They just slide off, once the clip is removed. Newer OBS trucks with stainless steel lines require special tool, but what you're working on ain't it. Jasper is likely correct in that the o-ring got stuck and stops the plastic fitting from simply pulling off the steel line. Try spraying some lube in there then twisting the plastic fitting a bit, should break it loose. Then again, sometimes they are so stuck the quick-connect fitting itself comes apart and the o-ring and its retainer remain on the steel line, while the plastic fitting slides off - good news is they can usually be put back together with no ill effects.