Hello everyone, Is synthetic dot 3 and dot 3 brake fluid compatible? Im having a rear brake problem, and im trying to rule some things out. thanks, Joe

By law, all brake fluid has to be compatible

I've never heard of DOT 3 synthetic. DOT 5 is synthetic. You should not mix synthetic and non synthetic in the same vehicle and never use DOT 5 in an ABS vehicle. What problems are you having??

so they can be mixed?

well over the winter, i replaced my axle bearings with green bearings and also my open to a suregrip, i also decided to replace the shoes and hardware,and wheel cylinders and top it off with prestone dot 3 synthetic from auto zone. I also asked if it was compatibleand they said yes. so i take it for a ride about 30 miles everythings ok. next day i take it to get exhaust put on and about 20 miles down the road smoke is pouring from the rear wheels. i get it home after the exhaust,pulled the wheels and its a mess, and wheel cylinders are seeping fluid. I replace the wheel cylinders and hardware and shoes, cut the drums, and topped off the fluid with napa brake fluid. go out today, get about 20 miles and can smell the linings smoking again. come home and on the way home it seems they are really dragging. take it apart again and fluid is seeping from the wheel cylinders, so that is why im seeking advice, Joe

my take on it is that you would be better off not mixing brake fluids but to do it will not be unsafe but could cause long term problems like corrosion etc.

Something is causing pressure to the rear brakes. I still say no to mixing, but I don't think is causing your problem. I had a '57 Chevy do the exact same thing. The rear brake actually caught fire while the customer was driving in a parade. I found the master to be the problem. Was not releasing all the way. I have also had cars that the brake rod is holding pressure. You can determine if this is your problem by driving the car and get the brakes HOT, jack up the rear, then as quickly as you can, loosen the bolts attaching the master to the booster or firewall, if the brakes are then free...you have a stuck master or a rod too long. Maybe, mixing caused the master to go bad????? Hope this will help. Jeff

Silicone fluid is DOT 5 , and can not be mixed with other fluids. Synthetic fluid is something different, and I would just follow the instructions on the bottle about mixing. I think your problem is caused by the brakes dragging, due to maybe the master cylinder rod adjusted a little long, or shoes not adjusted right or ? But not from mixing fluid if you did not involve Dot 5 Silicone.

I appreciate the help. I had a feeling i shouldnt have mixed those, and i also think it might be the master. Am i correct in my thinking, but when you pull back the boots on the wheel cylinders, there should not be any fluid seeping from the piston. The pistons on the passengers side wheel cyl. are also hard to move in and out. Im trying to keep the car original, do you know of any sources that may have an oe master? Joe I forgot to add they are manual brakes

The prob ain't happening that fast from fluid incompatibility. I'm thinking the OE wh cyls are worn out of spec (dia/pitting) and are leaking from that. Never heard of excessive psi causing them to leak PS are you using the correct size rebuild kits? Wouldn't think a person would not notice that (difference)in pistons/cups but just kicking out ideas to stimulate you

I have replaced the wheel cylinders after the first incident, I have owned the car for 6 years and have never had any problems until i did that work on the rear. fronts seem fine. should i bleed all the mixed fluid out and replace with fresh dot 3

Alright not worn OE cyls and w no prob until the rear brake work was done either there's a prob in the cyls or the fluid . Not sure on flushing it but I'd be inclined to do so rather than tossing any more parts at it which you have done to no avail. Maybe I'm wrong & mixing the fluid (types) CAN screw it up that fast.

Check the master rod. Pressure is not what is causing the leak, excessive heat is! You may be melting the piston seal causing the leak due to the brakes hanging up! Take care! Jeff

It could be you are getting faulty wheel cylinders. They should move with a fluid type movement and fluid should not be getting by the pistons. That Prestone dot 3 synthetic you used says it's compatible for ABS, disc and drum so I don't think that's the problem. Maybe take the cylinders back and tell them they leak, get you're money back and get the wheel cylinders from another source. Also double check installation of the shoes short in the front long in the back, with the adjuster in properly. Just thinking out loud here for ya.

Wht would i be looking for on the rod? is it something that can go bad or go out of adjustment

Could it be the wrong shoes, [to small], letting the wheel cylenders push out to far. Or to large of drum, same results. Maybe some thing installed wrong? I doubt the fluid would mix that far back, that fast, and even if it did, it shouldn't blow the seals out of the wheel cylenders. just my

today when i went to stop on the way home. you could tell the rears were hanging up. when i got home and pulled it apart, with the drums off i had my son push the pedal down slowly and the shoes would spread didnt seem to retract.

shoes are the same, i checked, and installed correctly. i did replace the drums when i first replaced everthing in the beginning.the old ones were out of spec, for cutting. thanks for the help