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Sorento 2003
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Discussion Starter #1
Recently aquired a 2003 Sorento, and the brake pedal seems to be intermittently going enarly to the floor. Usually it will have good brake pressure with the pedal 1/3 depressed, but then occasionally it will just go way further and need to be pressed nearly to the floor to get good brake pressure.

If I press hard on the brake pedal while stationary the pedal goes slowely all the way to the floor.

From what I've read it sounds like it is most probably the master brake cylinder. Is there any way I can easily confirm this to be the case before buying a new part?
 

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2007 Rondo and 2000 Tacoma
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Sometimes the master cylinder leaks out the back but most times it leaks past the seals internally so there is no visual signs.
If it was mine I’d try the cheap easy way first and bleed the brakes first......but it does sound like a bad master cylinder.
 

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I now drive a 2003 S-10 Had an 03 LX 3.5 Sorento
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Good advice from Old n Slow.
Also I would stop driving it until you get it solved.
I say that because you did not state that you have parked the vehicle.

:thumbsup: if you have.

Also :00000732: to the forum
 

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Sorento 2003
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Discussion Starter #4
Thanks for the replies guys.

I know I shouldn't be driving it with this issue.. But.

I've noticed that when taking off, after braking, I will sometimes hear / feel a bit of a clunk, and afterwards the soft pedal braking issue occurs for a short period of time.

It feels like maybe the rear brakes are sticking a bit, and then getting 'jolted open'? Which I'm thinking could mean the gap between pad and disc is much wider than usual when this happens, resulting in the pedal going much lower for a bit? The pedal seems to pretty much come back up after pumping the pedal one or two more times.

Any suggestions on that extra info?

I am going to bleed the brakes this weekend and see if that helps, but I have a feeling somethine else is going on, possible not the master cylinder either.
 

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2007 Rondo and 2000 Tacoma
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You might be right......What you describe has happened to me with drum brakes, if they wear down too far and are not adjusted out, the brake pedal will go soft for the first couple of pumps, until the piston finally extends out far enough to have the shoe make contact with the drum.

Let us know what you find this weekend.
 

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Sorento 2003
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Discussion Starter #6
Ok so on the weekend I lubricated and bled all 4 brakes. Gave each one a good long bleed to flush the brake system out good.

Since then the issue seems to have stopped all but entirely. I have still noticed the pedal depressing more than it feels like it should on a couple of occasions though =/

I'm thinking, the master cylinder probably is bad, but the fresh brake fluid has made it behave itself a little more normally, for now at least. I will set about replacing the master cylinder soon.

I have picked up a master cylinder from a wreckers, but I'm thinking it wouldn't make much sense to just replace it as it is, since it is likely old and on the way out anyway. So I'm thinking to buy a master cylinder seal kit and recondition this one.

Anyone have any thoughts on doing that? Am I being stupid, should I just buy a brand new master cylinder?
 

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I now drive a 2003 S-10 Had an 03 LX 3.5 Sorento
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Unles you have experience with rebuilding a MC you might consider buying a rebuilt or if your budget allows, new.

I have begun to understand the saying"Buy once, cry once"

The pedal can still travel more if the pads are worn.
 

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Sorento 2003
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Discussion Starter #8
The rear pads were on the low side but still OK. About 6-7mm left in them.

I wouldn't have thought that would be causing extra pedal travel though since the caliper should be adjusting down on them, yeah?
 

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I now drive a 2003 S-10 Had an 03 LX 3.5 Sorento
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Silly me. I was thinking of the fluid level when I wrote my post.
As the pad wears fluid level will drop. Pedal travel should remain the same.
Unless you are getting leak by on the MC or disk pistons, in which you will get fluid at the brakes,
in the form of a leak or fluid in the passenger compartment, sometimes.
 

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2007 Rondo and 2000 Tacoma
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...........

