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Brake fluid flush - brake sponginess improves, then feels like getting worse

I had a braek fluid flush done about 3 weeks ago, because brake feels kinda spongy…

my mechanic says he see air in the brake fluid line, not sure how they got in there… the brake feels good/tight after that…

however 3 weeks later, brake feels soft again, maybe not as spongy as before I had the brake flush done, but noticably softer than immediately after the flush, what gives?

Make ,Model, Year, and Mileage.


You may have more than one problem. A worn master cylinder will also give a “spongy” feel to the brakes.

I suspect the flush was not complete, but you also need a competent brake specialist to fully check out the brakes.

Pleas tell us your make and model of car and the mileage and how much brake maintenance the vehicle has had so far. It’s very difficult to give accurate advice without this infromantion.

2009 mazda6 3.7L V6 124k miles

the only brake maintenance is changing brake pads since I got the car at 70k miles, I didn’t machine the rotor, the front rotor is getting a bit thin but the car doesn’t pull to the side when I brake.

This can be caused by a caliper piston seal that’s leaking. And don’t mean leaking brake fluid but instead leaking air.

When you apply the brakes, the hydraulic pressure pushes on the piston and seal. This pressure causes the seal to expand so it doesn’t leak fluid. When you release the brake pedal the piston and seal retract back into the bore of the caliper. This retraction causes a negative pressure where a very small amount air leaks past the seal. So you bleed the brakes and the pedal is firm. But as time goes on and the more you use the brakes, air accumulates in that caliper where brake pedal becomes spongy.


so I need to replae the caliper?

If it’s leaking air, yes.



3 probable causes, as I see it, on order of likelihood . . . in my estimation

master cylinder
one or more calipers
brake fluid flush wasn’t done completely . . . you’ve still got air

In regards to the calipers . . . rack the car. Remove all the calipers from the brackets and carefully inspect seals. If one of them is wet, the caliper needs to be rebuilt or replaced

If the master is leaking internally . . . bypassing, that is . . . you’ll not see an external leak

Good luck, and please keep us updated

damn that’s so complicated, I need to take it to a brake shop and have them exam it see what the problem is, they will probably charge me an arm and a leg…

Good comments above, here’s a couple other thoughts.

It isn’t that unusual to have to re-do a brake bleeding procedure after a few days. Air can get in the nooks and crannies or just dissolve in the fluid. In normal use the air eventually returns to the lines and causes a return to sponginess. Re-doing the bleed a second time often solves this once and for all.

It’s unusual for the brake pedal to suddenly become spongy without someone having opened the hydraulic line unless some brake part or another is on the fritz. Before the flush, when you first noticed the pedal was spongy, I presume you or your mechanic inspected the brake fluid level. Was the brake fluid level at that point normal or low? If low, how low?

With used components in the brake hydraulic system – master cylinder, calipers, etc – over time they can develop a spur on the surface of the cylinders. This spur is never encountered by the moving piston in normal operation, but during bleeding the seal can move over the spur and damage it. That could be another explanation of what is going on. If so, the part containing that seal will have to be replaced.

In any event db4690 has pretty much listed out what needs to be checked.

I just thought of something disturbing . . .

If they didn’t use a diaphragm brake bleeder and the proper adapter, which is the preferred tool . . .

If the first guy built up pressure by stepping on the brake pedal, while the second guy cracked open the brake bleeders, the master cylinder seals may have been ruined

The brake fluid level was normal before the brake fluid flush… and it’s still normal I just checked.

so a re-bleed after a few weeks? is that common

Using the brake pedal to bleed the brake system is the normal procedure. Using a pressurized brake bleeding system allows one person to do the same job.

Once a brake system is bled where no air appears during the bleeding proccess there’s no air in the system. If air reappears in the system after it’s been bled, there’s a leak somewhere. And it’s usually due to caliper piston seals allowing air to leak back into the system.


I wonder if any of the bleeders didn’t get tightened down properly, since they were opened for the fluid change.

When you’re using the brake pedal to do a brake bleed, you’re pushing the master’s pistons further than they normally go. And the seals can get snagged on corrosion in the bore . . .

Granted, it doesn’t happen all the time

But it does happen from time to time

because the brake seem to have worsened gradually, it can’t be the master cylinder? it would have to be something that is happening gradually like water getting back into the line?

so most likely it’s a leak caliper like someone says??


It can be the master cylinder

It can also be a leaking caliper

Somebody needs to diagnose it

We can’t do that over the internet

We can give educated guesses, based on our experience, but that’s it

I don’t have your car on the rack

who lives in the LA area, i can bring the car over…

I drove it for two more weeks, now brake feels fine again… maybe there were some air trapped and they worked themselves out of the system??