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.
• On a split diagonal system, a pressure difference between the two front wheels indicates a sticking piston, air in the system or a problem with the proportioning valve. Side-to-side pressure readings might be found in a split diagonal system if air is in one half of the system. If a pressure difference is noted during initial readings, bleed the side with the lower pressure reading and then re-test.
• Low pressure on initial pedal application on one or both gauges and a low, sinking pedal may indicate that the master cylinder is internally bypassing.
• Low pressure on both gauges can indicate excessive rear drum brake shoe-to-drum clearance. If pressures on both gauges are low, hold moderate pressure on the brake pedal and apply the parking brake. If both gauge readings increase when the parking brake is applied, the most likely cause is rear lining-to-drum clearance.
A host of additional diagnostic tips relating to ABS is also included in the instruction manual.
Line pressure reading is uneven on the two gauges
Rear brake adjustment, stuck proportioning valve or frozen caliper pistons or slide.
Once the brake pedal is released, the pressure gauge needle drops slowly.
Imploded flexible brake hose or frozen caliper (if the gauge needle drops slowly, open the bleeder valve.
If the needle then drops quickly, the flexible brake hose is imploded and must be replaced.
If the needle still hangs when the bleeder is open, replace the caliper).
Pressure readings are low but even.
Both rear drum brakes are badly out of adjustment, the master cylinder is bad, the power booster is bad, or the ABS is in need of repair.
The load cell/gauge units measure about 10.5 inches in length, an ideal length for any disc brake application in terms of handling, access and readability. The pressure range is 0 psi to 5,000 psi (0-350 BAR). Each gauge is fitted with a protective rubber surround cushion shield, which is a nice touch.
Sticking calipers and imploded hoses
Calipers that stick or flexible brake hoses that “implode” (collapse) can easily contribute to uneven pad wear. If the piston(s) in the caliper cannot move freely, sticking in various positions or in the extended position will place excess pressure on inboard, outboard or both pads. Degraded brake hoses that tend to implode when the brake pedal is released can cause a restriction in the hose, preventing full release of the pads from the rotor disc.
If you suspect sticking pistons in a caliper, inspect closely for worn, degraded or burnt piston seals. None of us have X-ray vision, so it’s difficult to see what’s going on inside the caliper piston bores.
If a sticking caliper is suspected, remove the caliper, place it in a bench vise and insert a hefty shop rag inside the pad cavity and carefully apply shop air to the bleeder to push the piston(s) out. You may discover galled or rusty pistons and damaged seals. Never assume that what you can’t see can’t be the problem.
Hose implosion may be temporary, gradually releasing pressure after the brake pedal has been released.
This will cause residual pressure in the caliper, preventing the pads from fully releasing from the disc.
The result is obvious: If there is a pressure difference (wheel to wheel on the same axle), of if pressure does not release fully causing pad drag, premature pad wear is inevitable.
Our in-shop experience
For our test, we chose a 2001 PT Cruiser. The front brake pads are captive in the caliper (can’t be removed from the top with the caliper mounted), so temporary removal of the caliper was necessary by simply removing the two 8 mm caliper-to-caliper bracket bolts.
With the inboard pad removed from the caliper bracket, the gauge load cell was positioned against the caliper piston (black blank side contacting the piston, with round pressure pad facing the rotor).
Due to very limited access for the gauge load cell and gauge hose (the opening in the top of the caliper is very narrow and 90 degrees to the rotor), I slipped the load cell through the caliper top opening, then rotated the load cell 90 degrees. I then re-secured the caliper to the caliper bracket.
I did this to both front brake locations. The rotors were secured with a couple of lug nuts in order to stabilize the rotor and keep it in line with the calipers.
As an assistant pressed on the brake pedal, both gauges read 980 lbs. under very moderate braking pressure and climbed to 3,500 lbs. under heavy braking pressure. Both gauges held steady as pressure was maintained.
Upon release of the brake pedal, the right side gauge black needle dropped back to zero very quickly, while the left side dropped notably slower.
This led us to directly consider a sticking piston or an imploded hose issue on the left front brake (turned out to be an imploded brake hose).
Basically, these load cell tools are research lab-grade units that are designed for real-world in-shop use. Measuring load pressure at each wheel is easy and provides solid diagnostic information that can lead you to the source of a problem where the initial concern was one of inadequate braking, spongy pedal, uneven brake wear, brake-induced pull and more.
Being able to measure and verify clamping pressure at each wheel on the same axle is a real time-saver, leading you to a quicker and more accurate/intelligent diagnosis of the brake system.
I can enthusiastically recommend this tool. It’s a great design and quickly earned an honored spot in my shop.
It’s a keeper. ●
IPA (Innovative Products of America)
234 Tinker St.
Woodstock, NY 12498