My setup.....
Ford D60 front...dual piston brakes (disc)
Ford D60 rear with 3/4 ton Chevy calipers (disc)
Stock YJ prop valve/junction block (no mods)
'78 Grand Marquis cylinder (hydroboost version)

For some reason whenever I get on the brakes while driving then stop, fluid is always leaking past the seal on the master cylinder cap.
Reduced the fluid and still the same problem.

What is causing this?.....no sense to me based on my understand of the brake system
Do I need to remove the residual valve or modify something in the factory junction block?
With the cap off and when the brakes are applied, fluid shoots out of the rear brake port in the master cylinder...also normal??
Just a faulty cylinder?

Thanks,
Eric

Mine did the same thing. I had to swap the master cylinder to stop the leaking. I thought I may not be getting enough tension on the clip that holds the lid but it was actually the casting was too rough or something.

I changed out the MC and the leaks stopped.

BTW, my MC also squirted fluid like yours. Swap the MC and you will get better results. I decided to stop using re-furbished ones and bought a new one.

Thanks for the help.
I will try to find a actual "new" cylinder that does not cost and arm and a leg this afternoon. I thought the cylinder seemed to be acting strange....not used to it spitting fluid out....usually goes into the lines.

Did any of you guys remove the "residual valve" from the YJ junction block to help proportion the rear disc brakes somewhat?
Next plan will be to get an adjustable prop valve but I was something I could do quick to improve the situation. I have heard of people removing some type of seal also?????

I say you should remove the residual valve from the combination valve. The residual pressure causes the rear pads to drag. After swapping in rear discs on my '88 XJ, I noticed the inside pad was wearing faster than the outside pad??? I latter read that a residual valve would cause that. I since removed mine and the pad wear is now even.

It makes sense to me that if my rear brakes are locking up 1st, that removing the residual valve would improve this condition (since they hold 2 - 10 psi pressure in the lines all with no brake pedal applied...therefore actuating before the front).
Does anyone have a diagram or picture showing the exact location of the residual valve in the YJ (Jeep) junction block?
The rear line output appears to have something in between it and the block.
The junction block also appears to have a "bleeder" as well on the other end.

Do I need to replace the residual valve with a "plug" or "bypass" or is it simply internal and I remove it?

Anyone know what I am talking about?

I know there are write-ups on "gutting" the combination valve, and I did mine accordingly. As I recall, I completely disassemble the valve at each end, and removed a piston assembly and spring. There is also a center section that contains the differential pressure piston and o-rings (used to turn on the brake light if an imbalance exists between the front and rear brakes); leave this intact. You only remove stuff not needed. You don't add anything. I'd do a search here, and on Google for more info, but it it's pretty obvious what you do once you get the end caps off.

Take the front plug off the combo valve (be careful its spring loaded) and then remove the little black rubber washer off the plunger, reinstall.