I have picked up a master cylinder from a wreckers, but I'm thinking it wouldn't make much sense to just replace it as it is, since it is likely old and on the way out anyway. So I'm thinking to buy a master cylinder seal kit and recondition this one.

Anyone have any thoughts on doing that? Am I being stupid, should I just buy a brand new master cylinder?
I had no idea you could still buy rebuild kits anymore.....we used to rebuild all the time, the main thing is to have a good cylinder bore with no pitting, but even then we would hone them out with a brake cylinder hone.

It is just a matter of getting quality rubber replacement parts, a good master cylinder core and carefully noting how it came apart and putting it back the same way.

But like the toolman advised, there’s nothing like buying new.
 

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Sorento 2003
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Discussion Starter #11
Ok so, I ended up replacing the rear calipers (as I the pistons were very seized / tight) and bled the sh** out of the brakes. I am quite certain there is no air in the line.

I am also pretty sure the my master cylinder is actually OK because after bleeding the brakes the pedal is generally quite hard and does not go to the floor at all when the engine is not running, also when driving at low-moderate speed (less than 40kph, 25mph) if I depress the brake pedal fairly hard (or very hard) I get good brake pressure and a hard pedal.

The abnormal behaviour that is still occurring is this:

When driving at low-moderate speed (less than 40kph, 25mph) if I depress the brake pedal very lightly, just barely enough to start to slow the vehicle, then tightly release the pedal, then press lightly again, like feathering the brakes in traffic. Then the pedal will gradually get lower and lower with each press-depress-press cycle until it is nearly at the floor.

I can also reproduce this from being stopped with moderate pressure on the brakes if I release the pressure until the vehicle barely starts to creep forward, and then again press-depress-press around this level of pressure, the brake pedal will again go nearly to the floor.

When doing this, it gets to about an inch or two from the firewall and then it seems to find a point where it won't go any lower and at has some semi-decent brake pressure at this point.

Anyone have any idea what this could be? I'm thinking it might possibly be a bad ABS unit (or sensor?) rather than master cylinder, and that whatever is happening is only happening at slow speed with light braking (causing the ABS unit to allow some fluid past a valve, reducing pressure in the lines?).

If the Master was bad, you should be able to compress it with enough force on the pedal while the vehicle sin't running, right?

So frustrating..
 

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2016 Sorento LX V6 AWD
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Usually a bad ABS sensor/module will have the opposite effect.. you'll feel it kick in when you're just moving slowly. I wouldn't rule it out totally, but I would keep it at the bottom of the list for now.

Your symptoms point more towards an internal leak in the master. I assume you checked for external leaks in the lines, near the master, near the booster, and around the calipers.

You can try these tests to see if it's the booster:

Kia Sorento: Brake Booster Brake Booster Operating Test - Brake System - Brake System - Kia Sorento XM 2011-2019 Service Manual

Also, have someone press the pedal while you look at the brake fluid reservoir. You're looking for bubbles.

Another test is to run the car for a minute, shut it down, and press the pedal as far as it will go (with normal pressure being applied to it) then hold it there. It shouldn't drop any further. If it does, then it could be the master.

How many large bottles of brake fluid did you go through?

Also, are you sure the calipers are on the correct side? Bleeder valves are on the top? Please don't ask me why I'm asking this question ;)
 

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2019 Kia Sportage. SX with AWD. 2.0L Direct Injected Turbocharged & Intercooled Gas.
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Could be master cylinder. Could also be vacuum booster. Might not be a bad idea to R & R the booster along with the MC. My 2 cents....
 

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Discussion Starter #14
Usually a bad ABS sensor/module will have the opposite effect.. you'll feel it kick in when you're just moving slowly. I wouldn't rule it out totally, but I would keep it at the bottom of the list for now.

Your symptoms point more towards an internal leak in the master. I assume you checked for external leaks in the lines, near the master, near the booster, and around the calipers.

