Rad Caps??

I've ran a weak system cap (6 lbs) due to leaking freeze plugs. It helps slow the leak.
Stop leak products will not fix and upper tank leak. It cant stay up there long enough .
I should add... My brothers 93 Mustang has needed a new radiate for 3 or 4 years now. He is constantly having problems with the car.Battery terminals, starter, etc.. and doesn't comprehend the side effects of a continuous coolant leak.