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switching from dot 5 to dot3

Posted: Sun Jul 14, 2019 7:49 pm
by raynman1969
1969 Camaro with wilwood front discs and drum rear, stock gm master cylinder with prop valve. Cant seem to get a hard pedal. I have bleed these brake with a friend pushing very slow and trying a pressure bleeder and always the same. I like the dot 5 because its paint friendly but hard to foot brake the car if the brakes creep and will not hold.I have read just flush with dot 3 until the color is clear, also read to flush with denatured alcohol and also i would need to rebuild the calipers and wheel cylinders. Anybody have experience with this?

Re: switching from dot 5 to dot3

Posted: Thu Jul 18, 2019 2:49 pm
by Roundybout
DOT 5 being silicone based is slightly more compressible than DOT 3 or 4. It also has a higher boiling point and mainly used in applications where they sit for a long time and susceptible to moisture intrusion. That being said I don't think that's the issue with not being able to foot brake without some creep. It should produce enough brake pressure just fine. There are some DOT 4 that approach the boiling points of DOT 5 and probably better suited to your application. If you do switch they are not compatible and you do have to make sure everything is flushed completely. Denatured alcohol works but you have to make sure all of it is removed and not hiding in porportional valves and the like. Compressed air and leaving the lines open to evaporate works along with flushing it with DOT 4 a few times. Probably a good idea to rebuild all the rubbers (calipers, wheel cylinders, o-rings on the master cylinder piston ect.) as it will be hard to get rid of traces of DOT 5 there.

Re: switching from dot 5 to dot3

Posted: Sun Jul 21, 2019 7:50 pm
by Rick!
A Mightyvac goes a long way in jobs like this.
Suck everything out of the MC with a Mightyvac or turkey baster, pour in DOT 4 and suck until fresh on all 4 bleeders.
DOT 4 has a higher boiling point than DOT 3 and that's a good thing.
The rubber bits won't care if it's mineral oil or silicone and the little bit that intermixes won't be noticeable.
Do the calipers move freely, like really free or are they 4 piston units? 2 piston floating caliper units?
Pistons that retract too far make for spongy initial pedal until you pump them up. That is usually a piston o-ring issue.
Out of parallel rotors make for knocking the pads back too far and also creating pedal issues.
Calipers that don't float cause the outer pad to wear out faster than the inner pad.
Lastly, rear drums that don't self adjust properly make for a poor pedal. Maybe start off with several reverse - forward brake applications, aggressively, like lock up in reverse, then lock up in forward.
Good luck.

Re: switching from dot 5 to dot3

Posted: Mon Jul 22, 2019 9:46 am
by 77cruiser
I used one of these to change the brake fluid in my truck.
https://images-na.ssl-images-amazon.com ... L1100_.jpg

Re: switching from dot 5 to dot3

Posted: Tue Jul 23, 2019 6:19 pm
by cgarb
I just switched my race car and truck from dot 3 to dot 5.1 fluid. Dot 5.1 is glycol based like dot 3 and dot 4 but shares the higher boiling point and lower viscosity of dot 5. It can even be added to a dot 3 or 4 system with no issues. My local napa stocked it and it works pretty good so far for me.

Re: switching from dot 5 to dot3

Posted: Wed Jul 24, 2019 1:19 am
by englertracing
5.1
Personally I'd rebuild everything.
But I haven't ever messed with 5 myself.

Re: switching from dot 5 to dot3

Posted: Sat Oct 12, 2019 6:44 pm
by rebelrouser
raynman1969 wrote:
Sun Jul 14, 2019 7:49 pm
1969 Camaro with wilwood front discs and drum rear, stock gm master cylinder with prop valve. Cant seem to get a hard pedal. I have bleed these brake with a friend pushing very slow and trying a pressure bleeder and always the same. I like the dot 5 because its paint friendly but hard to foot brake the car if the brakes creep and will not hold.I have read just flush with dot 3 until the color is clear, also read to flush with denatured alcohol and also i would need to rebuild the calipers and wheel cylinders. Anybody have experience with this?
I use dot 5 on all the restorations and muscle cars I work on. paint friendly, and draws no moisture when they sit. I have never noticed a deference in pedal feel when switching between Dot 5 and Dot 3. Most of the time pedal feel is messed up with the diameter of the master cylinder compared to the surface area of the caliper pistons when sitting up the system. You have to take into account the pressure needed and the volume of fluid needed for application. Disc brakes require more volume than drum brakes.
This site explains how to do the math to get the right sizes.
https://www.joesracing.com/master-cylinder-math/