You can try these tests to see if it's the booster:

Kia Sorento: Brake Booster Brake Booster Operating Test - Brake System - Brake System - Kia Sorento XM 2011-2019 Service Manual

Also, have someone press the pedal while you look at the brake fluid reservoir. You're looking for bubbles.

Another test is to run the car for a minute, shut it down, and press the pedal as far as it will go (with normal pressure being applied to it) then hold it there. It shouldn't drop any further. If it does, then it could be the master.

How many large bottles of brake fluid did you go through?

Also, are you sure the calipers are on the correct side? Bleeder valves are on the top? Please don't ask me why I'm asking this question ;)
Thanks for your help, I have checked everywhere for external leaks, can't see any, also not losing fluid from the reservoir so I think I'm good there.

When bleeding the system (twice) I went through about 2 liters of brake fluid in total, which I think should be considered a pretty solid flush? Fluid is coming out clean at all 4 calipers.

Calipers are definitely on the correct side and bleeder valves are on top (I understand why that is important *gravity* :))

Booster passes those tests as well.

I guess we're back to thinking it is in fact the master then?
 

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2016 Sorento LX V6 AWD
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Yeah, I would see if you can isolate the master from the rest of the system. They sell plugs for it. If you can get at the brake line connections to the master, you can just disconnect the lines, plug up the connections at the master, top off the reservoir (as some fluid will leak out), then press the pedal.. again with normal pressure. If it still goes to the floor, the master is the problem. If not, I would have someone pump the pedal while you go around and look everywhere for leaks/bubbles.
 

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Discussion Starter #16
Yeah, I would see if you can isolate the master from the rest of the system. They sell plugs for it. If you can get at the brake line connections to the master, you can just disconnect the lines, plug up the connections at the master, top off the reservoir (as some fluid will leak out), then press the pedal.. again with normal pressure. If it still goes to the floor, the master is the problem. If not, I would have someone pump the pedal while you go around and look everywhere for leaks/bubbles.
Yeah I'm heading to the wreckers again this weekend to salvage a few more parts. I'm going to grab a couple of the short brake lines that run from the master to the ABS Pump Unit. Then I'll disconnect the existing lines from the two outlets on the master, and route a singe line to both ports on the master effectively isolating the master. If the pedal to the floor goes to the floor then I guess we will know it is the master, and I will go ahead and replace it..

Any tips on fitting a new master? After bench bleeding the master is it basically just as simple as sticking it in to the booster ensuring a good seal, reattaching the lines etc and then doing a big bleed?

Any tips for bleeding from the master down? I have heard it can be difficult to get any trapped air from the top of the system to move down to the bleed nipples on the calipers, do you have to pump the pedal quite quickly to move a lot of fluid down the lines (and any trapped air)?
 

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I think it's best to bleed the new master cylinder before you install it. You basically fill it with new fluid and pump the cylinder by hand. Then when you see fluid coming out of the brake line connection ports, cover them with your fingers (wearing a gloves), and you keep pushing the cylinder until it's all the way in, making sure to keep the fluid level topped off while you do this. You keep repeating that process until it's bled. Then you go and take out the old master and put the new one on. I'm sure there's a much better writeup on this process somewhere on google.

It can be a messy job, but it's not too complicated. You just have to think it through because you won't have 2 free hands during the process.
 

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Discussion Starter #18
Far out.. Ok so I got some plugs for the master cylinder, tested the pedal with the plugs in and it is hard as a rock!!

So the master is definitely OK. Now wtf is actually the problem? There's no leaks, no ballooning brake lines, I've bled the system to death and get no air bubbles..

Could it be the ABS system that is somehow allowing the pedal to travel further down, or the brake booster? It seems to be basically fine when the engine isn't running, but only when the engine is running will the pedal creep down to the floor... ???
 

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Discussion Starter #19
Oh also, after reattaching the lines to the master and bleeding again, still have the pedal dropping and now the pedal also makes a bit of a click sound when releasing the pedal..?
 

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