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Changing to Silicone Brake Fluid

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BobS76

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I am in the process of rebuilding the brake system on my Midget. I have not seen the need yet for R&R on the master cylinder and prefer to let sleeping dogs lie. I am replacing the rear cylinders and rebuilding the front calipers. Along with this effort, I will be switching from DOT3 to DOT5 silicone brake fluid. Yes, I know that there are pro and con opinions on this, but I ran my MGB for 20 years and 300,000 miles using silicone and still swear by it today.

Here is the question. Is there a preferred method for the change over? In the past, I simply drained the DOT3 and replaced it with DOT5. This obviously left some DOT3 residue. I think that I have heard someone suggest that after draing the DOT3, the system should be flushed with methenol. This would disolve out the the old DOT3 and any water. Blowing air through the system (using a pressure bleeder, brake pipes disconnected from the cyclinders) would cause any remaining traces of methenol to evaporate, leaving a clean system for the DOT5.

Does this make sense? Has anyone had any experiance with this? Is there a better way to do this? Or should I just do it the way I did it in the past - drain one and the add the other?
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I have not seen the need yet for R&R on the master cylinder [/QUOTE]
Put DOT5 in it & you will be shortly!
 
If there's even a chance that DOT5 might reach rubber in the MC's that's been tainted by DOT3, it'll eat it completely up...just my thoughts.
 
Last time I switched to DOT5 I changed all the rubber bit except the calipers. No problems. All the other bits were changed as a matter of rebuild.
 
On the cars I've switched to DOT-5 I've replaced all the rubber anyway as part of the brake system restoration. I've heard of people who have NOT done this, but as Tony suggests, it doesn't seem like a risk worth taking.

Even if you do replace all the rubber, you're left with old metal lines, unions, etc. When I've converted my LBCs, I've flushed the system with isopropyl alcohol and then left LOW pressure air flow through the pipes overnight before closing the system and re-filling it. That has worked well for me.

If your car has a hydraulic brake light switch, anticipate that it will fail after your change to DOT-5. Expect this even if the switch is new and has never seen any brake fluid. There appears to be some material compatibility issue that occurs more often than not when DOT-5 is used. The easiest fix is to mount a pedal actuated mechanical brake light switch and wire it in parallel to the hydraulic switch.
 
Thanks, all.

The only reason that I have decided not to do the master cylinder this time - it looks like it will be no easier than the removal / replace of the manifolds that I just went through. D__n!!

Your comments about the alchohol matches with what I had heard and makes good sense. Same with the switch.

I will see what happens.
Bob
 
I am a firm believer in DOT-5 also. But I always use it in an entirely new system including pipes, hoses, and rebuilt or new cylinders.
And even the special hydraulic brake light switches that are "supposed to be" compatible with silicone only last a couple months. So do plan on a pedal operated brake light switch.
A through flush with alcohol will clean out the old DOT 3.
I can not comment on the old master with new silicone.
But for me, 12 years and no problems makes me a firm believer.
And it doesn't eat paint!
 
I am currently dealing with this. My master was eaten up by the dot 3 and is being rebuilt. DOT 5 is the only way to go. I disconnected everything and flushed with DOT 5. Don't forget to rebuild or replace the slave.
 
Interesting topic, as I knew I had to fix a disk caliper on my 68-70 vw due to it not working, figured it was a seized piston due to sitting doing nothing for about 4 yrs.
Got a second hand one that worked. (Didnt bother checking original!)
Went to bleed it - no go!
Checked and put some fluid thru other working side, lots of like black dust in the fluid!
Mumbled things under breath!
Worked back to other side,found flex tube to caliper is blocked!, and just after auto shops closed for the day /bcforum/images/%%GRAEMLIN_URL%%/frown.gif
Collapsed flex?, or a prob due to non compatible fluid?
Reason I ask is because flex pipes are probably original vw and present (Put in about 5 yrs ago) fluid may be silicone, nothing else seems to be failing tho!
 
I'll be using DOT5 in my '63 Midget when its finished...but there's nothing left from the old brakes except the long hard line & that's easy to clean out....
 
I switched my Midget over to DOT 5, and have had zero problems despite having DOT 3 in it previously. That said, DOT 5 and DOT 3 DON'T MIX, so try to get as much DOT 3 out. I switched over when I replaced the master cylinder and brake lines. Do be aware that the front calipers on a Midget will trap lots of brake fluid in them; they won't just flush out through the bleeders. I blew out the pistons with my air hose (be careful if you try it, start with LOW pressure and have something to cushion the piston before it pops out), cleaned out the old gunk, and lubed with DOT 5 before reassembling. Ditto for the rears. No trouble at all with the old seals.
 
New front calipers, all new hoses, new MC, new wheel cylinders in rear - only thing being reused is hard line.
 
Come to think of it, I changed over the clutch system too, and didn't change a thing on that, just took apart the master and slave, and cleaned out all the old stuff. That was two years ago, and still no trouble.
 
So there is Dot3, Dot4, Super Dot4, Dot5, Dot5.1 as far as I can tell, so which is best to use on old systems?, have opted for a Penrite 525 Super Dot4 that is backwards compatible with Dot3,it states it is a non petroleum synthetic
 
zblu said:
So there is Dot3, Dot4, Super Dot4, Dot5, Dot5.1 as far as I can tell, so which is best to use on old systems?
Click to expand...

Yes. :crazyeyes:

As you may have figured out, there are all sorts of advantages and disadvantages, and lots of opinions.

DOT 3 and 4 (and "super" DOT 4) have a distinct disadvantage in vehicles that don't get used often, in that they attract moisture from the air. In my Midget, there's an air bleed hole in the master cylinder cover, so it's always exposed to air. When I first got my car after sitting unused for 18 years, the fluid was very brown and sludgy, and the rear brake cylinders were frozen with rust. The steel brake lines were rusty too.

Some brands of DOT 4 claim to be better at not absorbing moisture, such as Castrol LMA and Valvoline Synpower. I do not know if that's true of all DOT 4. The key advantage to DOT 4 over DOT 3 is higher boiling point. Both DOT 3 and DOT 4 will dissolve water in them.

DOT 5 is does not mix with DOT 3, DOT 4, or DOT 5.1, but has the advantage of not absorbing moisture at all. Some will say that because of this, if you get liquid water in the hydraulics, you will have a localized droplet of water which will cause corrosion, and you have a very low boiling point (equal to water). That said, the odds of getting water in it are pretty slim, unless you get lots of condensation under the master cylinder cap. It won't absorb moisture from the air or through the brake hoses because it repels water. For cars that sit for long periods, it's great. The fluid in my car is still crysal clear after 3 years, and has no rust or sticky pistons to speak of. The other downside is that it's slightly more compressible than the other varieties, especially when it gets hot. So if you do lots of spirited driving or racing, your pedal might start to feel a little soft. The other thing to keep in mind is that if you decide to switch over, you can only put DOT 5 in.

From what I gather on DOT 5.1, it's similar to DOT 3 and 4, but has an even higher boiling point. I don't know how it is from the moisture standpoint.

Hope that helps.

EDIT: Oh, one other disadvantage to DOT 5 is that it does not work with antilock brakes, so don't ever put it in a car that has those.

ANOTHER EDIT: Almost forgot; DOT 5 does not harm paint if the hydraulics happen to leak, but needs to be cleaned off thoroughly before something can be repainted. DOT 3 and DOT 4 will destroy paint in a short time if they leak. Don't know about DOT 5.1.
 
Good move Tony, wish I had done that.
 
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