Trailer brake and ABS

[snailexpress]

Need help to identify problem with trailer brakes. I got this trailer recently from auction and from day one brakes works not as it should be. I have done wheel bearings , new drums and shoes. No faults from ABS computer.
When I apply brake it start braking too hard then brake power is almost to zero then again it start braking hard. It tipping forth and back. really scares me when road is icy or in city traffic. It was few close calls almost rear ended because brakes power is unpredictable.
ABS unit is Meritor Wabco 2S/2M.
Is ABS air part is serviceable or need cleaning from time to time?
Any tps how to troubleshoot.