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MG TD TF 1500 - Brake Light update....

Its a good thing i'm bald... cause if i wasn't, i would now have no hair!!!
Ok... I replaced all the brake lines...cleaned all the t joints, replaced the brake switch, used silcone fluid ... Two weeks ago the brake light started coming back on after 5 - 7 seconds of releasing the pedal. (that is a rehash of the problem).
Ok..this morning... I adjusted the brake pedal as it didn't have the 1/2" play before engaging (this in itself was fun with long wrenches and an engine in the way). Still the same problem... 7 seconds after the brake light goes out, it comes on again.
I changed the brake switch to the old Lucas one on it when i bought it... brake switch works, but still the same...7 seconds and on it comes.
I looked carefully at the schematic in the manual and thought one of the two vents from the bottom resevoir might be clogged... looked at another master cylinder and the vents are huge...(will still get a wire and carefully probe throught the fluid and see if there is a "clog").
The only other thing i see in the schematic is the little valve right out at the brake line entrance...not sure how to test that???????
gordon.b.lawson '53TD

Two long shot thoughts: If your car has turn signals could there be an electical problem in the relay box? Or a wiring short? You could remove wires from the pressure switch and check switch with an ohmeter. Or,If pressure relieves to shut lights off, I don't see how the master could be producing more pressure in the lines. Could the shoes be adjusted loosely so the wheel cyl pistons travel out a lot, with the shoe springs pulling them back so far they pressurize the system enought to close the contacts? Lastly, can you tell by feel (free play) if the MC piston is returning all the way?
George Butz

I tend to think it is the MC... the thing that confuses me is the On/off feature.. i could understand "always on" or "always off"... but its the off , then on again. It means to me, that more then the 8 pounds "held" pressure (according ot the manual) is happening after i let off the brakes.
It, however, is weird as i have a slight leak in the right rear cylinder and you would think any additional pressure would escape...
Still think one of the valves in the MC is not working correctly.
Tomorrow morning (after as many of you can commetn) i will pull the MC...(groan...more bleeding) and check the rubbers. (new kits in June or so).
gordon.b.lawson '53TD

Gordon

See the 2002 thread on this issue:

http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=8&subjectar=8&thread=2002101014185624930

especially Conrad Sander's contribution at the end.

There have been cases I believe (and I admit to being hazy about this) that silicone fluid doesn't expand so much and so doesn't push the master cylinder piston back as far as it should go after brake release, thus keeping the port to the fluid supply in the top closed (see fig M.2 in the workshop manual), resulting in an increase in pressure in the breaking system which could result in binding brakes, depending on their adjustment, or even perhaps putting the rear break light on ...

There is a cure in the thread suggested by Conrad.

Regards

David
David Wardell

Thank you... will try that (didn't want to take off the MC again, but anything to cure the problem).

this was the posting i assume:
conrad sanders, Florida, USA, esane@msn.com

I have had this happen when the car was changed to silicone. The problem was solved by dismantling the master cylinder and removing .010" from the face of the aluminum piston. The rubber cup was too large and was just covering the small, fluid return hole in the master cylinder. By removing .010" from the face just behind the cup it allowed the rubber cup to clear the hole.Try it, you will like it.
Sandy

I suppose it is better to remove the .010 from the piston rather then trim the rubber cup?
gordon.b.lawson '53TD

Ok... just an update... Went in this morning and took off the top of the MC... crawled in and poked a wire down the relief holes...no obstruction.
Bit the bullet and removed the MC... took it apart, and found the little holes in the piston clogged... cleaned them out and re-assembled... bled the brakes.... and i still have the problem... now it is about 2 seconds before the lights pop back on.
Was hoping not to "remove" surfaces as you would think everyone who rebuilds a MC would have the same problem????
gordon.b.lawson '53TD

Gordon,
At first you indicated that the brake lights come on about 7 seconds after releasing the peddal,,, now it is down to 2 seconds after your MC rebuild,,, how long does it take for the lights to go out again ??? Or do they stay on untill you turn off the ignition?
SPW
Steve Wincze

They seem to stay on until i turn of the ignition...although they usually will still be on when i turn it on again, unless i have been away for some time....(i'm assuming the extra pressure slowly goes away...????)
gordon.b.lawson '53TD

Gordon

Not an expert in this area at all but I seem to remember that between the piston and the rubber there is a small "spring" washer. I remember reading that it was important how this went back in. As you have just taken the MC out, did you confirm that this was in place and correctly installed? Just a thought.

