97 suburban a/c

I have my 1997 chevy suburban K1500 5.7 w/rear a/c . 150k . Rear a/c works top and rear vents cold to(50) . Front will not cool past (70) at center bi level vent on high . Heater works on all settings front and rear . On the front a/c , vents will change to , def, bi level , floor , it,s just not getting cold .It does not seem to put out a lot of air at the outside bi level vents or under the steering wheel vent with the a/c and heat on . It will defrost and clear the windows summer and winter just not cold air . Last spring 134a freon was added at the shop and it worked in the front and rear two months then just the rear , still cold for everybody back there . Any help on what to look for ? Thanks ! P.S Thanks to everyone that told me about my abs wheel sensors . Everyone in the truck has stopped hitting the dash at stop signs and in parking lots .:banghead: :grinyes: