You don't. They are not difficult to replace.
Don't know that a leaking primer bulb can be repaired. Most likely, you have to replace it. You don't want to forget the routing of the fuel lines!! You might want to make a drawing of the routing before you remove the primer bulb.
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Primers are not waterproof. If you have a windowsill area that is leaking, you'll have to determine the path the water is coming from, repair the damage that is causing the leak then prime and paint.
get a coupling and the fitting that needs replaced. cut out the pipe that is leaking and reeplace it. use plenty of glue and primer. hold the fittings together for at least 1 minute after gluing.
First you have to identify the fluid that is leaking then locate the source of the leak before we can tell you how to repair it.
Some typical radiator repair jobs are leaking hoses, leaking radiators or loose connections.
Go to youtube, search for Davidsfarm, and he shows how to fix a valve.
Primer application is a part of the paint preparation.
Replace the leaking part
You will be better off replacing it that trying to repair it.
stuff it with paper
Replace the part it is leaking from.