Brake Fuild

So my 55 has front disks and factory rear drums brakes and I found a leaking rear wheel cylinder and need to replace it so heres my question: I am not sure what # brake fuild to use as the car had this setup on it when I bought it.

Nothing special required most just use Dot 3. I believe 3 mixes with 4 ok .I have heard of silicone fluid Dot 5 being used but not common and purple. On my cars I use 3 and change it out every 6-8 years. If you are worried about flush system and start over with your choice..

How long have you owned the car? Have you had other brake issues since purchase? Since you aren't sure of the fluid and do not really know for sure what the prior owner did, or when he did it, I would suggest a complete brake assessment which should include examination of the leaking wheel cylinder as to why is it leaking? Is the fluid dark? if so it need flushing anyway. Is the wheel cylinder 'rusty or pitted' inside where the piston/seals run? If it is, then probably the fluid was NOT silicone (DOT 5), or the brake build wasn't done properly before. Check both rear wheel cylinders... you are likely to find the same issues with each. Bleed all wheel cylinders (and calipers on the front). What does the fluid look like... very dark fluid generally implies rust/water in the fluid and your cylinders will be rusty. What does the fluid in the MC look like? If it was my car, I think I'd take the opportunity to *do it right* and *do it once*... ie. rebuild the MC, and calipers and WC's.. (the rebuild could be as little as replacing the rubber parts, OR it might involve more). Flush the lines then run 90% for better alcohol thru thru (blowing thru the open lines with air to dry before reconnecting to the newly rebuilt cylinders/calipers.

I've totally rebuilt a half dozen brake systems since 1988, and now I do all my old cars with Dot 5 (silicone) and have NO ISSUES with any of those since rebuild even though most of these cars seldom get driven.. WhenI Do drive them the brakes work and I have no rust issues due to the fact that silicone fluid does not absorb water. You should still bleed the WC's and calipers every year or two (although I generally don't because I have so many different cars)...

PS. Silicone fluid might be purple in color, or not.. I've purchased some the last few years which was not purple (I like the purple myself because the color does tell you what it is).

What BamaNomad and markm said.

DOT 5 is usually either purple or clear. With age it does not change appearance.

DOT 3 and DOT 4 are indeed compatible. They are clear or tan when new. They both are hygroscopic (attract and combine with water from the atmosphere). The water turns the fluid black. Few use DOT 4.

DOT 5 is not compatible with DOT 3 or 4. If you insist on changing over an existing system, you need to flush thoroughly with isopropyl alcohol. Better yet, replace all components that have rubber seals beforehand.

If you have a leaky wheel cylinder, you almost certainly have DOT 3 or 4. What I would suggest would be to replace the wheel cylinder on the other side too. If any doubt on condition, replace the master cylinder and calipers too. Flush the system with isopropyl alcohol before refilling. DOT 3 fluid is fine but you must drain and replace every few years, this is more important if the car sits a lot. On a driver the heat from using the brakes puts off the inevitable a bit longer.

You might also want to ad the DOT 5.1 brake fluid. This is compatible with the old DOT 3 & 4.
The 5.1 has a higher boiling point and a lower viscosity.

I do like to use the silicone, just bleeding the brakes take a bit more work and the pedal feel is a bit different.

Thanks for all the info, I was able to get back ahold of the last owner and he used DOT 4 so that's what I am going to use.