It's been almost two years since I posted this problem for the first time.
I have a SMI Stay N Play Duo auxiliary braking system installed in our Wrangler. The system's vacuum check valve allows the controller(with vacuum pump) to provide vacuum while towing and prevents it's vacuum from bleeding into the engine manifold when not running. The check valve that comes with the system has failed three times. It appears that the rubber diaphragm inside is swelling and blocking the vacuum in both directions causing no vacuum assist to the Jeep's master cylinder booster. This can be quite exciting driving the Jeep when it fails.
The first two times this happened, SMI replaced the check valves but said they have never had the first failure from anyone else. Our '08 is completely stock and it's hard to believe the braking gods have chosen my Jeep to be the only one 'cursed'. The only thing I can think of that is out of the ordinary is that the controller is mounted inside the pax compartment in the right foot well instead of the engine compartment.
The last time this valve failed was almost a year ago. The reason I waited so long to replace this thing is that I intended a change to synthetic oil instead of the dino hoping that if an oil mist was causing the problem, the syn oil would not have the swelling effect on the valve's diaphragm. Well, that's the scoop.
If anyone's still awake, has anyone's Wrangler with the SMI DUO system installed, have or have had the same problem with the check valve? I have no idea what's causing this. Otherwise, I'm really happy with this system...Thanks, Bob