AN MD90 FLC TAXIES THEIR ACFT TO THE GAGE WITHOUT GEAR PINS INSTALLED AFTER HAVING TO RETURN LAND BECAUSE OF A FAILING R HYD SYS AT ORD; IL.

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IN CLB; I NOTICED THE R HYD LOW QUANTITY INDICATOR LIGHT ON AND INDICATING A STEADY 3 QUARTS. THE R HYD SYS PRESSURE WAS NORMAL. WE RAN THE POM PROC; 'HYD QUANTITY LOW OR DECREASING.' THE POM DIRECTED US TO TURN OFF THE ENG HYD PUMP AND THE AUX AND XFER HYD PUMPS. WE THEN GOT THE EXPECTED; R HYD SYS PRESSURE LOW MESSAGE ON THE OAP. WE INFORMED THEM THAT A RETURN TO ORD WAS OUR PLAN BECAUSE OF FAMILIARITY AND AVAILABILITY OF MD90 MAINT AS WELL AS PAX HANDLING. WE TALKED OVER OUR PLAN TO RESTORE THE R SYS PRESSURE WHEN WE CONFIGURED FOR LNDG; AS PER THE POM. WE REVIEWED AND WERE PREPARED TO RUN THE R SYS LOSS PROC IF NECESSARY AND TO STOP STRAIGHT AHEAD. WE DECLARED AN EMER; REQUESTED A 15 MI FINAL FOR ORD RWY 4R FOR ITS LENGTH AND FAVORABLE WIND CONDITION. WE TOLD THE FLT ATTENDANT CREW AND PAX THAT WE EXPECTED TO BE ABLE TO GET THE GEAR DOWN NORMALLY; TURN OFF THE RWY AND MAYBE TAXI TO THE GATE. WE CONFIGURED 25 MI OUT WITH NO PROBS. AFTER LNDG; WE ROLLED OUT TO THE END OF RWY 4R AND TURNED OFF INTO THE PAD. MAINT TOOK A LOOK AND FOUND HYD FLUID STILL LEAKING FROM A NUT THAT SECURED ONE OF THE HYD LINES. THIS NUT EVIDENTLY WAS VERY LOOSE AND ALLOWED THE LEAK. I PLANNED TO HAVE THEM TOW US BACK TO THE GATE WHEN MAINT INFORMED US THAT WE COULD STILL TAXI IN WITH L SYS BECAUSE WE STILL HAD STEERING AND BRAKES. WE CONTINUED TO THE GATE WITH THE R SYS DEPRESSURIZED. I NOTICED THAT IT WAS MUCH MORE DIFFICULT TO TURN THE ACFT TO THE L AND THAT SLOWER THAN NORMAL TAXI WAS NECESSARY FOR L-HAND TURNS. IT THEN HIT ME THAT I HADN'T REQUESTED OR CONFIRMED GEAR PINS INSTALLED. WE MAY NOT HAVE HAD DOWN PRESSURE ON THE GEAR DURING TAXI IN. NEXT TIME I DON'T LET MYSELF GET TALKED INTO TAXIING WITH A HYD SYS NOT PRESSURIZED UNLESS THE GEAR PINS (R SYS OUT) ARE IN AND ONLY 1 DIRECTION FO TURNS IS REQUIRED. R TURNS FOR THE R SYS OUT AND L TURNS IF THE L SYS IS OUT. THANKFULLY; WE MADE IT TO THE GATE WITHOUT INCIDENT AND WERE ABLE TO GET THE PAX ON THEIR WAY SOONER BECAUSE WE DIDN'T GET TOWED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT WASN'T CERTAIN AS TO WHY THE ACFT ACTED THE WAY IT DID. INVESTIGATION REVEALS THAT THE PRIORITY VALVE WITHIN THE R HYD SYS DROPS THE NOSE STEERING OUT OF THE 'LOOP' WHENEVER THE PRESSURE FALLS BELOW 2000 FT PSI. THIS LEFT ONLY 1 VALVE OUT OF 2 TO HANDLE THE NOSEWHEEL STEERING; PWRED FROM THE R SYS. CREW DID NOT HAVE OR DID NOT NOTE THE REMARKS WITHIN THE CHKLIST FOR A SINGLE SYS FAILURE TO WHIT: 'NOSEWHEEL STEERING TO L IS SLIGHTLY REDUCED.' (THIS IS BECAUSE THE ANGLE OF TURN IS REDUCED.) THE GEAR PINS WERE NOT; IN THIS CASE; OVERLOOKED BY THE GND CREW AS THEY DID PLACE THE PINS IN PLACE PRIOR TO TAXI OPS. THE FLC DID NOT; HOWEVER; COORD THIS ACTION WITH THE GND CREW. THE CAUSE OF THIS WAS THE FACT THAT THE GEAR PIN INSERTION REQUIREMENT WAS 'BURIED' WITHIN THE AMPLIFICATION OF THE MANUAL GEAR EXTENSION CHKLIST; WHILE CHKING GEAR DOORS CLOSED; BUT NOT ON THE CHKLIST ITSELF. A PUB DEFICIENCY.