Any kind of silicone sealer that dries to a firm rubbery cosistency will do.
B&Q decorating stuff is ok if you have any around.
It's only to seal arond the outer body of the seal and hold it in place.
I used Loctite blue gasket silicone for cars.
If you mean the clip on the rod that goes through the brake pedal, no it's easy to get off/on.
Put a screwdriver through it and twist to open it up a little.. Now just slide it over and off the pin. Should be no need to get a new one.
An after thought to this is that it may not be neccessary to fully remove the seal.
Just pulling it back a few mm to get some sealer in the gap may be enough.
The metal housing that holds the seal will rip off some of the strips of rubber as you pull the seal out, so in effect leaving it in situ may be better.
The seal will have been moving in/out slightly as you have been pressing the brake but won't have been much.
Write out the method in short hand version and take a look at all the parts you can see to familiarise yourself with the job before you do it.
Will make it easier.
Just refer back to the full boring version if you get stuck.
Go for it. I've probably made it sound worse than it really is.