why is my master cyl. doing this

[blazer2007]

My master cylinder is weeping slowly from the fr. resorvoir to the back one. Is manual brake setup with drum/disc setup on a 80,s truck. The old one did this so I thought there was something wrong so I replaced it with a new one and this one is doing it too. Where is my problem ?

[mongocanfly]

I'm not sure I follow what your seeing....how can you tell?..,both sides should be full

[blazer2007]

the fr. fills up the rear one and then it leaks. This takes about about 3 weeks then I notice a spongy pedal and I then take a turkey baster and take some fluid out from the rear and put back in the fr. reservoir . The rear reservoir is full to the top and of course it leaks when I pull the lid off at this point. The pedal is now fine at this point

[mongocanfly]

Ok I understand what your saying now
There is a seal between the reservoirs..but 2 masters doing the same thing makes me wonder if the issue lies elsewhere.. I'm wondering if your proportioning valve could be transferring fluid to the wrong side..
Might be worth swapping it out

[blazer2007]

Ok I will have alook. At least it,s a starting point. Thanks

[geezer#99]

Is this truck converted from power to manual with the booster removed?

[blazer2007]

no

[geezer#99]

So it’s stock then.
I was thinking pedal ratio might be the difference.
Does your brake pedal have two holes in it where the brake rod attaches to it.