I have a 1986 F-150 4x4. earlier today every time I pushed on the brakes the reardriver side would lock up but it didn't feel like any of the other brakes were working. So I put new brakes all around and noticed that the rear passenger side hasn't benn working for a long time(long enough to rust up inside the drum). We tried bleeding the brakes, but now we don't have brakes at all in the back. We're not stupid people, we've been working on cars the last 40 years. This time we're stumped though. If anyone has ideas, please let me know. The master cylinder is only about 2-3 years old, however we'll replace it tomarrow and see if that fixes it.
Thanks for any help.

 

It sounds like there is a crack in the line somewhere. May be worth running new lines.

 

Highly doubtful just cracked line. Sounds more like like or master cyl. leakage and bad proportioning valve combo. Maybe look for bent/leaking lines to start. Bench bleed your master cylinder before you start (I'm sure you know that) and check master cyl. housing top edge for flatness with a straightedge (check the lid too). Some companies are manufacturing duds and the crooked housing and lid will allow moisture (rain water,etc.) past the seal and into the cyl. readily causing premature failure (and knocking out the whole system in the process).

 

Might have frozen (rusted) wheel cyls. If the pedel is firm then you may need to start loosening lines to find a blockage.