I did a complete overhaul of the brakes, new wheel cylinders, master cylinder, replaced some of the hard lines that were iffy. Now when I start the truck in the AM, the pedal goes almost to the floor ... after driving a while - the pedal gets tighter, and when I stop, the pedal barely goes down an inch or two, and the brake lights stay on for a few minutes after stopping. Some times they stay on and drain the battery - 1 out of 5 days - the behavior is so odd, I am clueless - is it a bad Master Cylinder >> HELP

 

the only way the brake light should stay on is ,,,ok first ..is it stock ? do you have power brakes ??when the pedal get tighter are the breaks dragging??do you have a pressuer switch or mechanical switch for the lights ?? we need more info ..then we can help

 

Standard brakes - drum brakes. The Master cylinder is manual linkage to pedal, with a pressure sensor on the back, with two poles, blue, red, which tell the brake lights to go on - no power brakes. When the pedal got tighter, and the lights were on - the back wheels would heat up as it appears it was pushing the shoes on the drums - like a ghost was pushing on the pedal

 

C1AZ-13480-A (replaced 91A-13480) .. Brake (Stop) Light Switch-Pressure Activated (Motorcraft SW-24).

Applications: All 1939/66 FoMoCo vehicles with "single pot" master cylinders-except trucks with air brakes.

This switch is notorious for getting gummed up. Pedal going to floor could be indicative of air in the lines, and/or leak in the system, and/or defective master cylinder.

FoMoCo went to dual master cylinders in 1967, so...there are myriad different switches...depending on applications.

 

have you checked the flex lines ?? the rubber flex line can collapse inside and hold pressure on the brakes ,and building up and getting tighter at the master ..either air in the system,or crud in the line blocking the return of fluid to release the brakes..so ..to start ..pump up the brakes and bleed at the rear and watch if the flex line changes shape..and how much fluid comes out..it is only supposed to hold about 2 or 3 psi to keep the wheel cylinders sealed ..because the light stays on ..I think the problem is the master it's self holding residual pressure..it has a small check valve at the line connection that could be the problem..or just something in the line blocking it from returning..sorry for the long run here going through my mind and all the mis-sreaks I have done to learn from..so your master is on the firewall right ??

 

Master is under the floor board, right below the driver

 

there is a picture of the brake master here https://www.ford-trucks.com/forums/1...r-rebuild.html

 

ok same as my truck,, the risidual valve is 2178 in the picture..that may be the problem..so ..I would start there..try pumping the brakes up and releasing the pressure at the master .if that works then it narrows the problem to the master ..and you can pull it down and see from there..