I have a reverb that is currently off at Loco Tuning to sort this. It has already been back to Loco about 3 months ago for the same problem. Unfortunately mine needed a new internals kit as air was escaping back through the internal seal and didn't allow it to hold air. It got sorted, then did the same thing recently so Loco are sending it back to RS to check the tolerances on the internals. So yeah, worth trying a bleed as its about £90 all in for Loco to sort it but it sounds like the internals might be shot