No brakes after stressing engine hard

I'm hoping to get your guys opinion on my problem. I was out wheeling this weekend and notices after I stressed the engine hard to crawl over an object then when to hit the brakes when the wheel got on top the brake pedal would go to the floor. I would pump it 3 times and I would have pressure. This only happened when I was doing something like this. I drove it 100 miles home and the brakes were fine but the red brake light is on now. The master cylinder is full of fluid. So I know I don't have a leak.

My thought is that the master cylinder has an internal leak and when I stress the engine hard the booster makes more boost and over powers the seals in the MC and how do I turn off the red brake lit

This is a 98 Cherokee with no abs and Grand Cherokee discs in the back.

Sorry for the long post but I’m looking for some input because this seems odd, thanks

sounds like bypassing master cylinder to me. you should be able to duplicate it anytime though. at idle if you push the pedal down, and then give it steady harder pressure you should notice it slowly going lower than it used to.

I tryed it with the engine running and held the pedal down hard and their was no fade it stayed like normal??? And the lights still on.

The pedal can only go to the floor when there is a loss of pressure, the light comes on when there is a loss of pressure.

I'm betting with jpdocdave that the pressure bypassed in the MC, or there has to be an external leak somewhere. If the fluid level is not going down, then it is a MC bypass.

Do you know the age/source of the current MC? New or rebuilt?

As far as I know it's the original one. Thanks

offrd3 said:

As far as I know it's the original one. Thanks

Click to expand...

That would be eleven years on the orignial--replace it just for grins and giggles.

Get a NEW MC, not a rebuilt, unless you really like changing them frequently.

ya, i still say start there