Bad Master Cylinder?

Car is a '85 tbird. Power Front disk/rear drum no ABS. Brakes have worked fine until yesterday morning. Suddenly the pedal needed to be pressed nearly to the floor to get any braking, and that was weak. I
checked the fluid and the front (smaller) section of the master cylinder was low. I filled it up but its not helped. I don't think I've ever had to add fluid in the 9 yrs I've had this car except for when I bled the front calipers after replacing the hoses some years ago. I "think" that perhaps only the rear drums are working at present but am not sure.
Does this seem like a master cylinder problem or a leak elsewhere? Why did topping off the low section not help? Since it came on suddenly, I don't think it can be pads or shoe wear.
Thanks JH
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JH wrote:

The fluid level will go down as the front pads wear. Check the calipers and rear brakes for leakage.
Bob
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I would have thought a booster failure would result in the pedal not going to the floor, just reduced braking effiency. To check front / rear braking, drive down a road (safely if you can!) with what braking you have on, then check the temp of the discs / drums. Front should absorb most of the energy, if front is cold and rear hot, you are not getting braking to the front, which would indicate master cylinder. Try bleeding the brakes again. Understand exactly how you braking configuration works, and test. You should figure it out and fix it before long. If not, go to someone who knows about brakes, before you kill someone!
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Does the pedal come up as you pump the brakes? If so, then yeah, the master cylinder has failed. If not, brake fluid is being lost through a seal or connection.
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Thanks for the replies. Pedal comes up normally when releasing. There is some braking, just lower to floor than before and not as hard. It does seem like the master. I had a booster blow out on a different car once and that one would push back when you pushed the pedal and extreme braking effort. This one is not like that at all. I also once had a '79 mustang with non-power brakes that the master went out and would lose fluid. It would go down in one reservoir and up in the other. That one would work OK if I kept up with topping it off though. This seems a little different than that. Hopefully It does not worsen as its too cold and snowy right now to dig into it, I have 18" of snow in my yard and it was -10F the other morning! I will see if there is any wetness around the wheels soon as I can though.
JH
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