Brakes got me Scratchin a hole in my head!?!?

Working on a 2000 XJ. I've seriously bled the brakes, got new brake fluid throughout! With engine off I can build pressure as normal. With engine running I push the pedal half way down it's 'ok' but any further I lose all pressure and the idiot light comes on. Not leaking fluid from anywhere; pads, shoes, calipers, wh cylinders are all good; even replaced ALL the hardware down to the self adjusters. I'm suspecting the master cylinder may have a 'nick' in one of the seal causing it to lose pressure above a point??????? It's $57 (NEW of course) so I don't wanna just throw money at it! Can I get any confirmation or suggestions

I'm thinking the same thing. Master cylinder

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Thanks! I've been working on cars for over 30yrs now, but I still second guess myself and I'm always open for suggestions! Especially when it's not my $, but my reputation is on the line!

Yup, sounds like the master cylinder is getting ready to go completely

You may be on to something here but let me ask a question quick. Does your Jeep have ABS? If you don't know then what rear end does it have? A D35 in that year will have ABS and if you're having issues with brake pedal feel there is a different way to bleed the system.

If you have a D35 and/or ABS you need to purge the air out of the ABS proportioning valve. It's the silver box right in front of the master cylinder under the hood. To do it you need a DRB Scantool which to my knowledge is only available at a dealership. If you don't have access to one you may have to take it in and have the dealership bleed the valve then you bleed the rest of the system out normally.

No, there ain't no ABS, Thank God! I know ABS has to be bled a specific way, and then you've got the newer cross braking systems like on my wife's car, gotta bleed in this order LR, RF, RR, LF, yep a bit different! Changed out the master cylinder and bled the brakes 2 more times, now he has brakes. When he hits the pedal the front end dips and the Jeep stops RIGHT NOW! I'm up in the Cleveland area, rained all yesterday high in mid 40's, you see I'd rather have extra input, get it done, and get back inside!

Good deal I'm glad you got it figured out. We're looking at a substantial snowstorm this evening. Bring it on!

I know we've been lucky so far, it has been cold and we've had plenty of rain, but sooner or later we're gonna have to pay for all this nice weather! Hopefully not all at once! I'm ready, the snowblower was tuned up in October!