Soft brake pedal.

[lrbraner]

A friend recently replaced the front calipers,rotors and pads on his 08 Aura after the front right
locked up due to constricted brake hose. He noted that the master cylinder was very low when he started the repair.
After the repair, we bled the brakes.
The brake pedal is solid with the engine off.
The pedal goes almost to the floor with the engine on.
We bled the brakes in the proper order, the calipers all move freely.
We tried bleeding the master cylinder. No improvement.
Until this incident the brakes worked properly.
Any ideas ?

[Bill Murray]

Two thoughts:

Are the calipers on the correct side? Not familiar with this vehicle but on some you can have the bleed screw not at the "highest" location if the calipers are reversed.

Assuming calipers are correct, you may need to visit a shop that has the computer setup to cyclic bleed the ABS system.

BTW that story about the hose acting like a "one-way" check valve brought back memories of my 85 Chevy S10 which suffered the same problem and took me FOREVER to figure out. The inner lining of the hose delaminates from the hose and it causes residual pressure on the caliper. UGH.

[lrbraner]

Originally Posted by Bill Murray

Two thoughts:

Are the calipers on the correct side? Not familiar with this vehicle but on some you can have the bleed screw not at the "highest" location if the calipers are reversed.

Assuming calipers are correct, you may need to visit a shop that has the computer setup to cyclic bleed the ABS system.

BTW that story about the hose acting like a "one-way" check valve brought back memories of my 85 Chevy S10 which suffered the same problem and took me FOREVER to figure out. The inner lining of the hose delaminates from the hose and it causes residual pressure on the caliper. UGH.

Thanks for the reply. Yes the calipers are on the correct side.
We are also thinking the ABS.
I have encountered the brake hose constriction myself.
In my case and that of my friend, the hose did not delaminate.
It was squeezed shut slowly by rust build up between the hose and the metal bracket that bolts to the car.
I have a post and photos on the subject on the L Series forum.

[imaddicted2u]

My Aura has always had a hard pedal with the engine off and the pedal drops a bit and goes squishy after I start it. I just figured that's the way it is once the booster is in operation. Might be wrong but have no trouble stopping.
I had the same issue with rust buildup choking the brake fluid flow. No de-lamination.

[lrbraner]

This pedal goes almost to the floor.
Owner said it didnt do that before

[imaddicted2u]

Mine drops maybe an inch as soon as the engine is started.

[lrbraner]

I would agree that is normal.

[fdryer]

^ You are correct. With engine off, standing on the brake pedal should have it travel about half way before firmly stopping. Without power assist (vacuum diaphragm canister between brake pedal and brake master cylinder operates on engine vacuum), hydraulic pressure may be around a few hundred psi but will stop a vehicle albeit at longer distances. With engine running, creating vacuum for the power brake unit to operate, power assisted braking multiplies leg power to higher hydraulic pressures resulting in the stopping distances we're all accustomed to.

When power assisted braking is applied while parked, the lower pedal and less firmness most likely are due to all the flexible hoses expanding slightly from higher pressures while remaining within rated specs. All wheel disc brakes and hard lines cannot expand but brake hoses can without visible indications.

All vehicles with power brakes should exhibit the same characteristics, firm pedal without power assist, less firm (never bottoming out) with power assist.

I was asked to help bleed brakes on an older Corvette. My bil, experienced as a collision shop owner but admittedly not a mechanic assured me this old Vette was a straight forward bleed procedure. Struggling a few hours with various methods of manual bleeding including vacuum bleeding before I came to help, this didn't end well. A week later, I was told this old Vette has upper and lower bleed screws. He didn't see the uppers and I didn't crawl under to look. We bled the lowers only. Guess why the brakes never worked? Once he found out two bleed screws per brake, he completed bleeding. My mistake was not crawling under to see for myself...... I never worked on any vehicle with upper and lower bleed screws.

lrbraner, until this thread, Vues were the only Saturns to have serious brake hose choking on the fronts with the hose brackets corroding, choking the hoses. Acting as one way valves, once braking was applied, the choked brake hose didn't release hydraulic pressure with some members witnessing extremely hot wheel rims and mystery dragging. One Vue member removed his brake hose and cut it to show how the hose was slowly strangled as corrosion built up. Now Auras have this issue too? Prying the clamp open would probably restore hose clearance for normal fluid flow.

Without mentioning mileage of this Aura, it may be possible for a worn master cylinder leaking around the piston seals. I know one example of a car with similar issues. Bleeding brakes went fine with a firm pedal after bleeding but occasionally the brake pedal went to the floor at lights. Zero leaks. I don't recall any solution to that car.

[lrbraner]

Originally Posted by fdryer

^ You are correct. With engine off, standing on the brake pedal should have it travel about half way before firmly stopping. Without power assist (vacuum diaphragm canister between brake pedal and brake master cylinder operates on engine vacuum), hydraulic pressure may be around a few hundred psi but will stop a vehicle albeit at longer distances. With engine running, creating vacuum for the power brake unit to operate, power assisted braking multiplies leg power to higher hydraulic pressures resulting in the stopping distances we're all accustomed to.

When power assisted braking is applied while parked, the lower pedal and less firmness most likely are due to all the flexible hoses expanding slightly from higher pressures while remaining within rated specs. All wheel disc brakes and hard lines cannot expand but brake hoses can without visible indications.

All vehicles with power brakes should exhibit the same characteristics, firm pedal without power assist, less firm (never bottoming out) with power assist.

