B737-800 Captain reported loss of A hydraulic system fluid; as well as the loss of one of the two B system pumps. The flight crew requested assistance from ATC and a normal landing ensued.

Narrative

The flight was an originator; and all systems were normal. We started the day with an uneventful taxi out; take off; and cruise. Prior to the start of the flight both hydraulic systems showed somewhere between 95-100% volume. Shortly before the Top of Descent the 'A' system engine driven hydraulic pump illuminated the 'LOW PSI' light. Looking at the lower we noted that there was no abnormal hydraulic pressure; but a slightly lower volume of fluid than in the 'B' system. We ran the QRH Checklist for the LOW PSI light which directed us to turn off the engine driven hydraulic pump and continued our descent. Shortly thereafter the 'A' hydraulic fluid volume began to decrease but seemed to steady out in the 26-28% range while showing pressure in the normal range. Further along in the descent; the 'A' volume dropped to 8% then 0% indicated. There was no increased demand on the hydraulic system that may have caused the fluid level to display a lower than actual volume.At this point we elected to treat this as an 'A' system failure and ran the associated checklist. While doing that; the 'B' hydraulic engine driven pump LOW PSI light illuminated while indicating normal volume and pressure. We ran the QRH for that system as well. After completing both checklists down to the deferred items; we had only one of the main four hydraulic pumps still selected on and the 'A' system standby pump on. For these reasons we [requested assistance] with ZZZ Approach and request the longer runway for landing. The Flight Attendants were briefed on the situation; then the passengers were also briefed via PA. Dispatch was kept in the loop via ACARS. Following the Deferred Items Checklist; we landed normally and under maximum landing weight (109.4). The brake cooling ACARS report said we were clear to taxi to the gate with no cooling delays necessary. The taxi in was uneventful. After shutdown there was fresh hydraulic fluid in the #1 main landing gear bay; and a fresh puddle of fluid under the #1 engine.The checklist was a bit ambiguous in the decision making process of is it/isn't it a system 'A' failure when one hydraulic pump light is on; but the other wasn't - even though volume dropped to 0.