An old-fashioned trick to test if the cyl. head gasket is ruptured from cyl. to coolant jacket is to tie a poly bag to the end of the rad overflow with a rubber band. If there's excessive pressure from a blown gasket like this, the poly bag will be full of hot coolant continually every time the motor is used. The excessive pressure gets through the rad. cap as it's only about 15lbs. for the spring in it.
If it is a ruptured gasket, you'll have to bite the bullet and pay. Don't waste your time with "miracle cures". When and if they work (usually waterglass) it's only for a few days, then you're back to square 1.
Quod erat desperandum.

Leedsman.