I am rebuilding all my brake components on my 1971 Shovel Head. If the system is completely dry can you switch from Dot 3 to Dot 5 brake fluid?

 

Why 5 it seems Motorcycle industry has been shying away for 5 ? From what I have been told yes if system is dry as you say.
My 2017 has DOT 4 in it so did the 2016.

 

When you say dry, ..completely disassembled, cleaned and seals changed in the calipers and master cylinder?

Ken

 

I’d make sure all the components are compatible with DOT 5.
Interesting from Wikipedia:

DOT 3, like DOT 4 and DOT 5.1, is a polyethylene glycol-based fluid (contrasted with DOT 5, which is silicone-based). Fluids such as DOT 3 are hygroscopic and will absorb water from the atmosphere. This degrades the fluid's performance, and if allowed to accumulate over a period of time, can drastically reduce its boiling point. In a passenger car this is not much of an issue[citation needed], but can be of serious concern in police vehicles, racecars or motorcycles[citation needed].

DOT3 has been all but replaced with the superior DOT4 as there is little cost difference between the two.

Boiling points

Minimal boiling points for these specifications are as follows (wet boiling point defined as 3.7% water by volume):

Boiling point ranges [2]
Dry boiling point Wet boiling point
DOT 3 205 °C (401 °F) 140 °C (284 °F)
DOT 4 230 °C (446 °F) 155 °C (311 °F)
DOT 5 260 °C (500 °F) 180 °C (356 °F)
DOT 5.1 260 °C (500 °F) 180 °C (356 °F)
DOT 2 Brake fluids DOT 4

 

The reason they went to 3 or 4 was ABS system where not designed for 5. So Harley did it across all models. On a motorcycle, nothing beats DOT 5 . Does not absorb moisture or hurt paint. Still need to change it ever few years by vacuum pump.. Never pump master.

 

Because DOT 5 cant be used with ABS and manufacturers buy in bulk.

 

Dot 5 because it's silicone base, it doesn't absorb water and is not harmful to paint or anything that I know of. My 89 and 2001 use Dot 5. I know it can't be used with ABS, I think it might have something to do with the rubber components (not sure).

 

Why can't you pump the master cylinder to bleed the brakes? I have a spongy front brake on my 2001 RK and Have bleed it both ways, (hand vacuum pump and pumping the master cylinder), with no improvement. Have to pump it twice to have a good front brake.

 

I've owned a new shovel which used DOT3, although I swapped to DOT4 and a 1990 from new with DOT5. I happily used DOT4 or DOT5 in that bike, out of ignorance (and DOT5 was difficult to find here in the UK in early days) and eventually learned the folly of doing so. I've used all the usual ways of bleeding brakes over 40+ years and had no difficulties, except the usual trapped air problems.

BT, I would only swap to DOT5 if you have stripped and cleaned your entire system, preferably using new rubber components throughout. I've done that with DOT4 calipers. An alternative is to use DOT4, which my other two bikes use. Despite the comments made about DOT5, the recommendation is the same as for DOT4 vehicles, to change every 2-3 years.

 

IYou can pump it. Obviously, you have air in the system or the seals are worn out and it has already gone past normal movement.

Why I said do not pump the master was the normal refill with new fluid and to get moisture out. This is not so mandatory on DOT5 but the new ABS system MUST BE CHANGED OUT EVER TWO YEARS..at least Harley's system.

When all is OK when braking, the master only moves so far and leaves a wear and crud build up at the end of the normal movement. If you break the bleed on the caliper and push the master thru that wear and crud edge, there is a good chance, it will screw up the lip seal of the master cylinder.
Also to add further confusion now, keep in mind the caliper seal is a square or sometimes quad seal. It will withstand pressure either way. The master is a lip seal. Pressure one way towards the caliper. The other way is very easy to leak by. So when you pull a vacuum on the caliper to pull fluid out, go slow and only about 5 lbs of vacuum. Pulling a whole lot will pull air laden with moisture into master past that seal.