Hello Loudette
Zone 14 should have enough winter for your lilac to bloom. Since you
(and I) are in an area of no summer rainfall and a very hot summer,
lilacs do need watering. You didn't say how old the plant is, but 4
gallons is not much water for a 6 foot shrub, when the temperatures are
hitting 100. That is only 4 gallon milk jugs of water! Barely wets the
surface..........
Give your lilac deep watering until you can dig down at least 6 inches
and still have damp soil.
I am watering mine twice a week in this heat, and once a week when it
gets below 90. Are you watering far enough out from the trunk?
If you're using drip, it usually doesn't spread far enough into the
root zone.

Stop fertilizing it until it becomes a healthy plant. Fertilizer is
not Medicine for a sickly plant. ( You wouldn't feed a sick person a 12
course roast beef dinnner. :) )I never fertilize my lilac.

The brown spotting could be a leaf spot fungus
Everything seems to have it after the long, cool, rainy spring. The
heat is helping some.

Do you use a mulch around the roots? That can really help retain
moisture and keep the roots cool.

Are you pruning it ? If so, when? It should be pruned in early spring,
soon after blooming.
I would remove the lower sucker growth.. It's taking energy from the
upper branches.

Another thought: soil. Are you in the clay soil belt near Sacramento?
Lilacs like to be in a good drainage situation. The pH should be OK.
We are mostly neutral around here.

Good Luck with your sentimental plant!

Emilie
NorCal