Brake bindage blues

Hello all. I have an 05 X-Type sport. I have owned for around 6 yrs. Its got around 210,000 miles on it and has been a great car. This year however, I got the car out of winter storage, and it seems to have developed an issue with the brakes. I started the car and pressed brake pedal and it went to the floor. I assumed it was broke brake line or whatnot. So upon looking under the car I could find no leaks at all. Had it towed to garage. They replaced master cylinder and told me when bleeding, fluid was not going back to MC. They want me to replace abs pump. I am not convinced that the abs module is the problem. New symptoms: Brake pedal feels good, but after a minute or two, all 4 brakes begin to lock/bind up. Loosening the 2 nuts that hold MC to brake booster(servo) will release them but they continue to lock up after tightening. Any help or ideas would be great.

The fact that the pedal returns to ‘normal’ after you release the M/c indicates to me that the push rod to M/c clearance is too tight, see if you can shorten the rod a small amount.

would it be safe to put shim between mc and booster/servo?

As a temporary check it might work, if it does then I would suggest correcting it at the pedal.