WITH R HYD SYS FAILURE; FLC PREPARED FOR EMER LNDG AND FOLLOWED ALL ASSOCIATED CHKLISTS. ACFT LANDED UNEVENTFULLY; BUT WHILE INSTALLING GEAR PINS THE CABIN BEGAN FILLING WITH SMOKE; APPARENTLY FROM THE LEAKING HYD FLUID IN THE ENG. PAX WERE EVACED AND SOME COMPLAINED OF EYE AND THROAT IRRITATION.

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ENRTE FROM HOUSTON TO LAS VEGAS AT FL280; THE R HYD PRESSURE LOW LIGHT CAME ON AND THE R HYD QUANTITY AND PRESSURE WERE NEAR ZERO. APPROX 1.5 QUARTS ON THE QUANTITY AND 200 PSI ON THE PRESSURE GAUGE. THE R ENG HYD PUMP WAS SECURED (TURNED FROM LOW TO OFF) AND THE FOLLOWING CHKLISTS WERE ACCOMPLISHED: 1) HYD PRESSURE LOW LIGHT ON. 2) HYD QUANTITY LOSS. 3) ALTERNATE GEAR EXTENSION WITH R HYD FAILURE. ZLA WAS ADVISED OF OUR PROB AND ASKED TO PASS OUR SIT ON TO LAS APCH AND LAS TWR. AFTER BEING HANDED OFF TO APCH WE WERE ASSIGNED A NW HDG AND APCH IMPLIED THAT WE WERE BEING VECTORED FOR THE N-S RWY AT LAS. AT THIS TIME WE REQUESTED RWY 25R AS IT IS THE LONGEST RWY. ZLA WAS ADVISED THAT WE WOULD BE STOPPING ON TO RWY AND THAT OUR GEAR PINS WOULD NEED TO BE INSTALLED AND THE GEAR DOORS WOULD NEED TO BE RAISED BEFORE WE COULD TAXI OFF THE RWY. WHILE IN THE DSCNT WE REVIEWED THE (UNSAFE GEAR; GEAR HANDLE DOWN) CHKLIST AND REVIEWED THE PROC FOR USING THE VIEWING PORTS FOR THE MAIN GEAR TO VERIFY DOWN AND LOCKED. WE STARTED EXTENDING THE SLATS AND FLAPS ABOUT 35 MI AND LOWERED THE GEAR AT ABOUT 20 MI TO GIVE US TIME TO TROUBLESHOOT ANY GEAR PROBS WE MIGHT ENCOUNTER. IN ADDITION WE NOTED THE EMER VEHICLES LINING RWY 25R AS WE WERE IN VISUAL CONDITION AND COULD SEE THE RWY FROM ABOUT 50 MI. THE FO FLEW THE APCH WHICH WAS ON GS AND SPD FROM ABOUT 20 MI ON FINAL. TOUCHDOWN AND ROLLOUT WERE NORMAL. THE LNDG WAS AT XA04Z AND THE FIRST DOORS FOR EVAC WERE OPENED AT XA05Z. DURING THAT MIN AFTER LNDG; INITIALLY ALL INDICATIONS IN THE CABIN WERE NORMAL; THE AFTER LNDG CHKLIST WAS INITIATED AND THE GEAR PINS WERE REMOVED FROM THEIR STORAGE COMPARTMENT AND TOSSED OUT THE COCKPIT WINDOW SO THAT THE GEAR COULD BE PINNED. THE COCKPIT DOOR WAS OPENED AND AN ANNOUNCEMENT WAS MADE TO REASSURE THE PAX THAT EVERYTHING WAS PROCEEDING AS NORMAL AND WE WOULD SOON BE ABLE TO TAXI TO THE GATE. OUR FIRST FLT ATTENDANT INFORMED ME THAT THERE WAS VAPOR OR SMOKE COMING FROM THE OVERHEAD VENTS; AND LOOKING BACK DOWN THE AISLE FROM THE COCKPIT; I CONFIRMED THAT INDEED THERE WAS SMOKE OR MIST IN THE CABIN. THE FO AND I HAD OUR COCKPIT WINDOWS OPEN AND WE CALLED DOWN TO GND PERSONNEL TO SEE IF THERE WAS ANY SMOKE OR FIRE ON THE OUTSIDE OF THE ACFT. WE WERE TOLD THAT THERE WAS NO FIRE. THE SMOKE INSIDE THE CABIN HAD GOTTEN WORSE IN THIS SHORT TIME SPAN AND THE FIRST FLT ATTENDANT SAID THAT THE PAX WERE STARTING TO EXPERIENCE EYE IRRITATION AND BREATHING DIFFICULTY. AT THAT TIME I ORDERED THE EVAC OF THE ACFT AND THE FO READ AND ACCOMPLISHED THE EVAC CHKLIST WHICH HE HAD BEEN HOLDING IN ANTICIPATION OF THE NECESSITY TO EVAC THE ACFT. EVAC OF THE ACFT WAS ACCOMPLISHED IN AN ORDERLY FASHION FROM THE 2 FORWARD DOORS (GALLEY AND PAX BOARDING); THE AFT GALLEY DOOR; AND THE REAR AIRSTAIRS IN LESS THAN 2 MINS WITH NO INJURIES. THE OVERWING EXITS WERE OPENED BUT FORTUNATELY WERE NOT USED. NOT HAVING PREPARED FOR AN EVAC WE HAD NO ABLE BODY ASSISTANTS STATIONED AT THESE EXITS AND GETTING OFF THE WING COULD HAVE CAUSED INJURIES. THE ONLY MINOR PROB ENCOUNTERED DURING THIS EVAC WAS THE RELUCTANCE OF THE PAX TO LEAVE THEIR BELONGINGS ON THE ACFT AND CARRY THESE ITEMS WITH THEM DURING THE EVAC. THE PAX WERE SUBSEQUENTLY TRANSPORTED TO THE TERMINAL BY GND TRANSPORTATION.