440, mild cam with good idle. Power brakes, drums all around. Brakes used to work fine, then two things happened. First, all of a sudden they became super-sensitive. If I just started braking gently, the brakes would grab hard. I could still drive it, but it took some real adjustment. Then I parked it for 10 years. After getting it started back up, the brakes were terrible and it was really hard to stop the car.

Here's what we tried:

Bled the lines to the wheels.
Replaced the master cylinder and brake booster assembly.
Adjusted the brakes.

Still, it acts like it has manual brakes and they don't even work well. The pedal is all the way to the bottom and the car doesn't want to stop very much.

The brakes when working had a nice feel to them, somewhere in between spongy and springy, and the contact point was somewhere in the middle of the throw. Now the pedal starts off already pretty close to the floor and has a short throw and is hard. It doesn't feel springy at all.

We checked the vacuum from the manifold and checked the hoses for leaks and everything seems OK. The car is tuned up with a new carb and plugs and it purrs and spins up fast and has no stumble at all.

I'm not sure where to go next with this. My mechanic is scratching his head and thinks maybe we could check the proportioning valve and also thinks maybe the brake pads have totally glazed over and don't grip at all. I have no idea about the valve, and while the pads may be shot, that doesn't explain the change in pedal feel and travel.

Any ideas on what it may be and/or how to continue to troubleshoot it?

TIA

For reference, here is the idle: https://www.youtube.com/watch?v=WlXs7GQbp0Y

what I do on a toughie is start at the MC. get 2 brass male inverted flare plugs for the MC ports from the Edelman cabinet at your parts house & take off the fittings/lines & cap the MC. seperate the MC from the booster & check the clearance of the pushrod round end nub to the bottom of the MC rear piston recess (want ~.020") then with MC capped/start eng idling & if the MC is good & bled out the pedal will be rock hard with minimal travel (stomp it hard & try light pressure, do both). If good there then hookup one half (I prefer the rear drums) & bleed them out as needed & you can cap any port in any line to any corner to help nail down the prob area. verify booster/mc first

you should first adjust all the shoes until there is light drag on the drums, then try it...
Same problem? then adjust the length of the rod coming from the new booster, it is not necessary to remove and rebleed the lines, just pull the master forward and inch or two and lengthen the pushrod adjustment until you like the pedal height and not to long as to have brake drag with the pedal at rest..

Brake systems can be difficult at times and troubleshooting a pain. As said start with the master and make sure its bled properly. Confirm sufficient vacuum at booster, wheel cylinders are working properly and adjust and you should be good. Are you sure the mechanic knows his way around brake systems? May want to confirm you have the correct master and booster assembly.