OT truck problems

You say that you have good pedal, I presume you are not “pumping” the brakes to get them to hold, so no, the worn drum theory isn’t the problem. As far as back pressure in the cylinder, this would have to be caused by spring tension returning the drum shoes or pressure from a front caliper piston being pushed back into its bore. The later is a possibility if you have a warped rotor, however I’m thinking it would have to warped so bad that you would have a very noticeable “pulse” in your break pedal when you apply your brakes.

Go with Allen’s comment, make sure the level is correct and have someone work the brakes. You should see the return back to the reservoir and make a small fountain. Make sure the gasket is flat in the cover and installed properly allowing it to “suck down” into the reservoir as the brakes wear. The spring clip that holds the cover should really snap down tight on the cover. If this all looks good, you likely need a new master cylinder.

I would pull the cap, have your wife work the pedal and watch that reservoir.

When the pedal is pushed, there should be a slight amount of artesian(ing) until the pistons move past the intake ports, but there shouldn't be a whole lot of pressure in there to speak of. If there is, I think you have a bad master cylinder.

Are you turning the baffle/cap seal in backwards on itself before you put it on? Lower fluid level over time should pull it outwards.

Hope you get 'er figured out,

Allan

Bend the cap flat if it is warped.
Use one or more stacked slivers of aluminum cut from a popcan to shim up the cap between the cap and diaphram in the area of the low spot if it is the cast body that is deformed.
Good luck, JimN