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MG TD TF 1500 - Another Dot 5 Query

My TD has been in storage a few years and after getting it out I find that the wheel cylinders are all corroded and need to be replaced. I have been vacillating about replacing Dot 3 with Dot 5 fluid.
I know I need to get all the old Dot 3 out but my question is can this be done without removing the M/C?
Is it possible just to pour a lot of acetone into the M/C and flush it out (via the clutch pedal) and get it clean enough? Pulling up the carpet and floorboard and disconnecting the removing the brake pedal assembly (not to mention removing the steering wheel so I can wedge my creaky old bones in there) makes me think replacing some wheel cylinders every couple of years might be a lot less work. Also, to properly clean all the brake lines, do they need to be removed from the car or can I just run some acetone through them as they are installed on the car. Thanks.
Joe Holtslag

Joe, I'm about to begin converting my TD from Castrol GTLMA (essentially DOT4) to DOT5. I don't see any way of doing it without removing, and totally replacing, all soft parts in the master cylinder along with a total flushing with acetone. Plus the replacement of the three hoses in the system. I'll be installing new wheel cylinders. I'll be purging the hard brake lines with acetone. Indeed, replacing some wheel cylinders every few years would be easier. BTW, I'm also adding a brake servo. Bud
Bud Krueger

I dont understand the use of Acetone to remove DOT-4 and clean the lines.

I have always been told that one uses either "methylated" spirits or methyl alcohol. Either of these are compatible with the existing rubbers and are a solvent for DOT-4. I am not aware that acetone is a solvent for DOT 4.

After cleaning min this way I flushed the lines out with dry nitrogen.

Jim B.
JA Benjamin

Regardless of what solvent is used to flush the system of existing DOT 4, ALL rubber parts (including hoses) MUST be replaced before refilling with DOT 5 fluid. Cheers - Dave
David DuBois

I think that if the TD in question has been in storage, and said storage has buggered up the wheel cylinders, I'd go through the master just for piece of mind. Not expensive and not that big of deal.

I'd also sleeve the wheel cylinders with stainless vs buy new AL ones. Stainless will NOT corrode again :) As for brake lines. I just used brake cleaner in a can and blew through all the steel lines, replace all rubber lines. Be carefull with the brake light switch though.

I've had not problems since. If you go through everything now that will be one less thing to worry about later.
L Rutt

Methylated spirits is an impure form of ethanol, it is NOT methyl alcohol.
Geoff Ev

They just make it that way so if you drink tax-free booze you'll go blind :-(

J K Chapin

This is getting close to home. I'm about to do the switch from DOT4 to DOT5. Chemistry was a 'C' subject. Why should I use acetone to clean the metal of the old system rather than (which form of ) alcohol? My only past experience with acetone was as a cleaner for soldered joints. I recall a time when I tried to see how much styrofoam would dissolve in a small jar of acetone. The level never seemed to go up. The liquid just got thicker. Bud
Bud Krueger

Acetone will dissolve the ethylene glycol "varnish" of DOT 3-4 better than "rubbing" ethyl alcohol, which is already diluted with water. Absolute ethyl alcohol (zero percent water, 200 proof) is hard to come by. Both will dissolve in water and dissolve the poly alcohol of glycol. The advantage of acetone is it is a better solvent and it is far easier to "dry" out than the alcohols you can get easily.

(I have a PhD in Nuclear Chemistry from UCLA , and a BS in Organic Chemistry) so I do understand the properties of alcohols and aldehides.
Acetone is an aldehide.

By the way, chemists alcohol (190 proof) without the denaturing stuff makes marvelous Martini's and many punches. Not available to us
peasants. Absolute alcohol is distilled with benzene to get the last 5% of water out to get 200 proof, and leaves traces of benzene which effectively denatures it so you shouldn't drink it.

Nothing will remove the various stuff that get absorbed by the rubber components exposed to DOT3 or 4 and that will react with DOT 5.
This is why you must replace all "rubber" components in the system.

It is true that DOT3-4 and DOT 5 are "compatable" in that they don't react chemically with each other and do not mix or dissolve. BUT this is the very reason you nust rtemove all traces before switching.

I have used DOT 5 in my 1962 MGB, 54 TF since it first became available and have never had a brake failure.
Don Harmer

I don't have a PhD in anything, but I did sleep in a Hampton Inn last night... Sorry, I couldn't resist.

I also used acetone to completely clean all my brake components prior to all my very successful switch overs to DOT 5. My reasoning was similar to Don's, I'm glad to have his assurance.

