WordNet (r) 3.0 (2006):

let up
    v 1: become less in amount or intensity; "The storm abated";
         "The rain let up after a few hours" [syn: abate, let
         up, slack off, slack, die away]
    2: reduce pressure or intensity; "he eased off the gas pedal and
       the car slowed down" [syn: ease up, ease off, let up]

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "let up":
   abate, ablate, arrest, backpedal, backwater, bate, be eaten away,
   brake, call a recess, check, clip the wings, consume, consume away,
   corrode, crumble, curb, decelerate, decline, decrease, delay,
   deliquesce, detain, die away, die down, diffuse, diminish, disjoin,
   disperse, dive, draw rein, drop, drop off, dwindle, ease, ease off,
   ease up, ebb, erode, fall, fall away, fall off, forbear, forgive,
   free, give quarter, have mercy upon, have pity, hesitate,
   hold back, hold in check, hold up, impede, keep back, languish,
   lessen, let down, let go, let up on, loose, loosen, lose ground,
   lose momentum, lose speed, melt, melt away, mitigate, moderate,
   obstruct, pardon, pause, plummet, plunge, recess, reef, rein in,
   relax, relent, remit, reprieve, rest, retard, run low, sag,
   scatter, set back, shrink, sink, slack, slack off, slack up,
   slacken, slake, slow, slow down, slow up, spare, stay, subside,
   tail off, take a recess, take in sail, take pity on, thaw,
   throttle down, unbend, unbrace, unglue, unlax, unleash, unstick,
   unstrain, unstring, unwind, wane, waste, waste away, wear,
   wear away