[Ironangel]

Are your existing front drum brake lines 1/4" like the rear? That will require using a 3/16" to 1/4" adaptor if you go ahead and use the disc-drum MC. I'm thinking rust, dirt and debris is in the piston bore of your drum-drum MC intermittently blocking the return port. The disc-drum MC you have shown will work for front drums but the front port is for 3/16" flare fittings, that's where you'll need the adaptor. The reason a drum-drum MC will not work for a front disc is the reservoir size, disc brake calipers use twice the volume of fluid than the wheel cylinders in drum brake applications. If it were me I'd spring for the correct drum-drum MC and be done with it. Take on the front disc swap when it happens. Trade the new disc-drum MC for the drum-drum MC and trade it back when you make the swap...Eliminate the variables.