Abnormal Brake Pedal Diagnostics

Abnormal Brake Pedal Diagnostics
A customer may complain of a brake pedal that ‘ feels funny. ’ It ‘s your speculate to determine whether it ’ s due to activate and/or contaminated brake fluid, excessive pedal travel or any early cause. Vehicles normally respond to our commands in a reasonably predictable manner. When we push on the accelerator, we go forth ; when we turn the guidance wheel, we change commission ; and when we press the bracken bicycle, we stop. Of all the things we ask of our vehicles, “ please stop ” is the most important request. The brake pedal should give the driver a reassure spirit that says, “ I got this. ”
The brake pedal is the driver ’ s physical connection to the brake system, providing feedback that the driver uses to determine if there ’ s a problem with his brakes. Spongy, bathetic and spongy are good a few of the terms that a customer might use to describe negative brake bicycle feedback. sometimes these concerns can be crafty to diagnose due to the numerous failures that can cause an unacceptable brake pedal feel .
The first step in diagnosing a low brake pedal refer is to determine the type of low brake pedal point issue you ’ re dealing with, and if other symptoms are give. It might be a adept mind to forgo the road if you ’ re uncomfortable with the serve of the brakes and settle for a park lot cruise rather.

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Besides a low or mushy bicycle, note if the brakes pull, whine or pulsate. Does the pedal lento stray to the floor when at a end, or does it stop firm, but besides close to the shock ? Determine if one or more wheels are locking up, and note if the red or amber brake warn lamps are illuminated. All of this information will aid in your diagnosis .
A depleted brake pedal is always caused by either a hydraulic or a mechanical malfunction. When a mechanical malfunction is the campaign, it ’ s due to a part moving past the range of gesture for which it was designed. A good model of this is out-of-adjustment rear brake shoes. The universally accept specification for brake shoe-to-drum clearance is .015 in. Every thousandth of an edge over specification causes surfeit brake pedal point travel. Less than .025 in. travel at the overlord cylinder pushrod can equal about ½ in. at the pedal point. Every fiddling spot of unnecessary movement adds up .
Abnormal Brake Pedal Diagnostics
Another exercise is a caliper piston that retracts besides far into the hold when disengage. rear calipers with integral park brakes are ill-famed for this. This causes the piston to travel further than designed to initiate contact between the inboard pad and the rotor. This surfeit travel can translate into a pile of campaign at the bracken bicycle .
Hydraulic concerns present the biggest challenge in low brake pedal point diagnosis. The smallest amount of air trapped anywhere in the organization will have a big impression on bracken pedal feel. Brake fluent converts the energy applied to the pedal point into the force required to engage the brakes. This happens because fluids are compressible only to a very humble degree, so any imperativeness applied to a fluent is transferred to all portions of the fluid and to the walls of the container it occupies. Since tune is identical compressible, atmosphere creates a decent, soft cushion for the fluid to lean on, ruining any chance of achieving solid hydraulic coerce .
Use the information you acquired during the road test to guide your initial ocular inspection. Watch all the calipers move while an assistant pumps the brake pedal. Worn caliper pins or incorrectly installed brake pads can cause the caliper to flex side to side. This lateral pass movement uses up brake bicycle travel. Be cautious of brake pads that are location-specific, or that need to line up with a locating pin during initiation. If these pads are incorrectly installed, they ’ ll causal agent the caliper to flex .
While you ’ re eyeballing the calipers, make indisputable they ’ re on the adjust english. When a caliper is installed on the incorrect side, the hemophiliac valve will be below the inlet. Since tune rises to the crown, good luck bleeding that caliper. Check meticulously for an external leak. Brake fluid leaks don ’ metric ton constantly present themselves as drops that puddle in the driveway. Just a little seepage at a fit can cause big trouble at the bicycle. Don ’ thyroxine forget to look behind the steering wheel cylinder boots while checking for leaks ; there should be no fluid there .
During your ocular inspection, besides watch the rubber eraser hoses as a assistant pumps the brake pedal. Look for a start in the hosiery that appears under pressure, then disappears when the bicycle is released. I ’ ve seen hoses twist while pressurized due to a defect at the union where the rubber hose meets the metallic crimp. Excess hosiery campaign causes surfeit pedal point movement .
Most negative brake bicycle feedback issues fall into one or more of the watch categories :
•A pedal point that stops hard at the bottom but has surfeit play at the top is normally the result of a master cylinder or pushrod issue.
•A pedal that ’ s spongy or bathetic can be caused by air out intrusion, contaminated brake fluid or a mechanical component moving beyond its design range of apparent motion.
•A pedal that continues to move toward the floor when applied is caused by a loss of hydraulic pressure, which can be caused by an external escape or an internally leak component. Contaminated brake fluid can besides cause a brake pedal to drift down .
An periodic exception to these rules—there ’ s always an exception—is the antilock brake arrangement ( ABS ) hydraulic control unit ( HCU ). A bad HCU will create symptoms that feel like atmosphere intrusion or a pressure loss and can very complicate your diagnosis .
Let ’ s look at a brake bicycle that stops hard at the penetrate but has excess meet at the top. A good way to determine if there ’ s besides much pushrod change of location is to loosen the master cylinder-to-power supporter attaching hardware. Have an assistant hold the unleash maestro cylinder firm against the supporter while you slowly apply the brake pedal. Determine how far the pedal moves before your benefactor feels the push on the master cylinder. excessive pushrod maneuver could be caused by a weak or break headmaster cylinder bore leap that ’ s not returning the piston all the means to the back of the bore, or it could be a pushrod or promoter issue. Remember, a fiddling play at the pushrod is a set at the bicycle .
many automakers use an adjustable pushrod. This adjustment is set at the factory and shouldn ’ t have to be messed with. If you need to adjust the pushrod, something ’ sulfur wrong .
We once serviced a Ford Escape with excessive brake bicycle locomotion at the top. Loosening the master cylinder determined that we lost about 3 in. of pedal travel before the piston moved. We removed the victor cylinder and discovered that the modest dome-shaped piece that was pressed into the end of the pushrod, which fits into the master cylinder bear, was missing, and laying at the bottom of the supporter. This added about ¼ in. of travel to the pushrod .
Diagnosing a spongy, mushy brake pedal point topic can sometimes be frustrating angstrom well as time-consuming. here ’ s an exercise of how crucial the initial road test is when diagnosing a spongy bracken pedal point .
While we were road-testing a vehicle with a depleted, spongy brake pedal point, we noticed that the brakes pulled to the impart. That told us that the left field front and right front calipers were not applying evenly. Using the results from the road examination, we started our ocular inspection at the front calipers. Both presence wheels were spinning freely and both stopped spinning when the brakes were applied. There was no excessive drift in either caliper or hose during brake application, and the brake pads were a healthy 9mm, with nice, beefy rotors .
Abnormal Brake Pedal Diagnostics

