Can anyone help? I use an Engl Z5 footswitch with my Screamer combo. All works fine, except when I try to switch reverb on when I've already got VLS and Hi Gain lights on. The reverb light switches on, but as soon as I play something the reverb light (and effect) goes off. It only happens with this combination of hi gain channel and VLS "on". Works fine with VLS off or with VLS on but on a different channel. It's driving me mad, as this is the setting I like for lead parts. Taking the cover off doesn't reveal anything that looks "broken"; it's all pretty contained. Any help appreciated! Thanks.
Check to see if any of the switches in the footswitch are making contact with each other, or with the casing.

You could be getting a short-out.
My Gear
-ESP truckster
-Ibby RG 7 string
-Mesa/Boogie Studio Preamp (Mk2C+ mod)
-Mesa/Boogie 50-50 Stereo Poweramp
-Mesa/Boogie Vertical 2x12 (C90 and EVM)