"deteriorating brake warning valve "
I assume that you mean the brake circuit balance warning switch? If so, did you bleed the brakes initially and if you did this using the pedal pump method, did you remove the warning switch contact? As noted by C1800, if not, the pedal pump method will cause the unbalance piston in the warning switch to move. If you restore pressure to both circuits the piston moves back to its balanced position; but, may jam the the operating pin on the warning switch. This normally results in the warning switch being on all the time; but, it is possible that the pin is preventing the balance piston from completely returning to normal. However, this should still result in pressure returning to normal . It is hard to see how a problem with the warning switch could cause a problem with the pressure not returning to rest when you release the brakes.
As noted by C1800, how did you check the brake hoses? Swelling of the internal liner cannot be determined by visual inspection of the outside hose jacket.
Finally, di you try inserting a feeler gauge between the inside and outside surfaces of the brake disk and the respective brake pads. That will indicate whether it is a particular pad / circuit that is causing the bind on whether both pads and circuits are the problem.
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