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.

 

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

 

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.

 

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.

 

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?

 

...........

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.

 

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.

 

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?

 

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.

 

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..

 

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?

 

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....

 

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)?

 

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.

 

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... ???

 

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..?

 

Then it's the pedal. ABS wouldn't cause the pedal to go to the floor like that (as far as I understand).

Kia Sorento: Brake Pedal Inspection - Brake System - Brake System - Kia Sorento XM 2011-2019 Service Manual
Kia Sorento: Brake Pedal Adjustment - Brake System - Brake System - Kia Sorento XM 2011-2019 Service Manual

edit: I know your model is older, but it should work the same way.

 

But the pedal felt fine when the master was plugged. I will have a look at the pedal, but I don't think it is the pedal.

I am back to thinking there is air trapped in the lines somewhere..

I am going to try a new brake bleeding technique today. Attach a long piece of tubing (5m) from the bleeder valve, all the way back to the reservoir on the master cylinder. Fill the reservoir then pump it until the cows come home..

 

I've been thinking it might be the brake servo or vacuum system. However, I have a diesel which has a slightly different system with a separate vacuum pump on the back of the camshaft. On mine the brake pedal will be hard/high with the engine off, but on starting the pedal will slowly sink (say midway). This is normal and brakes are very efficient.

Setting up new servo/master seems complex: (attached for earlier model with vacuum pump on alternator)

 

I think you might be on to something.

I have bled the system to the nth degree, pedal is hard as a rock with the engine not running. As soon as the engine is turned on it becomes quite soft (more-so than just the brake booster should produce). Still also getting the strange 'fading pedal' under feathered light braking pressure...

I am quite certain there is no air in the system at all, I have replaced all 4 calipers and bled the system to death. Thoroughly checked all areas for leaks or ballooning lines, all seems fine..

Another thing I have noticed now is that when turning the wheel a long distance (ie when turning in to my driveway), the pedal will depress further. This is consistently reproducible, unlike the brake fade under light braking which seems to be only intermittent..

I don't quite understand how the vacuum and brake booster interact, but I suspect that possibly the booster is losing vacuum pressure intermittently or under heavy turning? Would the power steering being activated cause a potential loss in vacuum pressure?

 

ahh.. it's one of the calipers.

When you're turning, either the hose to the caliper is too short, or the caliper is hitting the frame.. Have someone turn the wheel while you look at each caliper. Maybe the banjo bolt on the caliper is hitting a strut.. whatever it is, you should be able to spot it.

 

Hmm, OK I will have a look at the calipers (front ones only right?) when the wheel is turning to see what they are doing.

Still not sure how that would explain what's going on with the fading pedal in a straight line though?

Anyway, will have a look-see tonight and report back.

 

Hmm, brain wave. Just did some more reading on the fading pedal when turning issue. Few suggestions are for a bad wheel bearing in either front wheel causing the wheel to lock and the hub to cock slightly when turning (and possibly at low speed), thus pushing back the pistons and increasing brake pedal travel.

I have been getting really bad fuel economy, so that could also be explained (at least partly) a bad wheel bearing (or bearings??).

Thoughts?

It looks like the whole hub needs to be replaced when replacing the wheel bearings, is that right? Can't replace just the bearing itself?

 

Yep, they sell just the bearings. Easy test is when you jack it up and there's play when you grab the tire and try to wiggle it. It shouldn't wiggle at all.

Could certainly be a bad hub, but see how the front calipers do when you turn the wheel.

The bad fuel economy is probably due to something else.

 

Sigh.. I jacked up the front end and turned the wheels, both calipers seem fine, aren't hitting on anything or getting puled out of shape when turning. Both hubs seem tight and don't wiggle at all.. Guess it's not that after all.

I have found that if I hold the brake pedal down slightly and drive along a bumpy road, the pedal consistently go towards the floor. Seems like something is pressing back the pistons when the wheels are bumping around and then allowing the pedal travel to increase. Doens't seem like the front wheels though, so I guess it could still be the back wheels? I'll have another look at them tomorrow..

Any other ideas?

 

Could be a rear hub then.. the correlation with bumping points toward that.

Tough one.

The only other thing I can think of would be the booster.. I know you already did the tests for it, but maybe try to find some other ways to isolate it. I'm sure someone out there is more creative than me.

 

Yeah I'll have a look at the rear hubs tonight.

I had already isolated the master and booster from the rest of the system by plugging the master outlets. With the plugs in place the pedal was very firm even with the engine running, much firmer than with the outlets connected. So I think the issue has to be something more down-stream than the master and booster..

 

Shooting in the dark ..... check the flexible hoses between the body and rear axle.

 

If you have ABS maybe the ABS is recircing the brake fluid and allowing a pressure drop. Can you put the plugs on the downstream side of the ABS and test? I am no brake expert so if the idea is a dumb one someone more knowledgeable can speak up.

 

If you have ABS maybe the ABS is recircing the brake fluid and allowing a pressure drop. Can you put the plugs on the downstream side of the ABS and test? I am no brake expert so if the idea is a d

umb one someone more knowledgeable can speak up.

This has been suggested elsewhere as well, however it has been generally regarded as quite unlikely..

I'll check out the rear hubs first, and if they are OK then I think I will need to try this to rule out an issue with the ABS unit.

 

Yep.. might be time to move ABS near the top of the list as a possibility. It's rare, but this is an older vehicle so who knows.