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Brake fluid

uglyblue66

Subscriber
Last Subscription Date
11/07/2018
When I started fixing up my 47 Pontiac I was kinda under the impression that DOT 5 fluid would be good to use in it as it wouldn't corrode and gel up and so forth.
Well, I rebuilt the entire system. And bled it and it worked.It sometimes will be completely empty of fluid and I have to bleed it out again and so forth. There is NO leaks. Where it goes I have no idea.
Can I put dot 3 or 4 in there after a good blow out of the lines and such? I don't have dot 3 or 4 getting gone out of stuff sitting around here.
 

ulgydog56

Registered
I do know boiling points are different for dot 3, 4, 5, and propertys of each, I would evacuate the break system....blue
 

LCJudge

Subscriber
Age
60
Last Subscription Date
12/14/2019
I've had the same thing happen. I have a '46 Chevy pickup. The local bank was celebrating its anniversary and wanted an old truck to drive in the parade so I loaned them mine. About 2/3 of the way through the parade the driver of the truck hit the brakes and there wasn't any, the pedal went to the floor. They pulled it to the side of the road and we loaded it up and brought it home. I looked everywhere on it and couldn't find a leak. The master cylinder and wheel cylinders were all dry as a bone. I looked at every fitting. Nothing. I called the guy who rebuilt the brakes a couple of years ago and told him what was going on. There was no brake fluid showing in the master cylinder but no leaks could be found. He said he remembered the old truck well as he rarely works on a vehicle this old but did it for me. He said he knew they bled it well and used the best brake fluid in it he could get. He said it had to go somewhere. I've driven the truck probably 1500 or more miles in the last couple of years and not a problem one with the brakes. Where could the fluid have gone? I filled it back up with fluid and its been fine the 3 or 4 times I've driven it since.
 

FWurth

One Millionth Post
Last Subscription Date
07/29/2019
We had an older GM pick up that kept doing a similar thing. Finally figured out that the brake line was too close to the exhaust pipe. It kept boiling the fluid and then air lock, no pedal. Moved the line away from the heat source and no more problems. On another one with the power brake booster losing fluid. It was leaking past the cups in the master and going into the vacuum chamber and then getting sucked into the motor intake.
 

LCJudge

Subscriber
Age
60
Last Subscription Date
12/14/2019
We had an older GM pick up that kept doing a similar thing. Finally figured out that the brake line was too close to the exhaust pipe. It kept boiling the fluid and then air lock, no pedal. Moved the line away from the heat source and no more problems. On another one with the power brake booster loosing fluid. It was leaking past the cups in the master and going into the vacuum chamber and then getting sucked into the motor intake.
The '46 has the master cylinder under the floor board and its not an easy task to even get to it. There is a hard rubber plug in the floor board that you pop out and you can see the top plug on the cylinder. No vacuum assist on this one but the exhaust pipe situation you mention is interesting. I'll have to give it a close look when the temps warm up.
 

Steve Kunz

Subscriber
Last Subscription Date
07/10/2019
I don't think changing the brake fluid will help. I had the same thing on an old Ford, if you let it set for a few months the brake fluid would disappear. I could not find any leaks. I did find that if I refilled the master cylinder and let it set a few days without touching the brake pedal it would work the air out of the system on its own just setting there.
 

uglyblue66

Subscriber
Last Subscription Date
11/07/2018
It has not run in several years sitting in the shop.I just turn the engine over and several times ,no fluid no where.And that dot 5 aint cheap. it just goes poof.
 

Rick Keane

Subscriber
Last Subscription Date
09/06/2019
I'm in the middle of rebuilding the brakes on a 1950 Dodge power wagon. Everyone tells me that DOT 5 brake fluid is silicone based and will not absorb moisture but will find the tiniest spot and all leak out. They claim that DOT 3 wont do that. I don't know ...Just what I have heard. I'm using DOT 3 and having the wheel cylinders sleeved with stainless steel so they don't rust.
 

Oldtech

Subscriber
Last Subscription Date
12/14/2019
I think Rick is right. That is why you have to thoroughly flush and clean the system to change. I've been tempted but always ended up sticking with 3 or 4. Now you have 5 in it, just check it whenever you need to drive it. That plug ain't that hard to get out
 

uglyblue66

Subscriber
Last Subscription Date
11/07/2018
Well I know the master cylinder is dry as a pop corn fart and I rebuilt everything when I redid it. I have had several issues come up in the past couple days but I am at a point I can get to the car ok.I might loosen a bleeder screw and see if any purple comes out. I know my other vehicals set around for weeks and no brake fluid gets gone.But they are dot 3 and 4. My dads Sportster Harley did the same thing.It sat for a few months and he went to back it out of the shed the other day and had no front brakes. needless to say he was peeod.
We put 5 in it .
I just checked the rear brakes on his 58 duoglide and they seem to still work with 5 in them.I think it is 5.not sure.
 

pegasuspinto

Registered
DOT 5 aint your problem. It's leaking, not evaporating. DOT 3 will almost certainly do the same. DOT 5 is better in most respects but if i remember right it adds a slightly spongy feel and it can froth when used in antilock brake systems (antilock pump churning), that info may be old and outdated tho.

If you want to switch back, you need to rinse the system out with denatured alcohol, and then blow out the lines and refill. It's been a week or two but I'm pretty sure DOT 5 and 3 mixed turn into a disaster.
 

uglyblue66

Subscriber
Last Subscription Date
11/07/2018
I aint done anything yet,The shop leaking and some issues with my dads truck have taken up my time.But I appreciate the denatured alchol thing as I didn't know that. I would love to see where it is leaking.Most leaks are visable when you crawl all under something with a light. IS there a uv die I can put in the fluid if it is a leak?
 
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