i have a 1984 mustang gt with no rear braking power. every once in a while i can feel the rear brakes kick in but soon after the pedal just sinks down. i have tried pumping the brakes and got nothing. does this sound like a bad master cylinder?
i know there isn't a leak because, like you said, the pedal hardens up like a rock. its been about a year since i had the drums off so ill have to take a look. and i would hope the previous owner changed the original brake fluid out, but it definitely won't hurt to change it. thanks
It's also possible that there is an internal leak in the MC.
I've seen leaks into the brake booster from the rear of the MC. You can unbolt the MC and pull it away from the booster slightly and a leak will reveal itself there. Brake fluid here will damage the booster diaphram. Once any fluid is down in there it will be history in a matter of time.
On the MC's plunger, there are 2 sets of seals. One for the front brakes and one set for the rear. I have seen where the fluid from the rear reservior leaked around a bad seal toward the front set of seals and filled the cavity in between. So they can decieve you.