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MG TD TF 1500 - Effects of silicone brake fluid

Hi Gang;

Has anyone encountered long term problems with silicone brake fluid and the rubber seals from Moss or other source supplied brake components? I noted in an earlier
thread that I am having problems with an errant brake light system. The brake lights come on after I release the parking brake handle with my foot off the pedal and go off after a couple of seconds. After the car is driven for a short distance the lights come on when the brake pedal is pressed and come back on and then go off after I remove my foot from the brake pedal. As I continue to drive the time for the lights to go out after I remove my foot increases to about ten seconds. I replaced my Ron Francis brake light switch with a new Ron Francis switch but have the same results, eliminating the switch. To me this indicates that the system is not properly relieving pressure when no longer needed.

The obvious suspect is foreign material in the system. I have thought about flushing the system with new silicone fluid but I am concerned that the material may be from the breakdown of system seals which could ultimately lead to total brake failure even if I do the flush.

Thanks Again
Dick 20102
Safety Fast
Richard McCutcheon

Dick, before you do all that you might to check the archives about a little orifice in the master cylinder that, if blocked, can cause similar problems. It's only about .020" in diameter and is located in the main fluid chamber. If you look down through the filler opening you can see a small drilled out area. This orifice is at the bottom of that area. It's function is to relieve pressure into the fluid supply area. I'll hunt for an image. Should have one somewhere. Bud
Bud Krueger

I have used silicone exclusively in MGs and a Triumph since the late 1970s with zero problems.
D. Sander

Both Bud and David are correct. The small hole in the master cylinder between the reservoir and the actual cylinder is a pressure relief hole that bleeds off any remaining pressure in the system when the brakes are released. If that hole is not uncovered by the returning piston, the pressure will not bleed off. Check that this is not the problem you are experiencing.

Like David S., I have used silicone fluid exclusively in our TD and MGB and have never encountered any problems with the brake system. Cheers - Dave
David DuBois

Have you recently rebuilt the MC? Bud found that some of the rebuild kits had incorrect piston cups. Otherwise I'm with the guys above- no problems whatsoever from silicone fluid for years. Just in case something wierd is up with the switches, try cracking a bleeder valve and see if fluid squirts out. There should be no pressure. George
George Butz

In the past there was a problem with some of the brake light switches...they didn't last very long with the silicone fluid.
Gene Gillam

i've run silicon fluid for a issues. regards, tom
tom peterson

I've run silicon in an MGTD, Morris Minor, Austin America and Jaguar 3.8S for decades with no problems.
John Quilter (TD8986)

" In the past there was a problem with some of the brake light switches...they didn't last very long with the silicone fluid."

To this day, there are people who swear that silicone fluid is the cause of the brake light failure. This is probably due to some of the brake light switch manufactures stating that silicone fluid cannot be used with silicone fluid. Interesting because I am using a switch made by one of those manufacture that state that silicone fluid will cause the switch to fail. It has been operating fine for more than 10 years with a relay/arc suppression circuit, whereas it failed in a matter of months without the relay/arc suppression circuit. I also had a difficult time getting a replacement switch for our MGB that would not fail in a very short time. I got so frustrated that I installed a mechanical brake light switch from a late model MGB that operates off the brake pedal - no brake fluid ever gets near that switch - it failed in 2 weeks. Now both MGs have my relay/arc suppression circuit installed with the brake light switches and I have had no failures since. The only switches that hold up without a relay/arc suppression circuit added are the heavy duty switch that is sold by Ron Francis wiring and the new old stock (red box) Lucas switches (if you can find one). Cheers - Dave
David DuBois

In checking a Ron Francis switch that quite working I found that it still worked but that the ON resistance was in the neigborhood of 300 ohms. This of course is way too much. Remember that our switches are mounte3d upside down so all air will rise up out of the switch. Then if the gasket inside has any leaks the fluid will get to the contacts, silicone is a good insulator.

Just my findings and my 2 cents worth.
Bob Jeffers

...replaced my switch 3 times when i was getting it on the road... added one of Dave's relays and haven't had a problem since...
The silicon fluid is still the same i rebuilt with....there again, i had replaced all the lines and cleaned everything out of the original fluid....

"In the past there was a problem with some of the brake light switches...they didn't last very long with the silicone fluid."

this might be changed to '...they didn't last very long' period!

The small relief hole in mine was plugged and i had to clean it out with a needle (it is tiny, tiny) and you are searching for it blind as it is at the bottom of that 1/4" drilled hole...right in the middle of it.

