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MG MGB Technical - silicone break fluid

I have had silicone break fluid in both my TD and MGB for several years now. In the TD I have good firm break action. However in the B I have never been able to fully get the air out of the system to give me good firm breaks. Again this weekend I attempted to blead the system and while we got some air out breaks are still soft.
Has any one else had trouble satisfactorly bleeding out all the air from the system? What is the trick?
Thanks for any ideas.
Geoff Kimler

I have just tried the silicone in the B, seems fine and I didnt do anything that I didnt do on the TD, have you got a brake servo?, are you using a pressure type system to fill the lines? I use the gunsons easybleed filler fed by a tyre.
Tatty

Thanks for your quick reply, Mr. Tatty. I forgot to mention that this is a 1969 B with only the split break system, no servo. Yes I have tried the easybleed and this weekend the lads and I did it the old standard way. The only thing I can think of doing is put the whole bloody pint through those long lines to the rear cylinders.
Geoff

I take it that when you first went to Silicone, you changed all the cylinders etc, flexi brake lines?
Tatty

Hey Tatty, why would you go to all that trouble and expense if the existing parts are OK?
Dave
David

Wim Jenten, in his article on silicone brake fluid some years ago stated that it is necessary to change all the seals in a braking system when changing from standard DOT 3 or 4 brake fluid to silicon or vica versa due to the different absorbtion/desorbtion rates of the two fluids. When changing over to silicone fluid in both my TD and MGB, I completely disassembled the systems, cleaned everything out with brake clean, completely dryed everything and changed all the rubber components. I have never had a problem with the change over, while others, who have not done such a complete job of cleaning and change out of thr rubber components have had problems. This leand me to believe that Wim may have a valid point. The one area where I have had a problem with the silicone fluid is in the clutch hydraulics on the MGB. It is my understanding that silicon fluid will arreate (entraine small air bubbles) very readily when it is forced through small orfices quickly. This is why it's use is recommended against in ABS systems or systems that use equalizer valves and and this may be the problem I ham having with the clutch system. It only occures when I am on and off the clutch repeadley, such as when backing into a parking space. Both, or either of these causes could be the problem that is bothering Geoff. Good luck - Dave
David DuBois

I know this subject is a never-ending point of contention, but I have changed over several cars over the years and never replaced anything and never had a moments problem, including with my old '66 MGB that I have had for 9 years. All I have done is flush the systems out with silicone fluid. Just good luck on my part? Maybe. Maybe not. I do remember back when I was a teen in the mid 1970's having seals and boots turn to black goo in Lockheed hydraulics when I used good old 'Murican DOT III from NAPA.

David
David

Yes I had gone through the break system two years ago replacing all rubber. Some how I am still getting air in the system,perhaps its time to do a tear down again or maybe the master is causing this arreation problem that David is describing.
Well Thanks to all of you for your input.

Geoff
Geoff

My Roadster is fitted with automatic transmission, so the braking system has to be 'spot on' otherwise I simply won't stop....
When I rebuilt everything 10 years ago I renewed all the cylinders, fitted a servo unit, and filled it with silicon fluid as I have done on all my old cars (ranging from Bentleys to Morris/Wolseley).
Silicon has never, in itself, given me any problems. The only time I did have a problem was when I was building up a near 70-year old Morris and I had to use some 'old' wheel cylinders; I honed them as best as possible, but the surfaces were not perfect and I experienced 'soft' pedal and minor air ingression until I could renew the units.
Have you used '1st class' brake cylinders (wheel and master cylinders)? It is just possible that one of the seals is not doing it's job properly, hence your ongoing problem.
Of course, it could also be a defective hose but if this were the case you should see visual evidence of a leak.
Good luck, but don't discount silicon fluid. In my experience once the system is set up properly you won't have any problems over the years of internal rusting which destroys so many cylinders.
Jim Haines

Well if all the rubber and lines were changed, then maybe its a leaky caliper.
Tatty

A while back someone with a similar problem mentioned he'd found a rear wheel cylinder to be at fault. Didn't look like it was leaking, but was letting in air, and mimicing a master cylinder problem. Just a thought, and you seem to be going in the direction of rechecking all your rubber again, anyway. Best, Joe
Joe Ullman

This thread was discussed between 28/01/2003 and 30/01/2003

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