P51D PLT HAS A RWY EXCURSION.

Narrative

ON SECOND MAINT FLT AFTER ENG REPAIR AND REPLACEMENT; ACFT'S RWY ALIGNMENT WAS ADVERSELY AFFECTED BY LIGHT GUSTING XWIND FROM L ON ROLLOUT AT APPROX 70 MPH WITH TAIL ON GND AND LNDG ASSURED. I TAPPED L-HAND BRAKE TO RECOVER DESIRABLE RWY ALIGNMENT. L-HAND BRAKE MOMENTARILY LOCKED UP WHICH SLID L-HAND MAIN TIRE AND MADE MORE CORRECTION TO L THAN INTENDED. I THEN TAPPED R-HAND BRAKE TO RECOVER DESIRABLE RWY ALIGNMENT WITH THE SAME RESULT. R-HAND BRAKE MOMENTARILY LOCKED UP AND ACFT VEERED BACK TO R-HAND MORE THAN DESIRED OR ANTICIPATED. AT THAT POINT I MADE A CONSCIOUS DECISION TO NOT USE BRAKES FURTHER AND TO LET ACFT VEER OFF RWY AT 20 DEG ANGLE TO RWY CTRLINE TO AVOID THE POSSIBILITY OF GND LOOPING OR NOSING OVER ON DROP. DURING THIS FINAL ROLLOUT PHASE; OFF BRAKES; AT APPROX 25 MPH BOTH R-HAND AND L-HAND FLAPS AND R-HAND LNDG GEAR SPADE DOOR HIT THE RWY MARKING THE SIGNS. MINOR DAMAGE WAS DONE TO BOTH RWY SIGNS; BOTH FLAPS; AND R-HAND SPADE DOOR. NO WING OR AIRFRAME OR PROP DAMAGE WAS INCURRED. MY SUBSEQUENT OBSERVATIONS AND CONCLUSIONS: 1) ACFT HAD BEEN FITTED WITH OVERSIZED P-63 BRAKES AND WHEELS. 2) NO COMPENSATION IN MASTER CYLINDER VOLUME HAD BEEN MADE TO MAINTAIN NORMAL P-51 BRAKING FEEL AND EFFECT. 3) BOTH MAIN LNDG GEAR PNEUMATIC CYLINDERS HAD BEEN FILLED AT TIME OF PREVIOUS ANNUAL TO MAX HT FOR STRUT OP. 4) I WAS UNFAMILIAR WITH BOTH THIS BRAKE COMBINATION AND ACFT ATTITUDE RESULTING FROM THIS STRUT SVCING. 5) I GOT BEHIND ACFT WITH REGARD TO ITS ROLLOUT CHARACTERISTIC IN XWIND DUE TO MY 1200 HR TIME IN TYPE AND FAVORABLE EXPERIENCE WITH OTHER ACFT OF THIS TYPE.