I was asked to help bleed brakes on an older Corvette. My bil, experienced as a collision shop owner but admittedly not a mechanic assured me this old Vette was a straight forward bleed procedure. Struggling a few hours with various methods of manual bleeding including vacuum bleeding before I came to help, this didn't end well. A week later, I was told this old Vette has upper and lower bleed screws. He didn't see the uppers and I didn't crawl under to look. We bled the lowers only. Guess why the brakes never worked? Once he found out two bleed screws per brake, he completed bleeding. My mistake was not crawling under to see for myself...... I never worked on any vehicle with upper and lower bleed screws.

lrbraner, until this thread, Vues were the only Saturns to have serious brake hose choking on the fronts with the hose brackets corroding, choking the hoses. Acting as one way valves, once braking was applied, the choked brake hose didn't release hydraulic pressure with some members witnessing extremely hot wheel rims and mystery dragging. One Vue member removed his brake hose and cut it to show how the hose was slowly strangled as corrosion built up. Now Auras have this issue too? Prying the clamp open would probably restore hose clearance for normal fluid flow.

Without mentioning mileage of this Aura, it may be possible for a worn master cylinder leaking around the piston seals. I know one example of a car with similar issues. Bleeding brakes went fine with a firm pedal after bleeding but occasionally the brake pedal went to the floor at lights. Zero leaks. I don't recall any solution to that car.

You are correct, prying the clamp open restored normal flow.
As I mentioned I had this same problem on my 02 L300.
I posted some photos in the L series section.
I suppose it is possible its the master cylinder, however the brakes worked fine before the incident.
Also no matter how many times or how hard we push on the pedal with the engine off it is a firm as can be.

[fdryer]

As a rule, all braking without abs usually means the abs unit remains on standby - all abs valves are closed, preventing interference to normal hydraulic brakes. Standing on the brake pedal with engine off also means abs is off. Unless the abs was operating while bleeding brakes, little to zero air gets into abs units. This presumes all brake flushing, bleeding procedures are performed correctly with the engine off. The assumption of correct brake flushing/bleeding procedures precedes assumptions of intact abs units somehow getting air into it. I have flushed and bled my L300 and previous vehicles without issues, all having abs. The majority of diyers perform brake flushing/bleeding procedures the same way without issues, all with abs. The key to brake servicing is never having the engine running as the power assist unit operates from engine vacuum and interferes with brake bleeding with the possibility of abs inadvertently operating a valve. Abs is physically invisible with engine on or off with everyday braking. Only when abs becomes active with the foot massage indicating abs activation during emergency braking is when abs valves cycle open and closed to prevent wheel lockup. Abs effectively takes over braking to at least allow steering whether its too late or not. Crashing somewhere else instead of into another car as it were. In theory, 99.5% of us never have abs activation, driving within our limits.

There are zero procedures to service abs units by diyers without GMs scantool to provide programming functions for servicing, exercising, bleeding abs units. In effect, as long as diyers maintain brakes before abs became standard on most vehicles, no concern for abs is needed because abs remains invisible during brake services. Dealers and brake shops equipped to service abs may be the only places for safe abs services. I won't touch my abs system because it's not a diy component for diy repairs or service. And to determine if abs is faulty requires familiarity of how and when it activates, what it does in severe braking situations and if it fails to operate during these conditions. I don't know all the answers and can figure out one or two scenarios if abs fails, like a wheel or more locks up but rely on performing normal brake services to ensure zero air remains in a system before making the first drive to test brakes. The easiest abs test is on dirt or gravel roads to provide the lowest traction when braking hard. Rain soaked parking lots free of nearby vehicles and light poles, wet snow or ice are other alternatives. This allows abs to activate at relatively low speeds without traffic and distractions. If abs works, steering is retained while the car slows down without skidding or sliding sideways. This doesn't prove if air is in the system. Incorrect brake flushing/bleeding procedures leaving air in a system can eventually get air into the abs unit when abs activates. Before getting to this point of suspecting abs issues, servicing brakes must be clear without questions about air in a system that works its way into the abs unit when abs activates. My guess is 99.999% of everyone performing diy brake repairs are doing it correctly. The 0.001% are those completely oblivious/unfamiliar with brake service/repairs. Those are the ones getting into trouble or trying to fix a very old brake system with a broken abs unit.

[lrbraner]

Can you bleed the ABS with your tool ?

[fdryer]

I don't know as I'm not willing to experiment unless my abs unit fails its self tests. Even then, if the abs indicator turns on, the most likely error would be one of three external components; wheel sensor, sensor wiring or reluctor wheel. Long before I jump into abs unit diagnosing, I'd check for abs error codes first, never assuming the abs unit failed. To date, in these forums, no one has proven a dead/faulty abs unit from normal use. The S-series cars are the oldest here and very few threads about abs failure to the unit. Those failures are due to old age, resurrecting parts to restore an old unused car or meddling with one and finding out how difficult it is to restore abs function. More misinformation and assumptions are made about abs issues discussed than familiarity with how and when it operates and diagnosing error codes when most readers weren't sold yet to decode abs errors ten years ago. AutoZone is one store switching from older readers to better ones that can decode abs errors as more vehicles and and external components fail. Guess what abs error codes are displayed?

[bmp93]

Originally Posted by lrbraner

This pedal goes almost to the floor.
Owner said it didnt do that before

I had a recurring issue like this that surfaced each time I did a brake job. Out of nowhere (after replacing the brake pads), air would get into the front passenger side brake line. Initially, bleeding the brakes would solve the issue. Eventually it became a chronic issue that would reappear within days. Dealer replaced the master cylinder twice before I took it to an independent shop. They replaced the ABS hydraulic unit and the brakes have been problem-free ever since.