Need help with blinker not working

I have a 1986 GMC 3500 1ton dually that came with a diesel motor which i have converted to a 350, anyway the blinkers work slow and when i apply my brakes the left one quits flashing and the indicater arrow on dash stays on.I tried a larger flasher and the blinker speeded up, but when brake lights came on, left stoped flashing again.Please Help!!

check for a short, does it do different on the left or right????

chech for wrong bulb to left rear light and for bad ground left front

I had the same thing happen on an '85. It turned out to be the ground wire that attached to the frame had corroded in two. It was the one located on the driverside rear of the frame, I think it was a black wire held to the frame with a 10mm screw. Check that, it could be your problem.