Mine was so bad that the brakes wouldn't release....first drive I was dragging the rear wheels getting it home....

Removed the mc about 4 times before I found it...

When cleaned out, you can just see the end of the needle through the bore.....

gblawson(gordon- TD27667)

why do they make those holes so bloody small anyhow?? I have the same problem once in a while on one of my old motorcycles. Or at least used to with dot 3 as it can chuck up. no problems since dot 5.
L Rutt

Hi gang;

Crap! Made a tool to clean out the pressure bleed port in the master cylinder. Accomplished goal but no luck. The funny thing is that my problem seems to be associated with actuation of the rear brakes with the emergency brake handle. If the emergency brake is on I can press and release the pedal as often and as hard as I want and the lights go on and off following the pedal. If I release the emergency brake with feet off the pedal the brake lights go on and stay on for up to twenty seconds. Once the lights go off any pressure on the pedal causes the lights to go on, go off when the pedal is released and then go back on again without further pedal action. Pulling the emergency brake handle at this point turns out the lights and the system goes back to operating normally as long as the emergency brake is applied. It is pressure related as I can hear the Ron Francis pressure switch click with each action.

Can anyone tell me if a shoe brake system is designed to retain some level of pressure after activation? I wonder how long it takes for the pressure bleed port to bleed to the designed at rest pressure value? I wonder if the Ron Francis pressure switches are too sensitive to use with a shoe brake system although the first switch was in the car for two years and I can't remember of seeing this problem until recently.

Tis a puzzlement!

Thanks again.

Safety Fast
Richard McCutcheon

I have never personally experienced this, but I have head the the inner portion onf the flex brake hose can colapse and hold pressure in the line,,,, could this have some effect to your situation????


Pretty sure that is what the small rubber piece is at the outlet. Maybe it is sticking and not allowing enough pressure to bleed out? Pulling the handbrake takes all of the rear return spring pressure off of the hydaulics. Any way you could rig a pressure gauge to the system through a bleeder nipple or something? George
George Butz

Double amen to Steve's comment. Fooled around with mine for quite a while until I changed the rear rubber hose - problem solved!! When old it acts like a one way valve, MC can push fluid through but return springs cannot causing rear brakes to remain in contact and get quite warm.
GF Metz

Hi Gang, Me Again:

Thinking about bad brake lines I released the parking brake, the light came on and then I was able to move the car by putting my foot out and pushing while the light was still on. Obviously whatever system pressure causing the light to illuminate is insufficient to make the brakes function. Looks more and more like it may be normal residual system pressure taking time to bleed down through the tiny master cylinder bleed port and the Ron Francis switch is too sensitive for this application. Why this is happening now after a couple of years using this pressure switch with no problems is still a question.

Tomorrow I will install one of my spare NAPA pressure switches and see what happens. I prefer the Ron Francis switches as they illuminate earlier than the others which is always good when you use a gas tank as a rear bumper.

Thanks all

Dick 20102
Safety Fast
Richard McCutcheon

I have had Silicon in my TD for nearly 20 years without any issues with any brake or electrical parts. I think I may still have the original brake light switch.

Saying that. John Twist has said that unless you have a very nice car and do not want to risk paint damage, he does not recommend silicon. He says that at least in the MGB he has seen swelling that has affected the braking and or clutch system.

I have silicon in my B also and have not had any issues.
Bruce Cunha

Hi Gang;

Well I relearned something today. If you assume that it is mechanical it probably is electrical. I replaced the Ron Francis switch with a NAPA switch and the strange light operation problem went away. Seems the less sensitive NAPA switch covers for any residual system pressure after braking. The new problem was that the NAPA switch takes heavier pedal pressure than I want to illuminate the lights. The lights would come on after the car began decelerating and not at all under light braking raising the spectrum of the gas tank bumper. I tried two NAPA switches and they were similar. Then it dawned on this old dead head that even though I added brake fluid to the NAPA switch before installation I probably still had a bubble of air entrapped within the switch. I tried filling and then tapping the switch on the work bench until the fluid level in the switch lowered. I had to repeat this three times until most of the air was out. Operation with this switch is acceptable.

I may seem to be paranoid in relation to the operation of the lights but my truck has been hit in the back twice by women drivers, one reaching for coffee and the other texting, both in heavy traffic. As far as the Ron Francis switch I have no idea why the brake light operation characteristics changed after a couple of years of satisfactory operation. It has to be a change somewhere affecting the residual pressure in the system.

Thanks for your input

Dick 20102
Safety Fast
Richard McCutcheon

This thread was discussed between 19/08/2012 and 23/08/2012

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