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Master Power Brakes Brake System Diagnostic Kit - Brakes 101Dialing In The Boost From the April, 2009 issue of Custom Classic Trucks By Kev Elliott
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Master Power Brakes sent us one of their Brake System Diagnostic Kits, and rather than just run it in our new products section, we figured it'd be neat to actually put it through its paces on one of our rides. My '46 roadster pickup was the obvious choice, especially as I've not been 100 percent happy with the brake system since getting the truck on the road. While the front and rear brakes are stock Caprice, albeit rebuilt with new calipers, wheel cylinders, hoses, pads, and shoes, and the combination valve is also from a Caprice, the dual circuit master cylinder and booster were aftermarket items more commonly used on lighter hot rods. I didn't know the cylinder bore size and had a feeling the 7-inch booster was too small, but I'd selected it as a 9-inch version would have been too large to fit between the steering column and the inner fender panel. It should also be noted here that the brake pedal is the Caprice item, albeit modified to allow it to mount in a comfortable position, yet offset the pushrod two inches to the left to align with the booster on the firewall, though the ratio remained unchanged. The brakes had never felt "right" and it took considerable effort to bring the truck to a halt after the first, and easy, initial use of the pedal. All things considered then, the system needed inspection, so I put it up on the rack at the Source Interlink Tech Center and unboxed the diagnostic kit.  The Master Power Brakes Diagnostic...  The Master Power Brakes Diagnostic Kit includes a vacuum gauge and hose, pressure gauge, assorted fittings to hook the pressure gauge up to various calipers and wheel cylinders, a syringe to bleed a new master cylinder, and a master cylinder/booster depth gauge. Oh, and an instruction manual, which contrary to most guys' usual opinion, should on this occasion be consulted!  The first task was to determine...  The first task was to determine the amount of vacuum the booster was getting. Power brake boosters require at least 18 in-hg (inches of Mercury) from the inlet manifold at idle to operate correctly. With the vacuum hose disconnected, the gauge was fitted in its place, and the best reading achieved was 17 inches, though the gauge shows 15 1/2 here. A performance cam will affect the vacuum, but this is a stock rebuilt motor with an Edelbrock Performer inlet and carb, so the vacuum should be unaffected. I elected to conduct all the tests for the purpose of this article with the vacuum at 17 inches. Though Classic Performance Products offer an electric vacuum pump to remedy this, I'm going to investigate further as there has to be a reason the vacuum is so low.  The kit includes a number...  The kit includes a number of adapters to fit the pressure gauge, as well as blanking plugs and brass fittings to connect the gauge to junction points in the system if required. Here the gauge his fitted with the adapter for the GM calipers.  Installing the pressure gauge...  Installing the pressure gauge is as easy as removing the bleeder screw and fitting the gauge in its place with the correct size threaded adapter.  Moving to the front, and using...  Moving to the front, and using a different adapter, the gauge was fitted to the calipers, each registering just under 500 psi. Hmm, more pressure at the rear than the front? No wonder I'm having trouble stopping this thing!  I know I said I would conduct...  I know I said I would conduct all tests with 17 inches of vacuum, but I borrowed this bench tester vacuum pump and hooked it up to the booster just to see what difference more vacuum would make.  Talking over my brake problems...  Talking over my brake problems with the guys at Classic Performance Products revealed that they sell a double diaphragm booster in 7-inch diameter, so a quick trip to their shop saw me leave with what I hoped would cure the problem. While I had the master cylinder removed from the booster, but not disconnected from the brake lines, I used the depth gauge from the MPB kit to check the cylinder and booster. All was good, with 0.020-inch clearance between the pushrod and the cylinder piston. If this is adjusted incorrectly it may cause the brakes to drag if too tight, or give too much free-play at the pedal if the gap is too large. While the master cylinder was off, I also measured the bore diameter, and came back from CPP with a slightly smaller one in addition to the booster, dropping from 1-inch to 15/16-inch.  Here's the old (right) and...  Here's the old (right) and the new (left). Both share the same mounting bolt pattern but the double diaphragm version is a littler longer. While this wouldn't be a problem in many cars, it will cause interference issues in my pickup. Temporarily I unbolted the inner fender at the bottom for clearance.  With the double diaphragm...  With the double diaphragm booster fitted, and with the vacuum back at 17 inches with the motor at idle, the rear brakes showed almost 700 psi. A huge improvement! This compares well with Classic Performance Products' own specs which state that a dual diaphragm 7-inch booster with 23 inches of vacuum will provide 175 psi of 'free pressure' (the pressure the booster can provide, as opposed to 'manual pressure', that exerted manually on the pedal without the booster) with 100 lbs weight applied to a pedal with a 7:1 ratio and a 1-inch bore master cylinder.  The final step in my quest...  The final step in my quest for better brakes was to swap out the 1-inch bore master cylinder for the slightly smaller bore cylinder from Classic Performance Products, reasoning that the smaller bore might give more pressure at the expense of a little further pedal travel. CPP's cylinder is supplied with four bungs to seal the outlet ports during bench bleeding, as well as a rubber boot and bore adapter. Manual brake applications require the pushrod extend a couple inches into the master cylinder, in order to prevent it from accidentally falling out. The adapter is therefore needed in power brake 'shallow bore' applications.  With the master cylinder clamped...  With the master cylinder clamped in a bench vise, the bungs were tightened in the outlet ports.  CPP gave me this small syringe...  CPP gave me this small syringe with which to fill the bore of the master cylinder with brake fluid without trapping any air. Be sure not to mix silicone and regular brake fluid if you're working on an existing system.  Using a rod to act as the...  Using a rod to act as the booster pushrod, the master cylinder was slowly operated to release any air bubbles.  The bungs were next removed...  The bungs were next removed individually and the large syringe from the kit used to 'back-bleed' the master cylinder through the outlet ports. A surprising amount of air was released back into the reservoir during this stage. The bungs were then refitted in order to install the master cylinder on the car.  The master cylinder was next...  The master cylinder was next fitted to the booster, after checking the bore depth again. With permanent bungs in the left side outlet ports, the brake pipes were fitted to the right side one by one, each one left slightly loose and the pedal gently applied to bleed air. The whole system should be bled, but this sometimes can work, and did in this case.  With the booster and master...  With the booster and master cylinder fitted, I again checked the pressure at each wheel, the rears still showing 650 psi and the fronts up to 750 psi. Master Power Brakes recommends they should be 800 psi for disc brakes and 500 psi for rear drums, so the pickup's a lot closer than when I started, but not quite ideal. However, the brakes work much better now, and feel like those on a modern car.
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The Thinker??? - Haul Monitor
Ryan talks about the friendly badgering that goes on around the shop and overcoming obstacles that seemly pop up out of nowhere during a project....
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