i have a 97 F150 it started a couple years ago the rear brakes would lock up in the morning or when it was raining or damp outside it would do it a couple times then would be fine. i have changed the shoes, wheel cylinders, drums and all the parts in the drums new springs etc. still would do it when raining damp outside a couple times and work fine after that. now getting worse now only back passenger tire locks up everytime you drive it. try to get a new proportioning valve couldnt find one i have bled the brakes numerous times everything seems to bleed normal. could it be the master cylinder ???????? getting pretty dangerous now
Thanks

 

Sounds like the rear drums are out of round and need machining. A simple test, forget the hydraulics and drive slowly 10-15 MPH and apply the emergency brake slowly. If truck feels like it is surging forward and back, thats your proof. The shoes expand to the larger out of round, and lock up.

 

i tried the e brake trick back tire locked up then i held the e brake release and and tried same thing tire locked up. now when i use the regular brake the back driver side tire locks up instead of passenger side really confused now. i am going to get drums turned this week and new shoes and give it another try will let you know.
Thanks

 

I dont know if this applies to your F150, My Neighbor bought a Ranger and had troubles with his rear drum brakes. Come to find out the rear end had been replaced and the current drums were too small and would sit not flush causing all kinds of issues when braking.

 

I had the same issue, and also had ****ty pedal too. I just replaced the drums, shoes, and hardware kits, and cleaned my adjusters really well. Since the re-install I've had good luck with them and my pedal is a WORLD better. Now it hasnt really rained here yet, so I'm not sure of that. It did sprnkle yesterday, and I drove it right after that with good luck, so I'm hoping this is fixed..