PN 6474BHK-
You may recall back in the July 2011 issue (see "Disco Tech"), we installed CPP's 13-inch dual-piston brakes in the front and their 12-inch single-piston brakes in the rear, along with CPP's 11-inch booster/master cylinder combo. With everything all plumbed up and working fine (other than the high pedal effort), we saw no need to re-plumb and bleed the brakes again when we had a perfectly serviceable master cylinder and prop valve. The solution? Just get the Street Beast hydro assist ($389) and the hose kit ($115, PN HAHK-R). We'd save some money, and make the installation a lot easier by not having to redo or bleed the brake lines. We're guessing many readers would be in the same situation, so we took a cost-saving shortcut by unbolting the master cylinder and zip-tying it away from our booster swap. All together, we spent about $530, which also included a small inline filter ($20, PN 20-0038F) for the low-pressure return line of the booster.
The results were immediate and dramatic. In before/after testing, we got the same 60-0 stopping distance of 120 feet, but the difference in pedal effort was astonishing. We went from cranking all the pedal pressure we could muster, to using just light-to-moderate force. In traffic, we can now relax, knowing that even panic stops are easy at any engine load, speed, or rpm. This is one mod that really gives the green light to big cams and big power on the street!
More Video Online!
We captured the entire CPP Hydra Stop installation in-depth in a 16-minute video. Check out CPP's new building, their R&D shop, the final test, and a bunch of other things we didn't have the room to show you here! Go to our YouTube channel (YouTube.com/PopularHotRodding) and look for "CPP Hydra Stop Brake Booster."