Brake bleeding problems - silicone fluid

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Jeremy66
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#1 Brake bleeding problems - silicone fluid

Post by Jeremy66 » Wed Mar 28, 2018 10:29 pm

I’m struggling to get a constant firm pedal on my rebuilt brakes, as part of the cars restoration I’ve replaced the master, servo, all pipes, fittings, rear cylinders and fitted coopercraft 4 pots up front.

I’ve bled the system countless times, achieve a firm pedal but then within a few seconds the pedal travels to the floor once reapplied.

I’ve isolated the master and that holds pressure, no fluid is being lost at the caliper or fittings.

I removed the servo which had been reconditioned by a well known brake specialist and sent it back to them, they claimed the servo bellows chamber was full of fluid and they said the problem had been caused by using both silicone and conventional brake fluid, this is impossible as the system has only ever been filled with Automec silicone.

I have now refitted the re-rebuilt servo to the car and once again cannot achieve a sustained firm pedal. Before I remove the servo yet again and strip it down myself has anyone else experienced similar problems?

My fathers e type had silicone fluid in it for over 30 years and never gave any problems, I’ve also got other classics running fine on silicone.

Does this sound like a duff servo and excuses from the supplier?

Thanks

Jeremy

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1954Etype
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#2 Re: Brake bleeding problems - silicone fluid

Post by 1954Etype » Wed Mar 28, 2018 10:49 pm

Yes. We use silicone with no problems but have had issues with servo rebuilds. I am working with a seal manufacturer to put together a servo kit.
Angus 65 OTS 1E11497

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Gfhug
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#3 Re: Brake bleeding problems - silicone fluid

Post by Gfhug » Thu Mar 29, 2018 8:44 am

Silicon fluid isn't compatible with some of the seals in older cars designed for DOT4. Which might be the cause for some of your problems. Though it's interesting to hear you say your father's car ran with it.
If Angus can get a seal set for the E to change all of them to silicon compatible that would be good news. I hate using DOT4 with the fear of stripping paint.

Geoff
S2 FHC Light Blue

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mgcjag
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#4 Re: Brake bleeding problems - silicone fluid

Post by mgcjag » Thu Mar 29, 2018 8:53 am

Not the old Silicone debate again...ive used it for years in all my old cars......i havent had a seal damaged by it....its not ideal to mix different fluids and lots of blame has been put on silicone for damaging seals.....i have yet to see anyone that has put a good seal in a jar of silicone fluid and have it detiriorate.....what usually happens is someone fills up with it then when they have a problem with an old seal blame in on the silicone rather than just a worn out seal.......Steve
Steve
1969 S2 2+2 & Building a C type replica

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#5 Re: Brake bleeding problems - silicone fluid

Post by Gfhug » Thu Mar 29, 2018 9:56 am

Steve, I happily stand corrected.

Geoff
S2 FHC Light Blue

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Jeremy66
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#6 Re: Brake bleeding problems - silicone fluid

Post by Jeremy66 » Thu Mar 29, 2018 9:59 am

While I have known a new old stock master cylinder on an xk120 to fail to bleed properly on silicone fluid, requiring a new seal kit to make it work I put that down to the old stock seal having gone hard.

I really don’t want to go to dot4, the reason for putting dads car on silicone 30 years ago was to stop the newly refurbished brakes from corroding, they worked fine and sure enough when we stripped them down as part of the cars rebuild recently the brake cylinders were still perfect. While I appreciate this could be the case with regular changes of dot 4 it still poses the risk of ruining paint etc in the event of a leak.

I’ll take the servo back off again and see if fluid is getting past the seals in to the vacuum chamber, I’m getting some very expensive puddles on the floor with all this messing about!

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#7 Re: Brake bleeding problems - silicone fluid

Post by christopher storey » Thu Mar 29, 2018 10:58 am

I make this suggestion with some diffidence , but.... have the seals in the servo cylinder all been put in facing the correct way. It is appallingly easy to get this wrong . The first seal , nearest the vacuum chamber, faces with its larger lip facing forwards. So does the second seal / cup which is on the forward end of the primary piston assembly . It is the third seal which is the trap for the unwary . This lies at the rear tapered end of the secondary piston or shuttle, and has to face backwards. The fourth cup faces forwards

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Jeremy66
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#8 Re: Brake bleeding problems - silicone fluid

Post by Jeremy66 » Thu Mar 29, 2018 8:43 pm

Right, servo is back off the car and I’ve opened up the vacuum chamber, this time it’s dry inside so the next question is:

Is it theoretically possible for the servo to still be the cause of the problem but the pressure loss is now with another seal causing it to effectively back feed the servo supply reservoir?

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#9 Re: Brake bleeding problems - silicone fluid

Post by Hugo » Fri Mar 30, 2018 8:44 am

I don't like silicone fluid. I used it once, on a hydraulic throttle on a coach. It started leaking past the seals. As a temporary measure I topped it up with conventional fluid. As the silicone fluid leaked out and was gradually replaced by the 'normal' stuff, the leak slowly got better and eventually stopped altogether.
Somebody is bound to pipe up & say I am blaming the fluid instead of the seals, and they would be right. But the fact remains that silicone fluid is far less viscous and will leak under conditions that vegetable oil fluid will not. That alone is sufficient reason for me to avoid it.
Hugo Miller - rebuilding an imported Series II OTS & converting to RHD

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Series1 Stu
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#10 Re: Brake bleeding problems - silicone fluid

Post by Series1 Stu » Fri Mar 30, 2018 9:47 pm

I thought the problem with silicone brake fluid was it's lack of lubrication properties rather than it actually degrading the seal materials.

