If it helps I have been through similar. I brought a drag specialties master cylinder which had dot4 only on it. But I had been usuing dot5 before the master cylinder swap. After a few questions back and forth, the drag specialties guy said yes there unit would be ok. Heavy duty seals.
I'd guess dot5 wouldn't be as harsh as other fluids. Mine hasn't been installed yet but I have had the ok to do so.
The hard part was getting the question across that I'm not mixing fluid, I'm starting from new.

 

I wouldn't think the factory who assembled the master cylinder would install the seals dry. Does the DS guy know if they used something to lube the seals and if so what was it?

Ken

 

dot 5 and dot 3 will not mix, like oil and vinegar. although best to thoroughly cleaning system with alcohol, not necessarily have to, all depends upon what is left over. the big issue with mixing is that any moisture will form in the inter-phase zone between the two and will become acidic and corrosion will set in.
can dot 5 be used in an ABS, sure if it is designed for it. the big issue here is viscosity. we know that ABS will cycle the brakes extremely rapidly and dot 5 flow rate is less than glycol based.
can you use the master to bleed, sure, been that way from conception. in all actuality the fluid does and will flow past the cup lip on retraction since there will be a diff'l pressure and the lip will relax, look at the piston assy and how the fluid returns to the master.
the norm is to lube the components with the fluid you are going to use.

 

One thing since we are seeing some bad info and misunderstood is on the older TCs. Least mine before the bimbo systems. Harley is one of the few who call out for a caliper piston grease for lubrication. It's on the rebuild kit from the mother ship or a PDF on line version is available if you look for it at harley.com associated with there parts. Using just breakfluid for a lubrication will get you sticking pistons in short order. See it all the time. Also pushing the piston back at a pad change before cleaning it will cause them to stick or later see uneven pad wear in about half the milage as the first set of pads.

 

well the market is full of greases for every purpose and every mfg has diff opinions. i think subaru mentions grease for the front but excludes the rear. for sure, the grease MUST be compatible.
so are we talking the piston or the seal??? there is a big diff! also, is this item going into storage?? another big diff.
one thing for sure, the seal must have a lube for assy as a dry approach can have issues. also of interest is whether the piston is plain steel, composite,aluminum or coated. one thing for sure is if it is lubed with what it is going to be using, the issues are reduced to nil.
most old books say only to lube the seal (not just mc) since the back side is exposed to fluid anyway and the front side if contaminates enter the boot will stick to the lube. chrysler went to a composite puck which turned out to have issues, my 1977 warlock truck was rife with them so i went to a steel puck and issue resolved. after the caliper works awhile, the lube gets pushed aside so greasing the daylights out of them is counter productive.
now i have read where the BOOT gets a special grease and its purpose is to maintain the elasticity since the boot acts like a bellows.

 

They didn't say how they are assembled. I did my old one with a mechanic friend, we used a little grease, rubber grease I'm sure it was.

 

I have a 1966 pan shovel trike that has a bad master cylinder, I ordered a new wagner masters cylinder from bikers choice it came in and say use dot 5 fluid only but I have read so much crap about dot 5 and would rather not have to build wheel cylinder again. Question is can dot 3 or 4 be used in a master cylinder marked dot 5 only? Whats different about it if you can't?

 

For starters, DOT5 is fine, I've been using the stuff since 1990 with no problems. It doesn't mix well with the other DOT fluids, however, which can cause serious problems. The reason it isn't used on current vehicles is that it tends to aerate when used in ABS systems, so most if not all manufacturers use DOT4. I have a 1990 bike, DOT5 from new, but currently use Brembo calipers off later bikes. I continue to use DOT5, having purged my Brembos. They were all showroom take-offs, so hadn't seen much use. What is different about DOT5? It is silicon-based, unlike all other common brake fluids and doesn't mix with them - in fact over time the mixture will react badly, turn to jelly and also corrode your master and brake cylinders.

It is quite likely the rubber used for the seals is designed to be compatible with DOT5. You could simply use DOT4 (DOT3 is a poor alternative), or get a set of new DOT4 seals to replace those in your new cylinder.

By the way, DOT5 doesn't absorb water, one reason it is preferred over the others. DOT3 absorbs a lot of water, DOT4 absorbs less but still too much, DOT5.1 (which is NOT silicon-based!) absorbs least.