hi, my rear disc break has all of a sudden become very slow to react and "soft". could it be because some oil got onto the disc and has accumulated onto the pad, rendering it fairly useless? if this is the problem is it just neccessary to change the pad or can it be cleaned or tightened? i noticed it as i cycled through some tall wet grass, would that have possibly caused it? and if i do change the pad, should i clean the disc to ensure no oil residue is left??