Bitburger

Is the mixing of brake fluid and hydraulic fluid in a convertible top pump system a bad thing ?

redoldsman

If you have any brake fluid in the system bleed it out. If you get a leak it will eat paint instantly. Use ATF only.

Tedd Thompson

X2, you can't mix those two liquids. I also use ATF fluid..... Tedd

Bitburger

My car is a 69. I thought I had read in a manual somewhere that pre 70 Cutlass models used brake fluid and 70 and later used ATF. What about straight hydraulic fluid ?

redoldsman

I have never used straight hydraulic fluid and know nothing about it.

don71

Most everyone uses ATF...it is a type of hydraulic fluid. Many use it in the power steering on these older cars too. It works great and makes it convenient

I wouldn't mix them either.

BangScreech4-4-2

Does anybody know definitively the year they switched from brake fluid to ATF? I thought the use of brake fluid dated back to the late '50s or early '60s.

don71

I don't know the time line exactly. Trans fluid used to be a light mineral oil around 10w. and even motor oil was used too. I remember seeing cans of "type A" way back when.

Dexron dates back to at least 1967 for GM in transmissions.according to this article. https://www.hemmings.com/blog/2015/0...manufacturers/

Gm started using it at least by 2001 with TSB's retro active for use back to at least 1993 models with convertable tops.

Bitburger

The 69 Fisher Body Service Manual says Hydraulic fluid or Type "A" or Dexron transmission fluid.

I have disassembled the entire system and will flush the lines and the pump. Both lift cylinders are new. One was installed and the system tested and failed so it will contain a small residual amount of hydraulic fluid and brake fluid mix. The second new lift cylinder was bought today.

Thanks to all who responded. I plan to reload the system with Dexron.

Tedd Thompson

The main issue with brake fluid is that it draws moisture and will corrode cylinders and shafts and rune seals. My car originally had brake fluid and my cylinders were damaged beyond repair after it sat a few years. If you have any questions about your system you can give Hydro- E- Electric a call, good folk and very knowledgeable.... Tedd

Vintage Chief

As Tedd suggested brake fluid draws moisture. The primary reason brake fluid draws moisture is because brake fluid more closely resembles radiator coolant. The primary constituent of brake fluid is glycol - which is a derivative of ETOH (ethanol) i.e. polyethylene glycol, diethylene glycol, triethylene glycol, etc. Many variants of glycol comprise the various DOT formulas of brake fluid. Since they are derivatives of ETOH, they will readily absorb moisture. Brake fluids contain no petroleum. Transmission fluid on the other hand is petroleum based.

BangScreech4-4-2

I know, brake fluid is hygroscopic and can be quite destructive over the long run. I'm just trying to learn what year GM figured this out and stopped using it as factory fill in convertible top cylinders.

akspeedy

Ive always heard and used transmission fluid, I bought new pump and cylinders and they recommend brake fluid and give an extended warranty using brake fluid, after reading and finding the information that brake fluid draws in moisture I am going to opt to stick with the transmission fluid it has always worked.

BangScreech4-4-2

Here's another silly question: if brake fluid is called for, can you use DOT 5 silicone fluid?

troyd

You could probably use a number of things.....but why?

ATF is what pretty much everyone uses without issue....and it’s cheap and plentiful, so why not?

Cheers,

Troy

BangScreech4-4-2

I give up.

TripDeuces

Atf