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.