anyone had ever issues with prop.valve

hi - just messing around, with a mystery problem concerning my frontbrakes

10 inch front drums and rear drums on a dual-mastercylinder

i´d change everything inside the drums, in the meantime, know the last part would be the prop.valve

my problem:

if the car is cold - no issues

if i go for a ride on the motorway and the engine bay heats up - the car starts to drag to the left side.

if it sits for a while and will cool down, no problems - no dragging,
if it heats up or if it´s been driven for a while the issue comes back.

in the summermonth (June till end of August) it was much more quick to notice than now in Oct.

i´d talked to an Ford-Mustang Guy and he told me had similar issues - and have chucked the prop.valve replaced it with a T-fitting for the front brakes and an adjustable valve for the reardrums.

what do you think about a miss-working prop-valve

and the replacement option