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MG MGB Technical - Flushing out brake fluid

Hello again....
Okay I have spent the last few hours reading through reams of archives and I have to say I am more confused than before.... Heres the deal... I have used dot 3 brake fluid in my 73 b since I bought the car 3 years ago Because that is what the filler cap on my master cylinder says to use, and have had no problems but after reading all these opinions I have come to the conclusion that i should replace it with castrol lma... Okay fine, now, is there any reason why I cant just open all the bleeders and pump the new fluid in untill i am satisfied that I got most of it out??? Do I really have to clean it all out with paint thinner or denatured alchohol or jet fuel or whatever else some of these readers have come up with??? If I leave some residual fluid trapped in some of these small out of the way places and it mixes with the new fluid do I risk a violent death in a firey crash????
Please someone tell me this will be okay to do...
Thanks again
Bud
Bud Beck

Bud-
Paint thinner!?! Good God Almighty! No, no, no! Flush the hydraulic system with alcohol. It's not that hard if you use a Mighty Vac to suction the old stuff out. Just clean the master cylinder out as best you can and pour in the alcohol. You can get one from any auto parts store for about $25-$30. If a job is worth doing, it's worth doing right.

You can't just open the bleed valves and pump the brake pedal to get the old fluid out. You'll just get air into the system doing that. Starting with the right rear wheel, have an assistant push the pedal while you open the bleeder valve and close it just as the pedal reaches the bottom (have him yell as the pedal is about to reach the bottom so you'll know to shut the bleed valve. When you shut the valve, shout so he'll know to let the pedal up, drawing more fluid into the system. This makes for a memorable and colorful work experience, like slaves chanting as they work in the field. Keeping an eye on the level of the fluid in the master cylinder, repeat this tedious process until clean fluid comes out. Repeat the process again on the left rear wheel, then the right front wheel, then the left front wheel, until you've done the whole system. Or, get a Mighty Vac, it's worth it just for the time that it saves.

Castrol LMA is really good stuff, but Valvolene SynPower is better.
Steve S.

Bud; You will do just fine if you bleeed your system until you have replaced all the old fluid. Probably about 50 strokes or so per wheel. I work by myself so I have to do things without help. Take a jar or old BF container. Hook a hose to your bleader valve. other end in jar. open bleader. Pump slow about fifty strokes. Top up master as needed. Close bleader go on to next nipple. do left rear, right rear, right front then left front if left hand drive. Order not really critical. I use Lockhead Dot4 but NAPA dot 4 is fine if your out. The days of fluid type being critical went out with the fiftys. I agree do it right but my goodness there is no need to flush the system! Bob Thompson/International Auto
Bob Thompson

In addition to Steve's comments, consider using some well dried compressed air to flush the fumes left from the alcohol in the plumbing before replacing the brake fluid with any sort of new fluid. Denatured alcohol has been a flush of choice by brake shops for a long time for the reason that it does evaporate quickly when followed by either dried compressed air or simply allowed to set open for a few days.

I mentioned the process of using silicone removing painting solvents (NOT thinners) for silicone removal from systems, hoping that those with more experience on this BBS would comment, but no one tackled the subject. Even in the case of my outlandish process, the removal of fumes and the evaporation of residue via dried compressed air should most likely give a similar result to the use of denatured alcohol.

If you would like the reassurance of some sort of warranty against problems and failures (none of which should be too spectacular), you might want to simply take your car to one of the Professional brake repair outfits to have the fluid exchange done. I am sure their processes are well documented and benefit from years of practical experience.
Bob Muenchausen

This interesting often visited thread on brake fluids has kicked my very imperfect memory back to a time ( about three or catalogues ago) when I think I read a tech tip in the Moss catalogue, and I think it was by John Twist. Note the I THINK. I will not be disturbed if I am corrected! I believe the tip said that no damage would be done if DOT 3 were added to Si fluid, but in so doing, one would lose all the advantages of the latter, and especially the non hygroscopic and anti rust inducing properties. Now if this IS what I read, then it would seem that no flushing is needed for this radical step, and that switching between non-silicone type fluids would be even more benign, provided the dictum of continuing to put in fresh fluid until cloudy or discoloured fluid exited into the jar were followed.
Ken R
Ken Rich

I've got silicone fluid in my brake system that i'd like to change back to DOT4. Tried the silicone fluid in the clutch but it kept leaking past the seals in the new master. Put in DOT4 and has been fine. The last post has got me wondering just what is the best way to change from one type of fluid to the other. And how often does the fluid REALLY need to be changed. I've never flushed a system that was working good, only when something needed to be fixed but will soon be doing complete brake and clutch rebuild with all new parts except brake and clutch masters which are about 2 years old. Will changing fluid on a regular basis make the system last longer? Thanks.
James Skinner

Thanks to all who have written here... Of course I will take all of these opinopns into account and then probably take the easiest road (That is what I usually do Ha! Ha!) All kidding aside this webpage and all who write here provide a wealth of knowledge that we should all appreiciate.... I am going to " drain and fill" my system and see what happens...i have to agree with Bob T. I dont think that the small amout of dot 3 that might end up getting mixed with the castrol would cause any harm.... Another thing to consider is that I like to keep things simple so I am wondering why it doesnt make sense to flush the system with the right thing "Brake Fluid" After all dont we flush radiators with, "Water". Thanks again
Bud
Bud Beck

I think it's prudent to change your BF on a yearly basis. DEE's Gt has now been on the road for two years and it just turned up 40,000 miles. I am running silcone in both the clutch and the brakes. I ran a fresh quart through last February and will again this. Have had no problems with the Silcone and do like it. Many of your current manufactures tell you to change BF yearly I feel it makes sense. If for no other reason to flush the crud out. Bob Thompson/International Auto
Bob Thompson

The logic behind flushing out old contaminated brake fluid with alcohol is much like allowing as much of the old oil in the crankcase to drain out: It's nasty old stuff that needs to be removed. Also, Castrol LMA is a DOT 4 fluid with a higher boiling point than DOT 3 fluid. If the fluid gets hot enough to boil the remnants of the old DOT 3 fluid, then there's little point in mixing it with DOT 4 fluid as the lower boiling point of DOT 3 will be reached before the higher boiling point of the DOT 4 fluid, thus defeating the whole purpose of using DOT 4 in the first place. Also, Dot 3 is heavier than DOT 4 and so will eventually gravitate to the lowest points in the system, such as around the pisons in the calipers and slave cylinders, where it's old contaminated nastiness will do its worst to damage them. If you're not going to flush the system, at least bleed the old stuff out about a month after you put the new stuff in. This might get the worst of it out (note that I said might!).
Steve S.

I agree with flushing and refilling the system every year. The alcohol will also serve to dry out any moisture that might be in the system. BTW, make sure to use denatured alcohol, not rubbing alcohol. You can usually get it at home improvement places like OSH, or from a pharmacy.
Jake Nicholas

This thread was discussed between 21/01/2002 and 22/01/2002

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