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Master cylinder problems

put new master cylinder and bled brakes in 1978 jeep cj5 and now fluid is being forced out the rear brake reservoir in the master cylinder, if lid is off there is about a 1inch fountain of fluid that shoots up.

That’s not unusual. What is the problem you’re having with the way the brakes work?

it seems I have to pump brakes more and even with the lid on the master cylinder , the fluid is being forced out the lid and dripping down the sides

Adjusting the rear brakes might help.

Bad master cylinder; get another.

It is possible that the master cylinder is bad. To check for that possibility plug both outlet ports and pump the pedal repeatedly, fully releasing after each stroke to allow all air to exit. Then step on the pedal with both feet and try to break it, holding it down for 30 seconds. If the pedal remains high and hard the master cylinder is OK. If the pedal fades, even very slowly, the master cylinder is bypassing.

But unbled air will compress as will the return springs on the rear drums and when the pedal is pressed and released the fluid that compressed the air and springs will rush back out to the reservoir. The smaller reservoir on the master cylinder operates the rear brakes and I feel sure that it is at the rear.

my first thought was a bad master cylinder, so I replaced it and rebled the system but still have the same problem. so I will adjust the rear brakes, check the master cylinder as Rod Knox suggests rebleed and see what happens. any other thoughts let me know.

Did you bench bleed the MC before installing it?