So, the wife says-
"The brakes on the Jeep are getting pretty bad, dear, can you please put that on your list?"

So I buy new rotors, calipers, pads, etc, and proceed to install them.

Get that done, and take the Liberty out for a test drive, and WTF??? The PEDAL is going to the floor!

I say to my beloved- "Dear, you didn't mention the pedal dropping; I thought it was just the shudder
(probably from cheap Chinese rotors the PO had on) When did the pedal start to get soft?"

Anyway, I got a new master cylinder, put it in today, and voila! sez I, let's make sure that the brakes are all OK. Take it out for a test drive, and the pedal still drops under steady pressure. I'm thinking that this must be something to do with the ABS, and when I read the book it confirms my diagnosis, (No, I don't have a brake fluid leak.) but doesn't show me which of the two ABS components are candidates for replacement.

Has anybody out there had this problem?

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Grid power- the one true essential

2005 Liberty CRD- ARP studs, Fumoto, Hayden FC, inline Tstat, ETechno 7v GP, Platinum P1, in-tank lift pump.
1983 Volvo D24T
1981 Volvo D24
Dodge Cummins TD
Case 580B
Mitsubishi MT180 diesel
Kubota BX25 diesel

I noticed the same thing today on mine, my calipers, rotors and pads are all fine, I'm not sure what is is

Mine had a similar issue. Try bleeding the system starting at the master cylinder then RR wheel, RL wheel, FR wheel then FL wheel. when finish, you need to initiate the ABS self test to purge air from the ABS module, then repeat the bleeding sequence. This took care of an apparently brake loss problem caused by uneven wear at the front pads (off course the pads where replace after all this)

PD. You need a scan tool to initiate the ABS module test, otherwise the procedure won't be effective.

This effect is more than likely a Master Cylinder leak-back effect.
You either need to replace the MC after a good bench bleed or rebuild with a kit.
I am sadly quite certain about this one.
You can bleed the bjeebies out of it and it won't fix it - it will take on air.
................And if/when you bleed the MC don't take the piston travel past its usual stroke please.
To explain:
I installed three consecutive pre-assembled MC's in one vehicle and the same thing kept happening. I went thorough everything - ABS ........you name it.
I then got hold of a good quality rebuild kit and rebuilt the MC myself and its been serviceable ever since.
I do agree that the ABS needs to be exercised on the KJ and you need an appropriate scan tool for that.

I take it you bench bled with a good technique before installing in the vehicle....short limited strokes with feed-back to the reservoir with a blunt dowel as a pushrod with a stop ring on it.

If you can keep the air out (and that's not easy) then you might get away with it and by exercising it in a skid situation but that's hard to do when you have a pedal going to the floor. The problem is it is an internal leak that you can't see in the MC.

Check break lines for weak spots.....had line split and another had inner wall failure where there was a "bubble" that got bigger as pedal was depressed....brake pressure went to bubble instead of the calipers.

OK, I did a good bench bleed and got the air out of the MC, no problem there.
I did a good bleed with speed bleeders on the 4 cylinders, Farthest to nearest;
When the engine is off, the MC stays high and tight, but when it is running, the pedal
drops slowly but steadily. Pumping while running makes it harder, and the rate of pedal drop decreases but still exists.

I have not done an ABS purge because I do not have a scan tool (yet)

I suspect that there is either air in the ABS module or a bad ABS component or both.

Sounds like the next step is to purge the system with a scan tool. Thanks, fellows! Will report back.

_________________
Grid power- the one true essential

2005 Liberty CRD- ARP studs, Fumoto, Hayden FC, inline Tstat, ETechno 7v GP, Platinum P1, in-tank lift pump.
1983 Volvo D24T
1981 Volvo D24
Dodge Cummins TD
Case 580B
Mitsubishi MT180 diesel
Kubota BX25 diesel

Totally normal brake operation, I am pretty sure all ABS vehicles do this now a days, even my Sprinters do this.

