1997 freightliner brake pedal valve problem

OK guys when I mash the brake pedal I hear air coming around it front the valve so I replaced the valve same problem. .my brakes acting crazy some times as well. .when you mash the pedal to stop the back pass. Side Tandon slides the wheel never had a problem like this..new drums new site cam and seals new brake shoes and hardware across the back 2 axles..4 new slack adj. And 2 new chambers ..please help me out.

 

Almost forgot. Have another issue lol same truck Cummins motor fuller 8ll trans..just started this week. When you try to go from hi side back to lo you have to do it while truck is rolling if not it will not go to lo side...when going from lo to hi it grinds like crazy. .I'm thinking slave valve on side trans..every body says it's inside the trans.not the valve but the problem. .took both covers of the hi and lo piston clean them out and the cover that has the 2 air lines running to it..any ideas on this ..slave valve I can pick up but thought I would ask ..

 

The brake valve is a new dealer part..but still hear air coming around the pedal when mashed..are you saying replace the piston looking thing between the valve and pedal. .