Puzzeled by YJ brake problem

[M G D]

My YJ won't build up brake peddle, I know that's usually air in the system but after repeatably pumping fluid through it I find no evidence of air. It started after I replaced the front calipers when both became hot after a short drive, they clearly weren't fully releasing. After replacing the calipers I had a very soft peddle but no sign of air bubbles so I replaced the master cylinder with a NAPA re-manufactured. After that I used a vacuum pump to pull a total of a quart of DOT 3 through the system then pumped pedal and checked for air with no sign of any, still no peddle pressure. With the engine running the peddle is much easier to depress which is good right, means the vacuum can thing is working. Anyway before I tear the re-man mc off for a new one any suggestions or ideas what I could check? I was wondering if the little block under the mc ever acts up & if so what that would act like. I can make the brake idiot light go out by jumping it so that works. Oh, yeah there's enough system pressure to actuate the new wheel cylinders out and I can't turn the front disks with a bar with the peddle depressed but with wheels on no stoppage. Help. Thanks.

[69jeepcj]

Its possible that you put the calipers on the wrong side so your bleeder valve is upside down and you wont get all the air out. Im not familiar with the little blocks on the yj,s but on cj,s there was a tiny little rubber cap on the end of it that covered a pin. Try pulling the pin out or pushing it in, I cant remember which, but it allows or prevents fluid through the lines.

[Bounty__Hunter]

Sounds like the calipers are on the wrong side, make sure the bleeder valves are above the banjo bolts.

[jps4jeep]

also did you bench bleed the master before connecting?

[PoliceMonkey161]

Assuming that you have a correct setup, that little block is your portioning valve. It divides the brake pressure appropriately between the front and rear. It’s not likely the problem. Other than checking that the calipers are installed on the correct side as mentioned earlier (Bleeder valve will be at the top), another problem (which is especially true if your calipers are locking) since replacing the Master Cylinder would be an incorrectly install check valve at the MC.

If you have drum rears and disc fronts, only the rear portion or section of the MC would have a check valve between the cast body and the threaded tubing fitting which is the drum side of the MC. It’s worth a look.

[NewYJOwner]

I find my brake pedal seems soft if the rear drum brakes are not adjusted properly. I spin my adjusters to where it wants to stop turning (shoes are tight to the drum) then back the adjuster off a quarter of a turn.