My ABS and brake lights come on every time I hook up my trailer, till the vacuum builds up. (vacuum/hydraulic brakes on trialer) It also used to come on when I had a vacuum leak at the plug on top of the transfer case.
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1996 White, Extra-cab, 4X4, 5-speed, 4. 10 Model 80 rear, Laramie Pkg, Rickson 19. 5's w/ with Michelin 265/70 XZT's, 4" turbo-back full-straight pipe exhaust with through the bed stack, Mopar brush guard, "Psychotty Air", no silencer ring, Banks Boost & Pyro Guages 'tweaked' injector pump, 130,000+ miles
Rollin'-S 25-foot, 10 ton flatbed g/n trailer, with Vac/hyd brakes, 16. 5" tandem duals
'92 D350 Cummins, auto/OD, 3. 54, enclosed service body, 45,000 miles. unBOMBed as of now.