Since the road test suggested there was an topic with the front man brakes, it only made sense to check the presence calipers for air travel. We removed a nice pouch of atmosphere from the right front caliper, which temporarily fixed the bicycle. I say “ temporarily ” because we found no reason for that caliper to be holding air, as there was no sign that it was recently disassembled. Caliper piston seals and wheel cylinder cup seals can replicate a one-way check valve. Fluid can ’ t get out but air can get in. So we replaced the caliper. This scenario reinforces the importance of gathering information during the road test. If we didn ’ thymine feel the brake wrench, we wouldn ’ t have started our diagnosis at the front calipers .
Abnormal Brake Pedal Diagnostics
unfortunately, it ’ s not always that simple. If you ’ re faced with a moo, mushy bicycle and the ocular inspection yields no clues, it ’ south time to break out the blockers. Blocking off sections of the bracken system ( not individual components ) is the sure way to find the cause of the trouble. Thexton Manufacturing makes a master cylinder plug kit ( Part No. 803P ) that can besides be used on HCUs. It ’ south significant to use quality plugs when blocking brake atmospheric pressure. A modest leak at a hack will yield erroneous results .
Always start by blocking the master cylinder ports. If the maestro cylinder is effective, the bicycle should be rock hard—barely moving—with the ports blocked. If the brake bicycle moves at all, the passkey cylinder is leaking internally, there ’ s a pushrod topic or the brake fluid is contaminated .
If the master cylinder is commodity, most diagnostic procedures recommend blocking the HCU ports future. The problem is that the HCU tends to be hard to access, and the fittings are normally street fighter to get a twist on ascribable to their proximity to each early. so in the heart of streamlined diagnosis, it makes sense to check the easier-to-access components first, which would be the wheels .
Block each wheel individually at the steel line, before the arctic hose ; this way, you ’ re taking the hoses, calipers and steering wheel cylinders out of the equation. Do not block off the calipers by using vise grips to crimp the rubberize hoses. Damaging a rubber brake hose could cause it to work like a one-way valve, allowing pressure to be applied to the caliper but not to bleed off, causing a constantly applied bracken caliper. The Thexton block-off kit contains only male fittings, and you need a female match to block the steel line. Attaching a brass union to the male plug works great. Test the bicycle after blocking each wheel. We found a right raise caliper on a Ford Explorer that was causing a depleted, spongy pedal using this method acting. We never did identify the actual lawsuit of the caliper malfunction. It was n’t leaking, the pins were uncompromising, no publicize, all looked fine. But the pedal came properly up when we blocked it off. We replaced it ; trouble fixed .
Abnormal Brake Pedal Diagnostics
If you find no trouble at the wheels, you ’ ll need to entree the HCU. It ’ south best to block all the wall socket ports on the HCU at the like time. This is a set of work merely for a diagnosis, but at this sharpen there ’ s very little option .
If the pedal point is still broken with all the HCU mercantile establishment ports blocked, before passing sentence on the HCU, perform a service shed blood. HCUs seem to be a favored rest station for stowaway vent. even if you have no reason to believe that the brake system was opened to atmosphere, you should bleed the HCU before condemning it .
Abnormal Brake Pedal Diagnostics
ABS service bleed procedures are performed with a scan joyride and vary by manufacturer. Some procedures are a easy as pressing the brake pedal a few times and clicking a clitoris on the scan tool ; others will require you to open hemophiliac valves, which makes a very mess on the patronize floor. Whatever the routine, it ’ randomness very significant to bleed all the wheels after performing an ABS servicing shed blood .
