How do I isolate bad MC or other problem. I don't want to do a parts swap to find the problem. I am thinking bad MC too.

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Update: I ordered a Brake MC. Will install it tomorrow after I pick up the part. Should I completely drain the brake lines?

I am going to replace the Brake Master Cylinder. Seems was getting soft for a long time. As I remember with another car YEARS AGO, acts like the MC. Thanks
@ cruiser54 , And I agree with your video. Enough is enough.

You watched it? LOL.

Can I remove the Master Brake Cylinder without completely draining the lines. I would first see if I can flush down the line going to the rear brakes. That I know would need to be bled. As for the front, can I do a reverse bleed, pump back into the reservoir?

Yes on the MC removal.
Are you gonna replace the MC?
If so, you bench bleed it and then, with the MC installed, bleed the system.

Last question . I am working on the Jeep today this afternoon.
I want to be sure I cover what MAY be wrong. If the MC reservoir did empty, will I get that solid no more fluid pump from the reverse bleed in the back? I could do a bleed of the old MC on the Jeep before I do the change. While disconnected, could I pump fluid thru the rear brake lines and see if I get brake fluid flowing back and out? If I do, then it IS the MC for sure. If not, then I'd have to check else where. Does that sound reasonable? The MC may be bad anyway. If two problems, I'd like to sort it out. Suggestions?

If you have a source of air, you could disconnect the steel line at the MC, open the rear bleeders (or disconnect the line(s) from the wheel brake cylinder) and hit it with some air pressure from the MC end. This will confirm you have clear lines. That's how I cleaned out my lines due to all the contamination I had.

Personally if you are going this far in "rebuilding" your braking system, I'd blow out all the lines and with a new MC, bench bleed it like cruiser said, install it, fill it and proceed to bleed the system starting with the farthest away bleeder and working to the closet.

There is one component on an automotive braking system that I'm not well versed in: that is the brake booster. Other than leaking and causing a hard pedal(like manual brakes, only harder), I wouldn't know what kind of symptoms a faulty booster could cause. I'm sure other's on here know. I did find this test online:

Starting with the engine off, pump the brakes several times. (Brake booster design allows the mechanism to hold enough vacuum to engage the brakes once or twice without the engine running. This helps you stop the vehicle in the case of engine failure.) Next, press on the brake pedal lightly while starting the engine. You should feel the pedal give a bit and then firm up. If it becomes hard or if you feel no change at all, then the brake booster has failed.

As far as the proportioning valve, I found it simple to disassemble, clean thoroughly and reassemble.

This way in the end, all components of the brake system were clean and ready for fresh fluid.

This is getting WAAAAAAAAAAAAAAAAAY too complicated.

Well, if that was due to my post, my apologies. Brakes are easy to me but I don't always write like Shakespeare so I could be adding confusion instead of offering help. What's in my mind doesn't always make it to my fingertips and then onto the white page.

Old Man, your advice was spot on.

Maybe I shouldn't have made my forum name, "old man", lol.

I am old by some, but I don't feel old! (62)

You're 6 years younger than I am!!

I've had a chance to get back to the Jeep. Cant believe the silly problems I am having. It really is a Simple fix. Brake line repair done.
Here's where I am at. I was able to reverse bleed back into the MC. I had accidentally emptied the Rear Brake reservoir when I last worked on it. That went easy. I disconnected the brake line at the passenger rear drum and reverse bleed into the MC. I wanted to verify bad MC before doing it. I had a similar issue with my old chevy. Was air in the line.
Now what happened. The reverse bled back to the reservoir went great. Then connected the line back up to the drum and set to bleed thru the normal bleed nipple fitting. I could not get fluid thru it. Pulled the nipple fitting out and pumped fluid thru just the nipple. It is fine since fluid flowed thru the side hole in the fitting.
. Tried putting it back in, and pump fluid- Nothing. Should I spray Brake Cleaner into that thread for the nipple? I'm thinking its plugged. The old fluid was rather dirty. Now all really clean fluid. Except can't bleed at the drum with the bleed nipple.
I'm using one of those pump type bleed tools for one person to do it.. Using it to Pull fluid thru the nipple and out. I tried pumping INTO the line and can't so seems a plugged up at the nipple. Suggestions.
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I was going to change the MC anyway since I am doing brake stuff.. I have a new one. But couldn't get the fittings loose on the MC. Sprayed with brake cleaner. Still wouldn't budge. I did not want to break the fittings., the brake line, etc If I have to replace the MC, Any advice on the stuck fitting, Rear brake . I did not try the front brake reservoir fitting

Get your phone up in there and film down the hole with the bleeder removed to see if there is a rust plug in there. With the bleeder removed you should get fluid oozing out, and if you have the bleeder out while the cap is off the master you should have a decent flow, not gushing (don't touch the pedal while the bleeder is removed or it will gush), but you'll make a mess. If it is indeed plugged you could remove the wheel cylinder and try to rebuild it. Or you could replace it given the price and time.

Thanks. I'll check that. I have new wheel cylinders for the back that I was going to replace at a later time. They weren't giving me problems. Just thinking its old and have no idea if they are originals or not. I suspect they are.