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2006 LTD Bright Silver loaded with all the needed mods, CCV intact.
Proudly supporting CRD vendors, and their development of quality parts and accessories.
Equipped with HDS thermostat, plenty of heat, faster warm-ups, increased fuel mileage.

Without "feeling" your specific brake pedal operation all I can offer is every single ABS equipped vehicle I've owned or operated has a mushy brake pedal that if pressed long enough and hard enough will slowly descend toward but not to the floor. In contrast older style non-ABS brakes when pressed hard hit a hard stop if in good shape. As nearly as I can tell ABS brake pedals should be operated by pressing hard enough to stop soon enough and then just hold enough brake pressure to stay stopped.

Bottom line is if you press the CRD brakes to stop and they go all the way to the floor you have a problem. If you press the brakes and the vehicle comes to a prompt stop but the brake pedal keeps going down somewhat if you continue to press hard it''s probably normal.

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Sold to LOST member my 05 Ltd, GDE Stg II turbo + TCM tune, SunCoast TC w. Transgo kit, Steiger window regulators, Samcos, Fumoto valve, 2nd gen filter head with Lub. Spec. bleeder, Hayden clutch & 11 blade fan, inverted spare, P-1 battery, BF Goodrich Long Trail TAs, Etecno1 glow plugs, timing belt at 50K miles/8 yrs

Thanks, PapaIndigo.

Oh, it stops REALLY fast with the new rotors and pads. They don't drop when normal pressure is applied, nor do they drop when the motor is off (now) but if I up the pressure they drop. I only noticed it after I replaced the MC. I just have never felt brakes do that before. I think I will still go ahead and get the ABS system purged anyway.

_________________
Grid power- the one true essential

2005 Liberty CRD- ARP studs, Fumoto, Hayden FC, inline Tstat, ETechno 7v GP, Platinum P1, in-tank lift pump.
1983 Volvo D24T
1981 Volvo D24
Dodge Cummins TD
Case 580B
Mitsubishi MT180 diesel
Kubota BX25 diesel

You probably already know this but there are a couple of "tests" on the booster you can do if you're still concerned...
The following is normal behaviour:
With the engine stopped depress the brake pedal normally, several times.
and.....for this test the brake pedal must be depressed held down with normal foot pressure) before the engine is started in order to remove vacuum from the system.
With the brake pedal depressed start the engine. When the engine is started, vacuum is created and operates the booster. This causes the brake pedal to go down (fall away).
If the brake pedal only goes down slightly, the booster is operating normally. If the brake pedal does not move, the booster is not receiving vacuum or is malfunctioning.

You can also do this test for air tightness of the booster:
Start the engine and let it run for one or 2 minutes, then shut it off.
Now depress (in a normal manner) the brake pedal several times, applying normal pressure.
Be sure to wait about 5 seconds between each depression of the pedal.
If the brake pedal reserve distance increases every time the booster has good air tightness. meaning it falls away going further down with each press.

This is because the vacuum stored in the booster is diminished every time the pedal is depressed. So it should get closer to the floor each time the pedal is depressed with the engine shut off.... the amount of "assistance" is being depleted.

The brake pedal reserve distance will not change of the Check Valve is faulty. In general terms, the check valve is located between the vacuum booster reservoir and the vacuum source.

You could also check for the correct functioning of the one-way valve and the hose between the vacuum reservoir and the vacuum source.....look for hose leaks etc and make sure the valve works as a one-way valve for example if you blow on a hose connected to the valve, then it should not let air flow yet it can be sucked through (I don't suggest this as a test for the lungs) but if I'm truthful have blown and sucked on a clean hose struck into the end myself in my miss-spent youth.

Another note is that the check valve should hold vacuum in the reservoir for about a minute after the engine is stopped. When disconnected after that time there is a distinct air rush noise.

If all this checks out normally it could point to the booster.
Good luck.