Can't get heater hose valve to stop leaking

Everything 5/8”?

Larry

Thanks Larry.

Edited, just measured.
Barbs are 3/4" .

It's a tight fit on the hoses.

I’ve had similar problems when using silicone hose. The shop I bought the hose from suggested taking a scotch brite pad and roughing-up the surface the hose is being installed on. I recall thinking that was a dumb suggestion but it worked! It didn’t take much roughing-up either.

You should also be using a vacuum filling device, when filling your cooling system.

https://www.summitracing.com/parts/kti-70706

Thanks guys. David, I got one of those vacuum filler kits based on feedback here.
So my problem was two problems at once.
The heater valve was probably just not tightened enough. After removing and replacing it 5 times I finally realized that the air I was hearing escaping was from the thermostat to swirl tank hose which is close enough to the heater shut off that  that's what I thought I was hearing. Holding steady pressure now!