2004 Jeep Gr. Cherokee Overland brake issues

Have owned this Overland since last year. 132K miles. When it was first in the family, the brake light lit up, the brake fluid level was down dramatically with no sign of a leak,. We filled the reservoir in order to find the leak, and it never dropped again, so I thought it may have gone unnoticed when we bought it, and let it go at that. Fast forward to the present, we had a hard application on icy roads and there was a distinct chatter, sounded like near the booster, and the pedal dropped, fluid level dropped dramatically. Got it inside and went under, no sign of leaking calipers, lines, nothing, refilled the reservoir, but now the pedal bottoms out and , while it can be pumped up, it is soft. Fluid level is dropping again, but no apparent leaks anywhere. I have heard that the master cylinder can leak back into the booster and the fluid will disperse through the system, also a check valve on the booster can be an issue…any help appreciated since I do not want to start throwing parts at it. Before it gets mentioned, it is not being driven on the road. Thanks for the help.

When you first got the truck, the fluid level could have been low due to brake pad wear. The calipers self-adjust with brake fluid as the pads wear down. The brake light on could have been due to fluid level down because of pad wear.

However, now you have a leak. I assume that you have checked every caliper and brake cylinder by removing the tires and looking under brake drums. Also, have you checked the ABS module to eliminate that as a source? If the master cylinder is leaking into the booster, pulling the check valve and checking it for wetness is one way of looking for it. Also, the flex lines in front and at the rear axles are also suspect.

Knuckles, thanks for the quick response. We got it in the air, it has discs all around and we looked at over and around all 4 calipers with no outward sign of leakage, the flex lines are dry, have yet to check the ABS module, any special location around it where a leak would be evident? (I know every vehicle has it’s quirks and weak spots) I am really leaning towards the master cylinder, we parked over a full sized layer of cardboard and repeatedly pumped the brakes, with nothing making it to the ground, or laying on the frame that we could see. The chattering sound when hitting the icy patch leads me to think the ABS is working , or trying to. I just hate to start swapping parts without a clear idea of the most likely suspect. Again, thanks.

ABS will cause the brake to "chatter’ on ice. Sounds as if the Master Cylinder is bad. If you lost fluid and the MC was low it will have air in it and act soft. When you pull the MC you will probably see the fluid where it connects to the booster. Any fluid entering the booster will be sucked into the engine and burned.

Found it/fixed it, thanks for the help. it was, in fact, a minute leak that was in a line and spraying upwards. In the NY slop and slush on the frame it couldn’t be seen until we were up on the lift…even then it took a while.

Was it a rubber or metal line? if rubber you may consider being proactive and replacing them all.

Nope, metal, now they are all newly done with the copper alloy stuff…

Thanks for the feedback OP, good to know you’ve got it under control. A small leak that sprays upward when pressing on the pedal could indeed be hard to spot. fyi, when the MC has an internal leak, while the pedal may go to the floor, the fluid level in the reservoir doesn’t drop. That kind of leak is between chambers in the MC, not out of the system.