Hissing Brake Pedal

Hey All,
So my pedal developed a hissing sound over night. The hissing only happens when I press the pedal down at a moderate speed, like when you press down before you turn the truck on/off rather than slowing down to a stop. No visible leaks anywhere and the reservoir is still full after a full day of stomping on the pedal.

I have the OR with the hydraulic assist rather than the vacuum assist. One strange thing that also developed from this is that there's a very slight delay when the pedal is engaged to when the braking action happens. Its unnoticeable when slowing down slowly but I did some test emergency braking and the delay is between 1-2 seconds from the time I slam on the pedal. There is an initial bite that slows the truck but after 1-2 seconds it slams down like its supposed to.

I'm sort of chalking it up to the accumulator going out but would like to get some other feedback before I pull the trigger on a new assembly.