posted 12-13-2009 08:45 AM ET (US)
Brake fluid seems to attract water and rust, and it is quite common that the steel threads of the DICO actuator brake fluid reservoir will become corroded. The corrosion causes the plastic cap to become very tightly sealed to the steel threads. If the cap has not been removed in a long time, it can become very tightly fitted and difficult to remove.There is nothing particularly unusual about the cap and the threads. The treads are the normal right-hand threads, and the cap is loosened by turning it counter-clockwise.
To locate a source of replacement parts for a DICO Model 10 trailer surge brake actuator, I would inquire with retailers who sell that device. Most any trailer supply that carries the DICO brand will have a replacement cap.
Try using a strap wrench to loosen the cap. When you do finally remove the cap, you will probably find the steel threads are corroded. Be very careful when cleaning up the corrosion to not contaminate the brake fluid in the reservoir.