I just posted this to the B7800 with the same type of problem, i don't know if it will work on the B7510 but looking at the pictures you have they look very similar. Here is what i posted, hope it helps.
I recently had the same issue with my B7800. Look under the hydro peddle and you will see a rod that goes to a rubber boot. When you press on the peddle you should see a small steel cylinder that moves in and out of the rubber boot, that was where my problem was. Now the time consuming expensive part. You will need a can of WD-40 or an equivelant. Spray the steel cylinder with the WD-40 and press the peddle, do this a couple of time and your issue will probably be resolved, atleast it worked for me. I think the cylinder binds up on the ruber boot causing the safety switch not to engage.
I hope this helps.
Good Luck,
Piper