DOT 5

You can't mix the two. Even after flushing out DOT 3 you likely will not remove all of it and the DOT 5 mix will have negative effect on seals.. I believe modern systems that do use it have a specific type of seal composition as its a silicone-based fluid.

I looked into this recently and most will use DOT 3 however DOT 4 with a higher boiling point is fine. DOT 5 is fully synthetic and from what I read its best not to use for a car designed for DOT 3. Here is a decent writeup on brake fluid. Until just now I didnt know a 5.1 existed.

A. Why would you want to switch to DOT 5 brake fluid?
B. DOT 5.1 fluid, with it's higher boiling point compared to DOT 3 or 4 fluids, is compatible with DOT 3 or 4 systems and can be mixed with DOT 3 or 4 fluids.

I like DOT5 and have no issues in 25 years since I switched. For a car that sits in the garage all winter it is great since it can't absorb moisture and if you get a leak it doesn't attack your paint.
I made the switch when replacing both front calipers and master cylinder then flushed out the back.

It's true that glycol based brake fluids are hydroscopic but I've had various vehicles sit for years and never had an issue with water in the brake systems of any.

As for those fluids attacking paint, yeah, you have to wipe it off painted surfaces or else.

Neither has been enough of a motivating factor for me to swap out all the seals in a brake system to switch over but I can see where someone else might want to.

When I did my 4 wheel disc conversion I went back with DOT 5. Flushed the lines I think with alcohol. Lot of years and no problems. Never a seal leak, no moisture in the system. Brakes are good. Just my experience.

its kinda like putting regular PS fluid in stock systems...no no. ATF. The regular stuff I have seen will leak soon on steering box as its much thinner than ATF

A virgin brake system may be OK with DOT 5 if one feels the need if they get a multi piston large rotor/caliper brake system to autoX....otherwise it does not belong in these older DOT trucks, especially if currently running DOT3

There is a reason no vehicles come with DOT 5. Harley Davidson tried 5 for a couple-few years and discontinued it and uses DOT 4 now. FWIW dot 5 is NOT recommended in ABS systems, for reasons being DOT 5 aerates when applied very rapidly and you loose your brakes. Nothing to do with old classic cars maybe? This mentality is like saying the old single brake circuit design is fine. SMH.

We ran dot5 in the bush cars we built, and I have it in all of my old cars. Never have I had a brake failure from foaming or leaks. I guess I'm lucky.

I have it in my car. Everything was new or rebuilt so I did not have an issue with mixing the two. I have not had any issues but usually just take short trips around to car cruises etc. I put it in because I liked the idea of not damaging paint if I got some on the car. Probably not the best justification given that I have never ruined the paint on any of my daily drivers because I got some brake fluid on the paint....

What size are the bleeders' screws for the 4 piston calipers ?

Yes, you can't use DOT5 in ABS equipped vehicles. DOT5 is a Mil-Spec requirement for most military vehicles. We used it at my old job where we built military vehicles and had tons of gallon bottles.

what is the size of the brake bleeder screws for the original 4 piston calipers ? coupe