Refurbishing Steel Master Cylinder Reservoir Caps

I've had success refurbishing rusted English Ford steel reservoir caps with solder.

Refurbished Reservoir Cap

These should be sand blasted first to get rid of the rust and to observe the condition is void of any major dents and holes. It also preps the cap for solder.

Use a propane torch and 50/50 solder ( lead/tin ) used for repairing copper radiators.

Clean the steel cap with acid flux. Heat the cap hot enough so solder will flow once it's touched to the cap. Careful not to over heat the cap. Once the cap is covered completely with a thin coat of solder including the threads but not the inside, re-heat the cap and wipe with a cotton cloth to smooth or a small tooth brush size wire brush. Then use a bit of metal polish. It may take a bit of practice but the outcome is worthwhile.  This will not prevent the cap from rusting so it would help to wipe it with WD-40 once a month or sooner.  Make sure the vent hole is still open.

Aluminum caps that incorporate an inner steel insert can be cleaned with a small metal brush to remove the rust and wiped clean with a cotton cloth and a bit of WD-40.