What’s the drum - take II?

[ACE]

Over the past few years I have replaced clutch master cylinders on a 75, a 90 and several Alfettas. They have all proven difficult when removing the old master cylinder assembly from the housing on the firewall. It's not the bolts. While they can be hard to access, and may require a little persuasion to "crack" a frozen thread, they usually yield without the loss of too much skin ;-)

The problem is that where the back end of the Master Cylinder assembly passes through the 10 mm thick cast aluminium housing, the clearance space between the two is usually packed hard with a whitish caked substance. It often requires significant persuasion to free the assembly. But when it's out, it is easy to clean the housing as the stuff crumbles and can be scraped away, ready for the replacement to go back in. So this deposit is more of an inconvenience than a problem.

The mating surfaces of the steel assembly and the aluminium housing are basically flat with a small circular boss (~2 mm) which serves to positively locate the assembly in the housing. I have not noticed pitting or corrosion on either of these. But this may still be the site of a galvanic cell.

The major contact surface otherwise is the rubber boot (on the end of the assembly) where it sits inside approx 10mm of the housing. When new, the assembly will slip into the housing easily, it's definitely not a tight fit.

Anyway, the question(s) for the Brains Trust:
Given that the problem seems to be most prevalent where the rubber is in contact with the aluminium housing - what can be used to prevent the formation of this deposit next time around?

My approach is a nickel based anti seize on the bolts and the Fe-Al mating surface and to coat the rubber boot in rubber grease.

However, if it is a galvanic cell in the small space where iron, aluminium, rubber and air meet, then what is the treatment?

[bazzbazz]

You could try coating everything in the area with Copper Based grease.

[bonno]

Hi ACE
Find attached link to Penrite greases specification and application pdf document. Based on this information, the Marine grade Lithium based grease (p4 & p7) or Copper Eze (p4 & p10) would be suitable for the corrosion resistant application your after.
http://www.penriteoil.com.au/assets/pdf/tech/Nov2015/Greases.pdf