I have the same thing on my
1993 Civic Lx Non Abs rear Drums
I pick up some proportioning valve from Junk yard
end up finding my is 30/30 on the Car (but looks like someone swap it out so i dont think 30/30 is what the car came with reason i say that is all the 10mm screws are hacked up)
so close i found is is 30/40
i installed it and test drove it for 10miles brakes fine
the rubber plug was still there... but it leaking fluid from that area....
what could be the cause of this?....
Pedal to master Cly. clearance is too tight....
defect proportioning valve?...


Also Pic is not mine it Example of what i mean