My 2004 trail rated Jeep Grand Cherokee has 127,00 miles and has been a great vehicle, but lately I have occasionally found that I have had to push the brake pedal to the floor to get it to stop. This seems to happen after driving on cruise control on the highway for an hour or more, but last night it happened when I pulled it out of the garage after sitting for a few hours. Most of the time the brakes work fine. I took it in to my mechanic a couple of weeks ago, and they couldn’t find anything wrong except they said my calipers were grimy and possible sticking, and they cleaned them. They worked fine until I did some highway driving again. It has come to the point where I test my brakes before stopping to make sure I can! They also seem to build up pressure as I use them. Any Ideas?
Master cylinder is failing.
+1 to friedo’s comment.
I strongly suggest that the OP drive–very slowly and very carefully–to a competent mechanic for replacement of the master cylinder. This is not something that can be deferred.
Master cylinder is a possible. There is also a vacuum valve of some sort in the “power” part of the vacuum system. Most Jeep dealers are familiar with the problem. A friend with a Liberty just experienced these symptoms. He got the part and replaced it himself.
In the end I think a dealer will fix this for less than a general mechanic if that mechanic just starts throwing parts at the problem.
I differ with VDC only in that I suggest it be towed rather than driven.
I’d bet lunch…and dinner… that it’s a master cylinder.
The part Uncle T is referring to is probably the check valve for the brake booster. The booster is a vacuum-operated assist device, and if it fails on some systems ot can require a lot of effort to apply the brakes, but they won’t actually sink to the floor. The check valve enables the engine’s vacuum to pull the diaphragm but prevents positive pressure from the manifold from entering the booster. If when you’re braking you suddenly take your foot off the brake and floor the car, the pressure on the front of the diaphragm would go from low to high, withe the diaphragm’s backside vented, and that could backpressure the diaphragm, The diaphragm would then pull the pushrod going into and through the master cylinder backwards. The checkvalve prevents all this.
How much of a factor the checkvalve is depends on the car. Diaphragm sizes vary, and some cars even have dual diaphragms “stacked”. Every engine and every car has its own needs.
I think the master cylinder is highly suspect - tow it to a mechanic that can properly diagnose it and stop driving that jeep until it is resolved.
You don’t want to get hurt or hurt anyone else.
Thank you for your responses everyone! My jeep is getting a new master cylinder today!
And while we’re at it…
"I took it in to my mechanic a couple of weeks ago, and they couldn't find anything wrong except they said my calipers were grimy and possible sticking, and they cleaned them."
That means you need a new mechanic. This is a ridiculously easy problem to diagnose. Your mechanic should have thought of that first thing, and that he didn’t, indicates he’s not worth whatever you’re paying him.
Another possibility: a sticking caliper heats the brake fluid so much that it boils, preventing that brake line from functioning. Probably the master cylinder though.
Is temperature a factor in when the brakes don’t work? What if you pump the brake pedal?
If the fluid was boiling, the brakes wouldn’t build up pressure as you keep stomping the pedal.
Also, a caliper sticking that badly should create noticeable pull to one side or the other.
i think you need new calpiers and while youre in there do the hoses too . i think sadowfax has a exectlent point. calp our 50.00 per side and hoses our 40.00 per side m/c is 60.00 rbuilt 125.00 for a new one
Actually, my point was that I doubt there is anything wrong with the calipers - or at least I doubt that this particular symptom is due to anything being wrong with the calipers.
Of course, it’s not a bad idea to check, as long as you’re going to all the trouble of replacing the MC anyway.
If fluid was boiling in one of the lines, wouldn’t half of the brake system still be functioning?
Yes and no. The system is separate as far as fluid goes, but not heat transfer. The MC is metal, and the heat from one side will conduct over to the other side, so if it boils long enough, both sides end up boiling.
Plus, a stuck caliper sufficient to boil the brake fluid is squeezing pretty tight. You should notice at the very least a significant mpg drop, and most likely the vehicle will pull to whatever side the stuck caliper is on. (obviously, if OP has 2 stuck calipers, one on either side, then it could still be boiling fluid - but he most likely needs a new MC anyway since the seal integrity is now compromised).
On the other hand, a sinking pedal like this is a classic sign of a master cylinder who’s piston seals have either gone bad, or have gotten gunk between them and the cylinder wall and are therefore no longer creating a perfect seal.
A pretty good test for this,incidentally, is to slam on the brakes. If it’s just a bad piston seal, the brakes should stiffen up because the fluid is viscous enough that it can’t leak past the piston seal fast enough to cause a mushy pedal. If the brakes don’t stiffen up, then your fluid might be boiling (because now it’s a lot less viscous) - - of course, it no longer matters because now you’re about to hit a tree 
Long story short, I’ll keep saying what I said - OP needs a new master cylinder, and checking the rest of the braking system while it’s under the knife is a very good idea.
I have a 2004 jeep grand Cherokee, 95, 000 miles
I had Midas replace brake pads and rotors and flush brake fluid. When I picked it up all seemed fine…then I lost my brakes. Sometimes I have brakes, Sometimes it sinks to the floor. If i pump they will come back. The longer I drive the better they get. In the mornings after sitting all nite can be scary. Took it back to Midas, they acted like I was the crazy one. They flushed the brakes again. Still does the same thing. If i hit a dip in the road I lose my brakes. If the weather is cooler or it rains seems like it’s worse. It’s back at Midas, I never had this problem before I had the brakes and rotors replaced and brake line flushed. Any ideas of wht the problem is ?? I need help !!
Midas mechanics tend to be stupid. It’s your master cylinder. A real mechanic would know that right away. Take it to a real (local, independent, with good reviews) mechanic and stay away from the chain shops from now on. They’re more interested in turnover and BS upsells than in hiring people who know what they’re doing.
The problem is Midas. Find a real brake shop, this is too dangerous to mess around with.
Why would this happen only after I replaced pads and roters and had it flushed ?
When they flushed it, and then stepped on the brake pedal the first time when backing it off the lift, they probably moved the master cylinder piston farther than it usually travels because there wasn’t enough resistance in the lines yet. This made it pick up all the crap that’s been accumulating on the cylinder wall since 2004, and that gunk got between the piston seal and the cylinder wall and is preventing the seal from sealing.
So now fluid is flowing past the piston instead of getting pushed into the lines to squeeze the calipers. It’s not an uncommon thing to happen when you bleed the brakes, especially if it hasn’t’ been done regularly.
Thank you ! That does make sense 