MGB & GT Forum

Whats the difference of DOT 3 vs DOT 4 Brake Fluid

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RestorerMG Avatar
RestorerMG Silver Member Bill Schultz
1974 MG MGB
1974 MG MGB
Installing a new slave cylinder for clutch, Changed out out Yoke, pressure plate, clutch plate already. 20/50 in transmission.
All info points to DOT 4 for the system. Whats the difference from DOT 3?
Renewing all of brake system also, Shoes, drums, disk, pads,lines, etc... Should I use DOT 4 also?

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ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
4 is compatible with 3
Higher BP
You can use either

"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"

"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972

oleanderjoe Avatar
oleanderjoe Gold Member Joseph Baba
Fresno, CA, USA   USA
About $2.50

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Aphyle2012 Avatar
Aphyle2012 Silver Member Eric Pyle
Shenandoah Valley, VA, USA   USA
DOT 3 is bad for natural rubber seals. DOT 4 is not. First-hand experience with having seals blow coming to a T-intersection.
DOT 3 is nearly everywhere. DOT 4 is not.
DOT 3 is cheaper. DOT 4 is...not.
Both absorb water. DOT 5 does not.

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tahoe36c Avatar
tahoe36c Paul Hruza
Panama City, FL, USA   USA
1969 MG MGC GT "Little Red Rocket..."
1972 MG MGB GT "Tiny Dancer"
2002 Harley-Davidson Dyna

Now that you have heard all the scientific data and other blabber... moon The "right now answer" is use DOT 4. Best Choice. thumbs up


SCE Avatar
SCE Sven Erichsen
Renton, WA, USA   USA
1973 MG MGB "Betty"
DOT4 is one dot better.

ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
1971 MG MGB
DOT 4 has a higher boiling temperature, but shorter lifespan (due to higher moisture absorption) than DOT 3. At least, that is what my reading indicated. Practically, I've never noticed any real difference. They can be mixed.

Terry Ingoldsby

Grahamhaber Avatar
Grahamhaber Graham Haber
Dingman Township. Pa., NYC,New York, USA   USA
The chart attached shows the difference it is the rated boiling points, although you can buy products sold as DOT 3 claiming 450 degree boiling point. But those are the department of transportation minimum requirements for the rating. except for 5 they are all glycol based with more borate esters in 4 & 5.1 to increase boiling temp. The most important thing is to charge your fluid every 1-2 years as it absorbs water lowering your boiling point and corroding your broke system from the inside. Wet boiling point refers to performance after it has been in your system a year + and has absorbed water from the air.

Edited 1 time(s). Last edit at 2017-10-13 12:25 AM by Grahamhaber.

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Screen Shot 2017-10-13 at 1.13.46 AM.jpg

lgorg Avatar
lgorg Larry Gorg
Renton, WA, USA   USA
1966 MG MGB "Robbie"
You received the short answers above. For the longer answer, please refer to Brake Fluid Comparison Chart. There are 83 fluids listed in the comparisons. The author is into Porsche and Lamborghini, but the data is still good for MG's. Below the chart is an explanation of brake fluids that is an interesting read by itself. Like batteries that are made by Johnson Controls with many labels, brake fluid is also manufactured by one company that slaps different labels on them.

MGB567 Avatar
MGB567 Gold Member Barrie Braxton
Sunshine Coast, Queensland, Australia   AUS
1966 MG MGB MkI "Money Guzzler"
1979 MG MGB GT V8 Conversion "Darkside"
The recommended changeover is different too (as well as BP) - I'm sure Google knows but if you can't find it just go read the back of each bottle.

Convertible: CKD 11/66 first registered 8/5/67. Owned since 3/77. 90% original sheet metal. 18GB +40 balanced with almost all new internals. Peter Burgess big valve fast road head. Piper 285. Fidanza FW. Basil's followers and pushrods. TR7clutch. TT exhaust. ARP everywhere. 123 ign. Needham 4synchro c/r box.. Stock rebuilt/replaced suspension. Superpro bushes. New brakes all round including all pipes in SS flex. Interior redone. CAMS approved roll bar and side bars. Lots more. Hybrid of o/e and show/fast road car. Not for sale - it's my toy!

GT: UK car built/sold December '78. Stripped back to bare shell (with extensive bodywork to come). Powered by 'worked' Rover 5 litre V8 (ex TVR Chimera) with efi. T5 box. FC IFS. CCE rear attached to Salisbury axle with Quaife. And a whole lot more to yet to come. Stealth is the word.

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