Which brake fluid, Master cylinder

[Steelersman]

The brake pedal on my '85 seems to travel excessively before engaging, then all is good (normal braking...no sudden stops).  I have to mention that my daily-driver is a BMW Coupe and maybe I'm too used to its brakes immediate response).  Also, in a test I noticed the emergency brake of the Avanti does not hold too good when I have my car on my slanted driveway.  I bought the car about 2 months ago and have changed all fluids and taken care of general maintenance, and want to get the brakes up to speed, if there's a problem.  Brakes have been bled.  There's plenty of pad, front and rear and rotors/drums are fine; the adjusters on the rear wheels have checked out okay.  Now for the confusing part:  There is a black & white printed label on the brake booster that says to use "DOT-5" fluid...HOWEVER, on top of the master cylinder lid (where the bale holds it down) is an imprint that says "Use DOT-3.".   Hmmmm.  My mechanic says that "3" is in there now (showed me the color comparison) , and before using "5" he'd have to completely remove all brake components (lines, calipers, etc.) for a thorough flush/cleaning/drying because these 2 fluids are not to be mixed.  He also advised me that after his research,  he thinks that the higher compression of the "5" will make my brakes spongy. 

So: Which grade of fluid should I use?   If "5" is recommended, can I reasonably expect for the brake pedal to travel less?  Is all the dismantling/cleaning necessary?  Is the brake pedal travel I have considered to be  "normal"?

Thanks in advance.

[Avanti83]

I wouldn't use color as an indicator of fluid type. Take a small bottle of Dot 3 fluid and pour some into a glass. Add the fluid from the master cylinder to it. If it mixes, it's Dot 3 if you see a separate layer it's Dot 5. I know Dot 5 is purple but after some time it will lose the color in the master cylinder.

If it's 3 keep using 3, if it's five keep using 5. He's correct that it's extremely important to remove any trace of the other fluid if you convert. I run Dot 5 in all my classic cars but that's because I totally rebuild them and Dot 5 is less susceptible to moisture and corrosion than Dot 3.

The travel could be a poorly adjusted push rod to the M/C but more likely do to some leakage in the system possibly the M/c

As far as the emergency, that's an adjustment under the car to tighten it but it won't be like the BMW but it should slow the car and hold it on hills. 

[Gunslinger]

DOT 3 and DOT 5 are not compatible.  If they get mixed you'll experience a spongy pedal and potentially brake failure.  if you think the two may have been mixed you will need to flush the entire system before refilling with either fluid.  After so many years it wouldn't be surprising if the fluid has been inadvertently mixed to some degree.  Difficulty in bleeding air is common with a mix or even DOT 5 itself.

In 1970 Avanti Motors began using silicone fluid...later called DOT 5...as factory fill.  I don't know if they changed back at some point or continued its use.  That's probably why your sticker says one thing where the master cylinder cap says something different.  

I would suggest another complete flush of the system and replacing the rubber brake hoses if you don't know whether their ever been replaced.  Then you can use whichever type fluid you prefer.  

Brakes are simply too important to not keep in top shape.  

[silverstude]

 

I have to agree that the DOT5 silicone fluid ( No matter how careful you are)  will have microscopic bubbles suspended in solution and they will cannot and will not be bled out, causing less than a solid pedal.  The better side of the argument in using this stuff is  the long life of the components.  

One other damning problem with the stuff is, never get it on an unpainted surface or it will get absorbed and migrate through the strata  causing a parade of horrors with your paint.