I’ve been rebuilding master cylinders and brake systems since the 1960’s but this has me stumped. Need your opinions.
My 2007 ABS equipped Kia Rondo continues to act like it has air in the system after repeated bleeding. I’ve done the one man bleeder ( tube in cup) at least 5-6 times. I did the two man bleed recently ( pumped out about 24 oz. of perfectly clean new looking fluid) . I’ve activated the ABS system with my scanner 3 or 4 different times following the screen directions.
The problem. It consistently needs one or two quick pump of the brakes to bring the pedal up to normal height. Then everything is perfect.
With or without engine running the brake pedal will not slowly bottom out like with a typical bad master cylinder. It is low but never bottoms out. Brake booster seems good. No air leaks. It always stops fine but always needs that one or two quick jabs.
Can a bad master cylinder act like this or has this (new to this old mechanic) ABS system still got air in the system? Thank you.
( 138,000 miles, brakes bled every 3-4 years by me since new, never a problem before )
P.S. no external leaks……also, can one chamber of a two chamber master cylinder begin to leak internally possibly causing this ???
I’ve had quite a few single masters and dual masters go bad on me over the years but they always went to the floor and braking was exceptionally compromised. Like scary almost no stopping whatever.
This has been different, the pedal is low ( unless pumped up quick ) but still basically stops on a dime.
My next step was just going to replace the master but I was curious what others might think.
( never had a booster go bad so I also wondered if that was a remote possibility)
Make sure you don’t have any loose wheel bearings… I have seen even on the lift a loose wheel bearing give you the feel of a bad master cylinder…
May not be your issue but I have seen it stump a lot of mechanics…
One way to eliminate anything downstream from the brake hoses is to clamp off all the rubber hoses and you should have a very solid pedal, if so then unclamp one hose at at time until one drops more then the others… You may need to wait a while between each one since you can pump it up… But if you don’t have a solid pedal with all clamped off then it is upstream…
All excellent suggestions. I understand where you’re going. If I have a wobble and it pushes back on the disc cylinder it has to be reset constantly by a pump. I maintain this vehicle excessively. Everything is tight, pads & discs are well within specs. No pulsating in brake pedal and good stable stopping.
I’ve never power or pressure bled a system before but never had ABS until this vehicle. My feeling is I’ve still got air in the system (how I can’t image but anything is possible)
I’ve had single cylinder masters go and it was almost total brake failure. To the floor then pray and pump like mad.
I’ve had duals go and it was everything was squishy but it wasn’t quite the white knuckle panic.
But every time I’ve had master cylinder’s fail the pedal would eventually hit the floor. Now with or without the quick pump it never sinks to the floor it’s just a matter of how far it’s off the floor.
Excellent advise. The only reason I don’t think it’s a individual caliper ( sucking back air or bumped back by a wobble ) is because each time after throughly bleeding the pedal it does the same thing immediately.
If it was a bad seal in the disc piston it would take time to reintroduce air into the system, same with the wobble, I’d have to drive it to bump back the piston.
Just wondering….Is it impossible for the outer dust seal on a disc brake piston to get so old ( 16 years) that it starts to shrink, thereby pulling back the piston slightly every time the brake pedal is released ? Pads are about 1/2 worn.
Like with drum brakes and out of adjustment brake shoes….you had to pump the pedal to constantly reset the shoes until you got around to adjusting the star wheel.
I’m probably reaching but nothing else seems to make sense.
( Unless of course air is somehow still in the system after quarts of fluid being run through it )
I’m just a diy’er, but been installing replacement master cylinders and wheel cylinders many years on my onw cars. Years ago used to also rebuild both, more or less successfully. The symptom you describe doesn’t sound like a faulty master cylinder to me. Guesses, roughly in order of likelihood
ABS unit still has air inside. It may be impossible to bleed it without using a Kia pro scan tool.
Problematic brake power booster. I’ve never had one fail, so don’t know how to test them; but there’s a one-way valve in the input vacuum line that seems to be a failure area on some designs.
Air remains in system somewhere. You might try the gravity bleed technique. Takes quite a bit longer to complete, but not much effort needed on your part once started, so can be done while you are doing other things. Good time to for inspecting for missing exhaust hangers and cleaning the dirt, dust, & grime from the pleats in the CV boots.
If you have rear drum brakes, they might need manual adjustment to bring the shoes near as possible to the drum surface. Removing the drum is the way I do it on the Corolla, adjust by trial and error until drum barely clears the shoes.
Sometimes I’ll have unexplained low brake pedal after brake work that cures itself. Mysterious I know, but my theory is the air works its way up through the brake tubes & escapes via the master cylinder on its own with use, over the course of a week or so.
I replaced the master in my olds because the fluid level switch turned on the low fluid level light. Nothing wrong otherwise but had to replace the whole thing. Not that expensive or hard.
then let the car sit for some period of time, without any driving.
then try the brakes again.
Is the pedal still up, or do you need to pump it again to bring it up?
This could rule out any movement-induced reason, like the ones previously mentioned.
Agree. It doesn’t act like a bad master since it never goes to the floor.
My scan tool does activate the system (or at least it makes it sound like it’s activating the system ) and even though I’ve run a quart or two through it I’ve not done the gravity bled yet.
I don’t think it’s the booster since it acts the same whether left running or not running. Thanks
Leaving it sit or driving makes no difference that’s why I don’t think it’s motion induced.
Sitting in the driveway turned off, pump it up and come back later ( hour or so ???) and it needs a quick pump or two.
Driving same thing……pump it one or two quick pumps and I’ve got perfect brakes, forget to pump after awhile and hit a sudden back up and it needs one (or two) quick jabs and it’s perfect.
If I didn’t pump once or twice quickly the car will still stop but the pedal is at a less comfortable distance from the floor. Thanks
