A very handy specialty tool that I’ve recently discovered is Innovative Products’ Disc Brake System Analyzer, P/N 7884, which features a pair of precision mechanical pressure gauges.
Each gauge features a pressure contact block (referred to as a load cell) and a pressure gauge. By removing the caliper’s inboard pad and placing the load cell block in its place (between the disc and caliper piston), brake system pressure can be observed on both split diagonal and split front-rear systems. The information is useful in diagnosing a number of brake system problems, without the need to disconnect components and open the hydraulic system.
Each tester block features a round pressure “pad.” As the brake pedal is depressed, the pressure gauge black needle moves according to applied pressure. The red “telltale” needle features a tang that causes it to move along with the black needle. If/when the black needle drops, the red needle stays in the highest pressure that was observed to provide a telltale of the peak pressure that was obtained. For instance, the gauge provides an indication of a sticking caliper or imploded hose (if the black needle doesn’t return to zero upon releasing the pedal). The load cell block measures 1.5 inches wide by 2.54 inches high. Block thickness is 0.50 inch. Overall thickness (back of block to face of load cell pressure button) measures 0.585 inch (the round pressure button projects 0.085 inch from the block in a relaxed un-pressurized state).
The disc brake analyzer kit is useful in testing for a number of brake system tests — all without the need to crack a hydraulic line open. Tests that can be performed include checking for:
• Sticking caliper pistons
• Sticking caliper slides
• Air in the hydraulic system
• Proportioning valve operation
• Imploded/restricted flexible brake hoses
• Master cylinder internal leaks
• Rear lining to drum incorrect adjustment
• Uneven brake pad wear
Installing the pressure gauges
1. Determine the type of brake system (split diagonal and split front-to-rear). This is necessary in order to follow the correct repair procedures after obtaining test results).
2. Determine if the disc brake pads are of the type that can be removed without removing the caliper/caliper bracket. If the pads are captive and the calipers must be removed in order to remove the inboard pads, do so at this time.
3. Insert the pressure gauge load cell (rectangular block) in place of the inboard disc brake pad (installed so that the raised round pressure pad faces away from the piston, toward the rotor disc), and zero the red pressure tell-tale needle on each gauge by turning the center knob to the left until the needle contacts the black needle at 0 psi. Note that depending on the diameter and style of the caliper piston, it may be necessary to place a shim between the piston and the load cell.
Ideally, the load cell is centered to the piston (although perfect centering may not always be possible, depending on caliper design).
Specific procedures are outlined in the kit’s instruction manual for the type of brake system being tested (we don’t have the space here to go into all details).
Following are a few examples of diagnosis using this tool.
• When checking for imploded/restricted flexible brake hoses and/or sticking calipers, imploded brake hoses will cause the black needle to not return to the zero position immediately after releasing the brake pedal (for instance, it may slowly drop back to zero). The black needle should immediately return to zero once the brake pedal is released. If it doesn’t, this is an indication of either an imploded flexible brake hose or a sticking caliper. To determine the specific problem, after releasing the pedal, crack open the bleeder on the caliper. If the pressure returns to zero, this indicates that the piston is moving freely but the brake hose is imploded. If the pressure remains above zero psi, the caliper piston is sticking.
• On a front to rear split brake system, frozen or sticking caliper pistons show up as unequal pressure readings (comparing the readings on the two gauges). Also, both front wheel locations may reveal pressures that are less than expected if air is in the system.