Just got my car out of winter storage today and I'm experiencing a braking problem. It was converted to large bolt front disk brakes about 4 years ago using the parts from a 1974 Dart. When I went to brake comming out of the trailer the brake light on the dash came on and the petal went nearly to the floor. I can pump up the petal to stop but It goes right back down after that,(fluid is full). Could it be the proportining valve. Everything worked fine when I put it away for winter.

You have a leak somewhere. Take a close look at MC, and take off each wheel and inspect.

if the pedle goes to the floor but not lousing fluid has to be master cly. the fluid bleeds by the seal in side all the fluid stays in the master. the stm. was full of fluid neve emty if was emty air will make this hapen to.

Air. Bleed the m/c then the wheels in order if that dont make the pedal high & tight. the light coming on says there's a pressure differential in one half of the system which may b the m/c but do check the wheel cyls for leaks

Thanks to all for the help. I will try bleeding the system first chance I get. Just thought it was funny, braking system worked fine when i put it away for winter storage.