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In addition to Mike81K10's comment, if you replaced the spindles you also (at the very least) collapsed the calipers to R&I them. When you reinstalled the calipers and depressed the brakes, the caliper pistons had to move outward until the pads reseated against the rotors. If the M/C ran low on fluid while recharging the calipers, the fluid ports in the floor of the cylinder may have been exposed to draw in air. So, yes! The place to begin is by filling the M/C and bleeding the brakes. Be sure to inspect the lines, hoses, and connections closely for fluid leaks in the process. If the symptoms don't correct easily by bleeding, you may need to remove the M/C for bench bleeding followed by gravity bleeding the lines.