I started using a brake pressure bleeder a match of years ago and I ’ ll never go back to the old pedal-pumping method. pressure bleeders work bang-up. They attach to the overlord cylinder reservoir in place of the cap and put the brake system under consistent atmospheric pressure. The biggest advantage of this—besides not needing an assistant—is that you can open a hemophiliac valve and just let it flow, pushing all of the air out out of a line from the overlord cylinder to the wheel cylinder or caliper with good one turn of a bleed emergency room valve. If you don ’ thyroxine already have a atmospheric pressure hemophiliac, I highly recommend getting one .
It ’ sulfur crucial to properly benchbleed a new master cylinder before installation. It can be tough to bleed the tune from a passkey cylinder once it ’ s installed on the vehicle. Mount the overlord cylinder hard in a vise and screw bleeder fittings into the exit ports. Attach hoses to the fittings, with the other ends of the hoses in the reservoir, submerged in brake fluid. An effective kit for bench-bleeding a victor cylinder comes with check valves, which allow the air to be pushed out while not allowing air travel to get sucked in on the back stroke. Thexton, Dorman, NAPA and others offer such kits .
If you don ’ t have check valves for the hoses, the procedure will silent work. Since the hoses are submerged in brake fluid, fluid from the reservoir rather than air travel will be pulled back into the dominate cylinder. The problem is that air that doesn ’ t make it to the end of the hose gets sucked back into the cylinder on the spinal column stroke. Check valves prevent this from happening .
With the hoses in place, slowly push the piston into the master cylinder, then allow the piston to return to the lie position. You ’ ll see air bubbles exit the headmaster cylinder through the clear hoses. Do not push the piston past its normal compass of apparent motion ; doing then can damage the piston seals, rendering the overcome cylinder useless. When using check valves, all the air is gone within seven to ten strokes. Be certain that the fittings are nasty, as a poor cachet at a fit will pull vent into the master cylinder .
The heart of a hydraulic brake system is the fluent, and when that goes badly, thus does the pedal. Most automotive applications use DOT 3, DOT 4 or DOT 5.1 fluid, all of which are glycol-ether-based. DOT 5 is silicone-based and should not be assorted with glycol-based brake fluids. The most important property of brake fluid is that it maintains a stable viscosity and compressibility throughout its integral operate on temperature range—very cold to very hot. The negative property of bracken fluid is that it ’ second hygroscopic, which means it absorb urine .
Water in brake fluent greatly lowers its boiling degree. The dry boil target of DOT 3 is 401ºF. The besotted churn point, defined by the temperature at which the fluid boils after absorbing 3.7 % water by bulk, is 285ºF. big difference. Since brake fluent temperatures at the calipers can well exceed 200ºF, this could be a problem. The fact that water freezes besides tends to complicate things a morsel. This is why it ’ second recommended that brake fluid be changed every two years.

When brake fluid is contaminated with water it turns a blue tinge. If the brake fluid looks contaminated, or if you ’ re diagnosing a vehicle that ’ s more than two years old, recommend a brake fluent sluice. Use a pressure hemophiliac or brake fluent flush machine to push all the old fluid out through the hemophiliac valves. Be certain to check all the hemophiliac valves before selling the brake bloom. If you think the valves are going to break off, you need to know this ahead .
A match of more things : A customer might describe an intermittent assumed ABS energizing event as a low bracken pedal. But if the pedal feels very well to you, check for DTCs in the ABS module. besides, contaminated fluid can damage an HCU. Brake fluent is designed to protect alloy bracken parts against corrosion, an attribute it loses when weakened by water. so when replacing an HCU, it ’ second imperative to flush the fluid .
It seems that the furthest thing from anyone ’ south beware as he ’ s driving down the road is the bracken arrangement. The song on the radio or the set of the climate restraint system occupy more brain distance than the brakes do. So possibly problems like a low brake pedal point and/or noisy brakes should be looked upon as an significant base hit admonitory. It ’ s the brake system whispering, “ Hey, remember me ? ” to the procrastinator who should plan on getting his brakes checked very soon .

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Category : Car Brakes