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  #1  
Old July 11th, 2018, 14:37
lordoeuf lordoeuf is offline
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Brake pressure issue when easing up on brakes slightly, then pressing down further

I have a 1999 XJ with 33" tyres. Steel braded lines all around. Disc brake conversion in the back, I think from Teraflex, circa 2004 (was told that it is the same as the Ford Explorer rear discs). Removed the o-ring from the proportioning valve to increase rear braking ratio and this worked great. I'm using drilled and slotted rotors on the front, and I forget what style is on the back. I think I'm using Green and Yellow Stuff for pads. It has been many years since I've had to work on the car, aside from fluid service.

As of about a month ago, I've started to have an odd braking issue. Perhaps someone here has run into this before. If I am slowing down to an approaching red light, I hit the brake pedal, sometimes harder at first but then I want to ease up a little on the pedal to coast a little faster, before braking harder again. When I go to press the pedal down the second time (while it is still being pressed after having eased up), I have to push the pedal down a bit further for the same stopping force than had I not eased up on the pedal to begin with. Its as if the pressure which had built up in the brake lines during my first pedal press isn't being held during the ease up period. Its as if whatever holds pressure in the line, e.g. a check valve, is opening when I ease up on the pedal. Is there something which acts like a check valve in the brake lines? Is it in the master cylinder or is it the check valuve on the vacuum-operated brake booster?

If I sit with the car on at a red light and do a series: press down, let up half way, press down, let up half way, press down, let up half way, etc, I can get the brake pedal down pretty darn far. On the other hand, if I just release the pedal all the way up again at any point, then press down, the car brakes just as it should with the expected amount of pedal travel and force. The car, in fact, stops really well. And if you drive such that you never ease up half way on the brake pedal when coasting to a stop, then you might not notice that there is anything wrong. My wife doesn't notice anything and she drives the car regularly. I, on the other hand, have been driving the car since 1999 and noticed something immediately.

Do you experts have any idea what is going on? The symptoms are very repeatable. The issue seems to be a little worse on hot days with the A/C running. Perhaps today or tomorrow I'll follow one of these youtube videos on how to test your brake booster/check valve system with car on/off tests.

If I pump the brakes with the car off, the pedal stays hard. I bled the brakes last week, but it didn't help. The calipers aren't very old - perhaps 3 years old. The car only gets about 5,000 - 7,000 KM per year put on it. The brake fluid level isn't decreasing, that is, the mark stays at full.

Some ideas for failure I have are:

Brake booster check valve & gromet
Brake booster unit
Brake master cylinder
Moisture in brake line when hot
Air entering brake lines during during pedal ease up, but which gets dispersed into the resevoir at full release
Leaking tiny amounts of fluid, but not detectable
Master cylinder nuts loose
Is there a resevoir cap vent hose? Could it be clogged?
Soft or swollen caliper seals
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  #2  
Old July 11th, 2018, 17:36
Eeyore Eeyore is offline
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Brake pressure issue when easing up on brakes slightly, then pressing down further

Slide pins? Just a guess. Thinking the first press and the pads are against the rotor... Let up and the piston retracts but the pad stays... second press, the piston travels back and maybe the pedal goes a little lower?

That said, I thought a bad booster gives a hard pedal but less stopping power.

Sent from my SM-N920P using Tapatalk
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  #3  
Old July 11th, 2018, 17:47
lordoeuf lordoeuf is offline
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Re: Brake pressure issue when easing up on brakes slightly, then pressing down furthe

Not sure about the slide pins.

I recall reading somewhere that a bad check valve on the brake booster can cause air to enter the brake system.

One other aspect of my braking system that I should point out is that I have a line lock (I think the brand is Jamar) knob mounted in the centre counsel of the Jeep. It has been on there for 13 years. About 3 years ago, the knob ceased - that is, I can no longer press it in to lock the rear brakes when stopped. The brakes have worked just fine for the 3 years after the cease, so I don't think the cease is related to my current problem.
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Old July 11th, 2018, 20:41
trippled trippled is offline
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Re: Brake pressure issue when easing up on brakes slightly, then pressing down furthe

It really sounds like you have a master cylinder on it's way out. It's possible there's wear in the middle of the bore, and easing off the brake pedal part down is allowing some fluid to bypass the seal, causing the pedal to travel further when reapplying. If it was mine that's where I would start.
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  #5  
Old July 11th, 2018, 20:56
RCP Phx RCP Phx is offline
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Re: Brake pressure issue when easing up on brakes slightly, then pressing down furthe

^^^ This and no the booster can't introduce air into the system.
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  #6  
Old July 11th, 2018, 22:00
lordoeuf lordoeuf is offline
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Re: Brake pressure issue when easing up on brakes slightly, then pressing down furthe

OK, so a bad or partially defective check valve on booster pump cannot cause the symptoms I've described? If not, then I'll just order a new master cylinder. It is the original part from when I purchased the Jeep new in 1999.
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  #7  
Old July 12th, 2018, 19:48
lordoeuf lordoeuf is offline
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Re: Brake pressure issue when easing up on brakes slightly, then pressing down furthe

Any brand recommendation for a Cherokee brake master cylinder? Looking on rockauto, I see Centric (13167018), Centric (13067018), Cardone Select, Raybestos, Omix-Ada (most expensive), ACDelco, or Fenco/Fenwick.
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  #8  
Old July 13th, 2018, 16:43
Afireinside Afireinside is offline
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Re: Brake pressure issue when easing up on brakes slightly, then pressing down furthe

I'm gonna 2nd or 3rd. Bad master cylinder.
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  #9  
Old July 13th, 2018, 16:44
Afireinside Afireinside is offline
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Re: Brake pressure issue when easing up on brakes slightly, then pressing down furthe

I have always had good luck with centric
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  #10  
Old July 13th, 2018, 17:46
trippled trippled is offline
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Re: Brake pressure issue when easing up on brakes slightly, then pressing down furthe

Quote:
Originally Posted by Afireinside View Post
I have always had good luck with centric
X2 used a bunch of their stuff at work seems to work ok.
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  #11  
Old July 21st, 2018, 23:58
lordoeuf lordoeuf is offline
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Re: Brake pressure issue when easing up on brakes slightly, then pressing down furthe

Well, the local auto parts stores didn't have most of those brands listed on RockAuto. I didn't want to wait for shipping or deal with customs, so I went ahead and picked up whatever was in stock locally. I ended up with a new A1 Cardone Select brake master cylinder. Is this a crummy brand for this part? It is made in China.

Anyway, I bench bled it and installed it. The instructions asked to bench bleed it with the proportioning valve installed. I felt that was excessive, so I bench bled it without the proportioning valve. Installed the unit and bled the brakes. Initial test drive shows no issues, however, I'll need to test the brakes during the day after much driving with the A/C on to say if the problem is solved or not.
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