When Roscoe breaks the van out of the impound lot, it is obviously damaged in the right front headlight area. Later, as he is driving around, even after colliding with the other agent in the same spot, he drives up to the pirates, the damage is gone.
Durng the final showdown with the mercs, the rear corner of the General is clearly shown to merely have been lifted upwards slightly and then dropped back down to imitate a blown tire; the tire does not actually go flat, and the tires are fine when Luke backs the General out of the path of the oncoming scoop-loader.
Obvious use of artificial water-jet under the van's partially-submerged tire, since the water continues to blast backwards even when the tire is not spinning.