I'm a fan of replacing the hoses in this process, flushing the pipes, replacing the REAR cylinders, and honing and rebuilding the master and the FRONT wheel cylinders. Unless pitted, the future use of stainless sleeves is unnecessary with DOT 5. Anneal or replace the copper washers

A shop in Minneapolis does a great job of turning drums, arcing the shoes to match the drums, and sleeving when needed. You can ship to them by USPS flat rate shipping, and they turn their work quickly. Jay Leno (and Dave Braun) are loyal customers. Google Brake & Equipment Warehouse.


Dave Braun

Dave, Someday the undercarriage of my TD will be that clean and shiny - Wow!! I'm impressed and envious.

J K Chapin

I don't blame you. It was awfully pompus!

Don Harmer

Thanks, Don. Is there a 'final' process to use before installing the DOT5? Is there something to neutralize the possible effect of acetone on the new hoses and cylinder components?
Bud Krueger

Here's some good info on DOT 5 silicone based, DOT 4, and DOT 3 glycol based fluids.

DOT3 Brake Fluid Grade - Min dry boiling point=205degC or 401degF. Min wet
(saturated w/water) boiling pt = 140degC or 284degF. Aliphatic
polyether-based. Can be mixed with DOT4 and if so, characteristics will be
(linearly) in between DOT3/DOT4 parameters. Cheap, most widely used, eats
paint, absorbs water easily, eats natural rubber seals (this is bad). 1%
moisture can lower the boiling pt to 369degF, 2% to 320degF, 3% to 293degF.

DOT4 Brake Fluid Grade - Min dry boiling pt=230degC or 446degF. Min wet
boiling pt = 155degC or 311degF. Borate ester-based. Can be mixed with DOT3
and if so, characteristics will be (linearly) in between DOT3/DOT4
parameters, 50% more expensive than DOT3, eats paint, significantly increased resistance to moisture absorption
compared to DOT3 (but still absorbs water), does not eat natural rubber
seals (this is good). Most DOT4 fluids actually start out with a 509degF dry
boiling point.

DOT5 Brake Fluid Grade - Min dry boiling pt=260degC or 500degF. Min wet
boiling pt = 180degC or 356degF. Silicone-based. Cannot mix with other
fluids, turns to jelly if done so; colored purple. Used mostly for antique
cars and some motorcycles. Contains more dissolved air than glycol-based
fluids, aerates more easily when pumped rapidly through small orifices ala
ABS. Is more compressible than DOT3/4. Twice as expensive as DOT4 (and thus
3 times as expensive as DOT3) and has limited availability. Does not damage
paint. Isn't really suitable for a car that actually gets driven, definitely
not suitable for a car that gets driven hard or raced. Has low lubricity,
which increases the wear on sliding surfaces and may cause galling. Becomes
corrosive toward brake seals when hot, more specifically, leaches the
plasticizers out of nitrile rubber seals (which are very common in brake
systems) and causes them to become brittle. Doesn't absorb water but also,
doesn't stop it from getting into the system. It then collects in the low
points of the brake system. Absorbs air the way other brake fluids absorb
water, and releases the air at temps well below the rated boiling point
(fluid itself isn't boiling, but it's giving off air bubbles and giving you
a spongy pedal just the same). Viscosity at room temperature causes it to
entrain air when you pour it, which means you're going to be pouring air
bubbles into your brake system. See SRF.

DOT5.1 Brake Fluid Grade - Min dry boiling pt=260degC or 500degF. Min wet
boiling pt = 180degC or 356degF. It's a stupid name (the 5.x bit) as it's
synthetic & glycol-based (borate ester) like DOT3/4 but has NO silicone.
Named in the 5.x series as it beats the DOT5 performance criteria. Absorbs
water, eats paint, more expensive than DOT4, difficult to locate.

cj schmit


You use the acetone on the lines before the new rubber components are installed. (it shouldn't hurt the rubber anyhow) but the intent is to use it before installing any new rubber so the new rubber never contacts any traces of non-DOT5 fluid, even the residue dissolve in the early flush volume.

Blowing air thru will remove all traces of acetone which evaporates easily.

A tip: prefill (prime) the master and wheel cylinders with DOT5 before installing to make it easier to later bleed the system.
Don Harmer

Thanks, Don. That pretty well answers the questions. Can't really start until after the 4th of July parade. I'll see if I can't document this and add another page to Ttalk. Bud
Bud Krueger

This thread was discussed between 24/06/2013 and 27/06/2013

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