IPA Tools 7884 Disc Brake Analyzer - Manual (Page 2)

2. After installing the #7884 on both front brake calipers, apply and
release the brake pedal in the vehicle by using a moderate amount
of pressure (similar to normal braking) on the brake pedal and note
the pressure readings that are obtained. The pressure readings are
recorded by the red needle on each gauge. The red tell tail needle
remains at the high pressure reading recorded on each gauge.
Initial Analysis:
1. FRONT TO REAR SPLIT BRAKE SYSTEMS
(a) On a front to rear split brake system, frozen, sticking or restricted
movement of the disc brake caliper pistons will show as unequal
pressure readings on one of the two Disc Brake Analyzers.
(b) On front to rear split systems, both front wheels may show
pressures less than expected if air is in the system. Because
hydraulic pressure is equal in all parts of a closed system, the
result will be a low pedal. This may cause insufficient stroke to
obtain normal pressures on both wheels.
2. SPLIT DIAGONAL SYSTEMS
(a) On a split diagonal brake system, a pressure difference between
the two front wheels may mean a sticking disc brake piston, a
proportioning valve problem or air is in the system.
(b) You may notice a considerable difference in side to side pressure
readings on split diagonal systems if air is in one half of the system.
If a pressure difference is noted on the initial readings, bleed the
side with the lower reading and retest before performing any
additional testing.
3. ALL SYSTEMS
(a) Imploded flexible brake hoses will cause the black needle on the
pressure to not return to zero position immediately after releasing
the brake pedal in the vehicle. Note—the black needle may initially
show some “held” pressure and then bleed off to zero a short time
later. This is an indication of a problem. The black needle should
immediately return to zero after releasing the brake pedal. Sticky
calipers will also cause the black gauge needle to not return to 0
psi after the brake pedal has been released. To determine which
condition is occurring, do the following: After releasing the pedal,
crack open the bleeder nipple on the caliper. If the pressure
returns to 0 psi on the #7884 this means the piston is moving freely
and the brake hose is imploded. If the pressure on the #7884
remains “held” above 0 psi this indicates the caliper piston is
sticking.
(b) Low pressure on initial application of one or both gauges and low
or sinking brake pedal may indicate that the master cylinder is by-
passing internally.
(c) Low pressure on one or both gauges may indicate excessive rear
lining to drum clearance. If pressures are low, hold moderate
pressure on the brake pedal and apply the parking brake. Have an
assistant observe the gauges. If the pressure readings increase
when the parking brake is applied, the most likely cause is incorrect
rear lining to drum clearance.
Advanced Analysis:
1. Integral ABS systems use the two pistons of the master cylinder for
the front brakes. The rear brakes are controlled by the
accumulator based pressure. If unequal brake pressure readings
are recorded, frozen or sticking caliper pistons, worn or bypassing
master cylinder seals or air in the system may be the problem.
Rear lining clearance should not affect the readings on this type of
ABS brake system.
2. Most Add on ABS systems use conventional master cylinders and
usually use a split diagonal brake system in the non-ABS mode.
On these systems the same diagnostics used in the first approach
may be used.
3. If a vehicle has air trapped in line in an add on ABS brake system,
it may experience a low brake pedal but show near normal
pressures on the #7884. The add on ABS brake modulator valve
assemblies have a normally open inlet valve, which allows brake
fluid to pass through and apply the brakes. The unit’s outlet valve
for each wheel is normally closed. If the unit is allowed to run dry
during brake service or because of loss of fluid, air may be trapped
in the modulator valve assembly. It will be impossible on some
systems to remove this trapped air without cycling the valve (outlet
valve). This may require special equipment. This condition of
having air trapped in the modulator may be diagnosed by the near
normal pressures but a low pedal which will pump up slightly.
4. On trucks with rear anti-lock brake systems, if low pedal is
experienced but the pressure on the #7884 is normal, plug off the
rear brake lines at the master cylinder and retest. If the pedal
height returns to normal, connect the combination line to the
master cylinder and plug off the outlet of the combination valve. If
pedal height is still low, the vehicle has a bad combination valve. If
pedal height is normal, connect the line to the rear ABS valve from
the combination valve and plug off the outlet from the ABS valve. If
the pedal height is now again low, the problem is mostly likely a
leaking accumulator in the rear ABS valve (Electro-Hydraulic Valve,
EH Valve, RWAL, RABS).