Brake Fluid - Mixing dot 3 and 4?

[Mayor Goldie Wilson]

i need a second opinion can dot 3 and dot 4 be mixed?

my brake resevoir says use Dot 4 only - my service history from previous owner has a receipt for Dot 3!!

should i top up resevior with dot 4?

i am a rookie mechanic - and not over confident at changing the fluid completely - but i can follow instructions!!

any one got any advice for me?

thanks,

1988 1.6TD caravelle / DIY camping facilities on the way!

[toomanytoys]

Didnt think you could get dot 3 anymore...
Pretty sure you can mix, but I would consider a system flush with fresh dot 4, when was it changed?

[Titus A Duxass]

I second Toomanytoys:
Go for a complete flush, it should be changed regularly anyway.

[cozmik]

I second Toomanytoys:
Go for a complete flush, it should be changed regularly anyway.

Hi M8..care to give us a quick runthrough on how to do complete flush?
Cheers
Cozmik

[Syncro G]

DOT 3 is like DOT 4 but has a lower boiling point so can take less presure. Might age a bit quicker too. They can be mixed but obveously a pure DOT 4 system will be higher performance if not aged.

Brake fluid absorbs moisture and so should be changed every 2 years.

To change it its like bleading the brakes (follow workshop procedure for more details). First remove the contense of the resovware by some sort of suringe thing, then refill with fresh fluid. Then start bleading the brakes, shouldn't get any air out of it but keep the fluid flowing untill loads has come out so you know the pipes have been flushed. If the fluid was old it'll be cloudy anyway so you'll see when the clear fresh stuff comes through.

Personly though I understand how brakes work and can and do work on them I hate doing the hydrolics so prefer to pay someone I trust do to that bit - If nothing goes wrong (blead nipples can snap, then you've got issues) you'll be looking at an hours labour + fluid so its not that painfull.

[Red Westie]

As with all car fluids, technology improves and standards move onwards.
DOT3 and 4 are mixable but the newer standard means higher boiling points (not higher pressure as was indicated)
Brake fluid is Hygroscopic, which means it attracts and absorbs water and even when operating withing a relatively sealed enviroment , such as a braking system, water will be absorbed over time.
Why is this so critical? well look at the WET BOILING POINT figures for just 3% water content!!!! and then the much higher figures when the fluid is not contaminated.

Dry Boiling Point Wet Boiling Point
DOT 3 401ºF 284º F

DOT 4 446º F 311º F

Brake fluid change every two years or 24,000miles which ever is soonest.
Use 1 full litre of DOT4
Start with the furthest wheel cylinder away from the master cylinder (that's the N/S/R for a R/H/D or O/S/R for a left hooker)

You can try and suck most of the old fluid from the resevoir before you start, otherwise just bleed it out before the first top up (be careful not to bleed dry) or else air will be drawn into the system.
Put a good 300/350ml through the first cylinder, then around 200ml through the other wheel cylinder and front calipers, working you way towards the closest brake calliper.
I tend to go back to the furthest away at the end for just a few pumps to take the fluid level just below max on the resevoir (the level float, if it has one, will cause the fluid to rise 3/4mm in the resevior once refitted)
Don't over-tighten bleed nipples
Remember that brake fluid damages paintwork so be careful not to spill any.
Resevoir is under the speedo binnackle (tip it from the w/sreen side towards the steering wheel to remove)
As I said earlier, be careful not to allow the resevoir to run dry
Pump brakes slowly....around 2 second to push down and the same up.
Use the method of.....
Open nipple say 'OPEN' have your apprentice push the pedal down and say 'DOWN' you lock off the nipple and say 'UP' he lets the pedal up and says 'UP'
So on and so fourth.
We have power bleeders in the trade (attachments so we can put the resevoir under low fluid pressure) so we don't require the second person in the vehicle or to be endlessly topping the resevoir up but there you go.
PS FAO just in case you didn't realise DOT5 is silicon based so altogether different and not suitable.

Best of luck
Martin

[Titus A Duxass]

Nicely described Red Westie.
You'll know when its enough, you'll get clean brake fluid out (or relatively clean).

[Red Westie]

Nice clean brake fluid that has been stood on a shelf in your garage for a year could contain a dangerous amount of water.
Soo simply saying bleed until the fluid goes clear isn't really good advice.
That's why you need to discipline yourself and put a full litre through the system, this will gaurantee you are seeing the BRAND NEW fluid and not some of the clear but old fluid that was in the resevoir.
Martin