While not too common on the F150 I have seen a few bad master cylinders. I had one recently and made a video of the process. Hope this helps anyone who comes across this concern.

 

Thanks for the video. Never knew this.
Thinking of replacing MC on 97 F150 4x4, 4.6L 6cyl, 88k miles.
Has new front calipers, rotors, pads, flex hoses, hard lines, new rear cylinders, old lines. Rear cylinders were frozen up and got a strong shimmy when braking, similar to the feel of warped front rotors. It is way better now but shimmy still shows up slightly now and then. Thinking there may be some bypass in the MC, reducing effectiveness of rear brakes...

 

I see you have changed allot, are the rear drums old and rusted up?

 

It's much easier to install the new master cylinder on the vehicle. Then, install the plastic nipples and hoses running them into the top filler hole with the retaining clip in place. Then, fill up the master with the proper grade of brake fluid. Get in the truck with the engine off! and slowly depress the pedal while watching the fluid through the gap between the open hood. Keep depressing the pedal "slowly", never fast (that will allow air to remain/airate the system/MC). Always wear safety glasses while doing this to avoid getting the extremely caustic fluid in your eyes. Sometimes the hoses can fly off while bleeding in this manner. Always stop and check that the MC is full of fresh brake fluid so as to not re-introduce more air back into the MC. Do this over and over slowly until no more air bubbles are seen in the clear vinyl bleed hoses. Then you can safely remove the hoses and fittings quickly replacing with the brake lines.

Ideally, you want to bleed the lines right where they screw into the MC before fully tightening them with a helper. Then, once bleed and tightened, bleed the rest of the vehicles lines at the wheels in-order of furthest wheel away 1st, to closest wheel last from the MC. Do each of them around the vehicle at least twice, in a circle (you might say) to ensure all the air is out. After bleeding and cleaning all the brake fluid off the frame, knuckles and backing plates with brake cleaner, let dry and then apply a small amount of "Sil-Glide" brake lubricant to the (already tightened) threads of each bleeder to keep them from rusting. Apply only after your done bleeding the whole system and the bleeders are clean and tight! Additionally use fender covers to protect the paint on the vehicles body from Brake fluid contamination, as this will eat right through your paint down to the metal in a short time.

 

The rear drums are OEM, thus 17 years old.
Clean on the inside though as of a couple of years ago.
I removed them myself and cleaned thoroughly.
If they were bad, I assume that the Ford dealer would have flagged them.

Update - the shimmy has virtually disappeared. Perhaps it is a "reset" of the rear drum path with the new and properly functioning rear cylinders.

 

Yes they will need time to bed in for sure.