I've used DOT 5.1 in my cars. It's fine.

Regards
Stuart

If you can't make it work, make it complicated!

'62 FHC - Work In (slow) Progress
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#11 Re: Brake bleeding problems - silicone fluid

Post by Hugo » Fri Mar 30, 2018 10:32 pm

Series1 Stu wrote:
Fri Mar 30, 2018 9:48 pm
I thought the problem with silicone brake fluid was it's lack of lubrication properties rather than it actually degrading the seal materials.

I've used DOT 5.1 in my cars. It's fine.

Regards
I don't think it will harm the seals - it just creeps past them, like anti-freeze does compared with water.
Hugo Miller - rebuilding an imported Series II OTS & converting to RHD

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#12 Re: Brake bleeding problems - silicone fluid

Post by mgcjag » Sat Mar 31, 2018 9:28 am

Silicone dosnt creep past seals.....iv never had this happen.......obviously it will if you have a damaged seal.. Steve
Steve
1969 S2 2+2 & Building a C type replica

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#13 Re: Brake bleeding problems - silicone fluid

Post by Hugo » Sat Mar 31, 2018 9:36 am

Well, it leaks past them then. As I said in an earlier post, I had a pretty bad leak on a hydraulic throttle. Topped it up with the old-fashioned stuff till I could repair the leak, but I never did - it functioned perfectly for many years thereafter.
I'm sure if your seals are in perfect condition it won't leak, but the fact remains that silicone fluid will leak in situations where the proper stuff won't, in the same way that anti-freeze will leak out where plain water won't.
Hugo Miller - rebuilding an imported Series II OTS & converting to RHD

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#14 Re: Brake bleeding problems - silicone fluid

Post by 1954Etype » Sat Mar 31, 2018 3:09 pm

Utter rubbish!
Angus 65 OTS 1E11497

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Mich7920
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#15 Re: Brake bleeding problems - silicone fluid

Post by Mich7920 » Sat Mar 31, 2018 3:40 pm

20 years with pure silicone and absolutely no brake or clutch problems.

Mich
Michel
1963 E Type OTS - Angus Restoration
1965 E Type FHC - Front Restoration

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#16 Re: Brake bleeding problems - silicone fluid

Post by Hugo » Sat Mar 31, 2018 4:06 pm

1954Etype wrote:
Sat Mar 31, 2018 3:09 pm
Utter rubbish!
Which bit is utter rubbish? Are you suggesting I invented the story? Or what?
Hugo Miller - rebuilding an imported Series II OTS & converting to RHD

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#17 Re: Brake bleeding problems - silicone fluid

Post by cactusman » Sat Mar 31, 2018 5:05 pm

A minor point but dot 5.1 is not silicone based. It is still glycol based and is equally good at stripping paint. I know...I use it in my mg. Silicone fluid is dot 5. No point one.

Dot 4 and dot 5.1 can happily be mixed. They both absorb moisture. The only difference is dot 5.1 has a slightly higher boiling point. Both are extremely flammable.

Dot 5 will not absorb moisture. I can't see why it would attack seals as it is pretty inert, non flammable and won't eat paint. It should not be mixed with dot 4 or 5.1. People I know who use it in mg's have no issues with leaks but subjectively feel the pedal is a bit more spongy..
Julian the E-type man
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#18 Re: Brake bleeding problems - silicone fluid

Post by Gfhug » Sat Mar 31, 2018 5:07 pm

If one wants to change to silicone fluid what is the suggested way of doing it and clearing out DOT4?

Geoff
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#19 Re: Brake bleeding problems - silicone fluid

Post by cactusman » Sat Mar 31, 2018 5:26 pm

Not sure. I would open the bleed nipples and get all the old fluid out. Then I am afraid lots of rinsing through. Trouble is the silicon fluid is not readily miscible with most things one might use as a rinse and is very expensive to use as a rinse. Those I know who changed did it at the same time as rebuilding their brake systems with new hydraulic lines and either new or thoroughly cleaned and refurbished cylinders etc. It does give a more spongy feel.
Aside from the huge ability of glycol based fluids for removing paint replacement with silicone offers little advantage in my view other than it is non hygroscopic and there for lasts longer. Unless you are rebuilding your brakes anyway I would stick with glycol based.....but that is just my take...
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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#20 Re: Brake bleeding problems - silicone fluid

Post by Mich7920 » Sat Mar 31, 2018 6:00 pm

Rinse out all the circuits with methylated spirit.
Wait for dry and put silicone (blue liquid)
Put a bottle into the car, it's difficult to find during a trip.
Mich
Michel
1963 E Type OTS - Angus Restoration
1965 E Type FHC - Front Restoration

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