Turn signal and brake problem

I have a 1977 Cutlass Supreme. When I am using the turn signals, in either direction and apply the brake the signal slows to about one flash every 5 seconds and the side that is not in use flashes with the side in use but very dim. I relplace the flasher and both tail lights flashed together nice and bright until I applied the brake and then the stopped but the brake lights stayed lit. The front turn signal lights don't have the same problem of flashing together. all 4 bulbs and sockets look good in the tail lights. Any Ideas.

Dave

check for bad grounds in the rear then suspect the signal switch. Look at the bulbs closely also, sometimes the filaments cross over each other.