Bias Pedal Box Trouble

Can anyone give me some advice please?

I've got a pedal bias box and it's worked fine for about 5 years. Both master cylinders started leaking so I replaced the pair of them for some nice Wilwood ones of the same size and at the same time put on some EBC Yellow Stuff Pads and some nice drilled/grooved discs and braided fuel lines on the front.

Strange thing now is sometimes the brakes are binding to the point where 3 of us couldn't push the car but then after moving it briefly it can be pushed by one person one handed. This happens to all 4 wheels. Even lifted the car and the wheels can't be rotated by hand, but then try again later and they can be easily. Also since fitting the master cylinders and bleeding the brake pedal is so firm, to the point it takes a large amount of effort to get to the point of the brake light switch engaging. 

All of this discovered on literally a 30 second round trip to the MOT station and back. 

Any help or advice really greatly appreciated. Never experienced anything like this before. 
Cheers