dot5 in system....very sludgy....converting back?

[nzaldivar]

Hey Folks,

I have had a 911 for 8 years and the previous owner converted to Dot5 so I had been following suite and all seems to be fine. I have very good pedal everyone is always impressed.....BUT!

Well after diagnosing an alignment problem with sticking front brake, I have realized that my brake system needs a rebuild. I have sludge build up everywhere, calipers and reservoir.

If I drain the reservoir and clean out the master cyclinder as well the caplier cleans am I going to be ok when converting back to non silcone based brake fluid like ATE (ie dot3/4)? Is there anything else I need to do?

or


Should I just put dot5 back in?


Thanks,

Ned
79 911SC Targa
182K miles

[vash]

from my hazy memory, i think my owners manual says, NO dot 5.

(honesty i dont even know what the number means.)

[john walker's workshop]

curdles nicely when mixed with the regular stuff.

[Charlie V]

I had dot 5 from PO. Brakes felt like crap. I would only use dot 5 in a hydraulic clutch application.

I drained and flushed the system last winter when I reconditioned the brake system (take apart, clean and check). Now running dot 4. Much better feel with the brake pedal - not spongy. Took a couple of bleeds to get all the air out.

This winter I'm installing Motul 5.1 and air cooling kit. Want to reduce brake fade during DE events and very spirited mountain driving.

Cheers.

[DG624]

I used Dot 5 for several years in a VW GTI with no probems...did you have a leak? I used Dot 5 to reduce brake line water absorbtion and because it was non toxic. I was thinkinng of switching. I didn't notice any problems with the VW...I used it for Solo II and as a daily driver...over 165,000 miles.

[Early_S_Man]

Ned,

It sounds like PO didn't rinse out the system or clean the calipers or mc before converting to DOT 5 fluid. A really bad scenario because the old fluid and absorbed water just sit in place and each clump is free to react with any metal nearby, which could be steel lines, cast iron calipers or mc!

Regardless of whether you convert back to glycol-based brake fluid or not ... calipers and master cylinder should be removed and disassembled, cleaned thoroughly! The lines need to be flushed with isopropyl alcohol and blown out with dry compressed air! Every trace of that old contaminated glycol fluid needs to be removed from the system, or you are at at risk of catastrophic failure becuse of corrosion!

It is your option to continue using the silicone fluid if you are satisfied with the 'feel' with the old DOT 5 fluid. Long-term relibility is enhanced, but only when installed into a clean, dry system, and that was the serious failure in the PO's actions!

If you choose to rebuild the calipers at this time [a very good idea, IMHO] be sure to use plenty of Dow Corning 4, 111, or 112 silicone grease on the bore and piston when reassembling the caliper. It is compatible with any kind of brake fluid.

Good luck!

[beepbeep]

Err...you DON'T use DOT 5 in a system that previously used DOT 3 or 4. As far as I remember, they are totally uncompatible. DOT 3/4 are ester based and DOT 5 is silicone (?) based.

DOT 5 is only used in modern cars with ABS and it seems that even manufacturers are converting back to DOT 4 on new models.

Mixing those two will make them cludge.

As I remember, good thing with DOT5 is that it's not hydroscopic, bat thing is that it offers somewhat spongier pedal.

[Tom F2]

One other advantage of DOT 5 is that it doesn't harm paint.

But, I had my brake lights stop working, except under heavy pedal pressure, because the brake light switches on my master cylinder were filled with rusty brake fluid/water. This is in a 77 Turbo that was converted to DOT 5 around 1980, and ran the same load until now.

I am lucky that someone let me know about the brake lights.

So, if you're switching back to DOT 3 or DOT 4, you can expect to find corrosion somewhere in your system, unless the DOT 5 was put into a dry system.

P.S. My brakes felt fine with the DOT 5. I guess if I were building the system completely, I would consider the DOT 5. It was nice to see a brake booster with all its original paint intact.

[Dennis Kalma]

I have had Dot 5 in my car since 1993, no problems at all, but then again it was a complete rebuild of everything. In terms of bleeding, it is more difficult, lots of microbubbles it seems which make it spongy, but if you persist in bleeding it a few times (with some mileage in between to shake the bubbles loose) and use a gentle means of bleeding (gravity bleeding is best) then it gets to be very acceptable across the board in terms of feel and temperature capabilities.

I would never mix the two, but you can safely flush the Dot 5 with conventional fluids, but then you are on the regular replacement routine. The issue is as described, they are OK together, the small remnants of conventional fluid harbour water in teh various crevices and then you get rust. Other way around is fine, the Dot 5 mixes OK and if it lives in the crevices, no problem, it does not absorb water, but you need to flush the system.

Dennis

[Wil Ferch]

Don't confuse DOT 5 ( silicone) with other versons of DOT 5 ( would that be 5.1 ?) that is ester-boron based ( also ??)...and that one IS compatible with DOT 3 and 4.

Simplest and safest answer is DOT 5 silicone should not be mixed..as stated here.

Wil

EDIT...beepbeep..you mean DOT 5 ( silicone) is not hygroscopic ( not hydroscopic)...yes..it won't absorb water, but it has an affinity for air bubbles that are hard to purge.