brake fluid is gone

[marks bronco]

To start with this is a 76 with fact. front disc brakes and power brakes, I bought this truck last Oct. and its been siting in a cow pasture since 1992. I have been replaceing a little bit at a time since I have 3 bronco projects at once and when I moved it this weekend I had no brakes again. I have replaced all the brake lines and hoses, proportioning valve,master cil. and have inspected every other part of the brake system.I cant find any leakes,have pulled the hubs,and still the back (larger half) of the master cil keeps going empty when siting for more than 3 weeks. Is the fluid in the back for both or is there a divider there for front and back brakes or is it a resorve for emergency stoping when theres a leak . I need help stoping the leak if I can find it!!

 

[70_Steve]

Back half of the master cylinder (large reservoir) is for the front brakes. Look inside the brake booster for the lost fluid!

 

[Doleauto]

Mark
I have seen a brake caliper leak at the seals. I would almost imagine you have checked that already. You have to make sure the combination valve is not leaking also. If you have checked EVERYTHING visible to the eye, then 99% chance your master cylinder is leaking. What happens is the seals start leaking in the bore and a little bit at a time weeps out of the back----right into the power booster, which is engine vacuum actuated. The brake fluid ends up being burnt when the engine is running and usually leaves no trace.
Make sure you check all externals first, if you have to get someone to start vehicle and hold their foot on the brake to find a leak, do it. Hope this helps.

Tom