Do I need to fully drain lead acid battery before charging them?
No, you should NOT fully discharge a Lead-Acid battery.

The normal reason for wanting to fully discharge a battery is because some batteries have a so-called "memory effect" - old NiCd cells are notorious for this.

But Lead-Acid does NOT suffer from this effect.

In addition, you can cause permanent damage to some of the individual cells within the battery if the battery is discharged too deeply - the polarity of the weaker cells can actually reverse polarity.  This causes permanent damage to those cells.

Depending on which exact Lead-Acid battery you have, end of life discharge voltage for a nominal 12 Vdc battery (6 cells) ranges from 10.5 Vdc to 11.5 Vdc.  The manufacturer of your particular battery will specify what the minimum allowable voltage is.