Brian
Brian Smith

Yes... i found it and noted the manual description and which way it had to go......
gordon.b.lawson '53TD

Gordon;
Did you check both holes? One is much smaller than the other. That is the one that usually gets covered up.
Sandy Sanders
conrad sanders

When you changed over to silicone, did you replace ALL rubber in the system, including brake lines, and thoroughly flush the lines with acetone or alcohol?

If not you will have little bits of old rubber in the system and they will migrate to clog up the holes in the master cylinder. Any rubber that was exposed to DOT 3 or 4 will eventually disintegrate with silicone and leave debris in your lines.

I'll bet your holes are plugging again.
Don Harmer

Ok... it is getting even better then that....
Went out this morning and hooked up an alternate brake switch (will post something at another time about that)... I then went for a drive and lost what little play i had in the pedal... and the brakes kept staying on harder and harder. By the time i got back to the barn the linings were gettin' smelly.
I jacked it up and took off the MC again... this time i looked from the inside of the piston cylinder part and couldn't see that little front vent hole... checked it again with a wire and couldn't get it through...like it is solid. I'm thinking it was honed smooth. (however, i don't know how i could have driven 1000 miles with it like that...might have something to do with the one leaking cylinder however...).
Anyway... i am now trying to find out how large it should be... in the manual it looks like it tapers (i don't just want to drill it out the same size as it looks to be from the reservoir....)
Anyone have a MC sitting around who can drop a drill through to the piston cylinder????? I know the back hole is really large....?
gordon.b.lawson '53TD

Gordon

The systems sound exactly like those described by Conrad and others in the 2002 post. I bet the piston in the MC is not going all the way back and is covering the vent hole. I suspect that the cause of this is a combination of silicone fluid and 'old' rubber seals - or the rubber cup on the piston in this case.

I suspect if you went back to the original fluid, you wouldn't have the problem.

Regards

David
David Wardell

There "is" no vent hole visible...
When i changed to silicone, it was in all new brake lines and rebuilt wheel cylinders... there was no old fluid left in the system.
gordon.b.lawson '53TD

Gordon;
The vent hole is very close to the larger one. It is very small I would venture a guess that it is approximately .010 or thereabouts. Some times the replacement cups are a tad deeper and combined with a slight swelling they cover the small hole completely.Probe around right behind the larger hole and you will find it. It's there ,just hard to find.
Sandy Sanders
conrad sanders

Thanks Sandy... but i have the MC apart and have looked through the top with light shining in the bottom... can see through the large hole, but nothing at all through the small one. When i look along the cylinder part i can see the edge of the large hole, but absolutely nothing in the place where the small one should be? It is completely smooth... i am thinking that when honing it got full of particals. (again, don't ask me why it worked for 1000 miles).
gordon.b.lawson '53TD

Didn't know what size to make/clean the vent hole... sent a quick email to Joe Curto last night and he responded this morning...

Gordon if that hole is clogged than your brakes would lock up in a short period if time, it is a compensating port and as the brake fluid gets hot and expands it allows the fluid to go back into the reservoir, if clogged it applies the brakes, which sounds like your problem. As for the size I do not know what it is, I would use the tip cleaner for an oxy-acetylene torch and use the smallest in the set and go up from there.

Certainly sounds like it is a lot smaller then the opening i see down in the bottom of the reservoir. Am off to the shop to give it another go.
gordon.b.lawson '53TD

Update...

With that to go by, i used a safety pin straightenout out and happened to hit the "little" hole inside the little hole and got it to push through... it was clogged. Assume it was clear 1000 miles ago and slowly filled up.
Will now attempt the re-assembly and will touch up the end of the piston as suggested.

thanks all...
gordon.b.lawson '53TD

Interesting... could get a "pin" (tailor's pin) thought the hole...and a small safety pin... but nothing larger... That is a 'tiny' hole.
Also found under a high intensity lamp, that there were two rubber washers way down in there... the schematic shows only one??? hmmmmmm
gordon

Ok... this is the last addition to this post i will make...
Took out the extra rubber washer (it was the old original one) and reassembled.
Got the master cylinder back in and all 4 brakes bled in an hour and 10 minutes...
While i was pumping the brake pedal I would get a nice little "spit" of fluid straight up through that vent hole when the fluid got low...
The brakes are now back to their original feel and the brake light goes on and off (and stays off).
Thanks for all the help everyone... sorry to have this drag on so long.....
Am off in the morning to an MGT register get together in St. Jacobs, Ontario.
gordon.b.lawson '53TD

So it was a combination of a plugged hole and an extra washer ?????? And please Gordon,,, you have not dragged this out, I believe we want to find the cause how ever long it takes,,,


SPW
Steve Wincze

I concur. This is the kind of thread that gives us all the kind of information usually accumulated only by the pro that worked on these cars every day in years past. I vote for more of the same.

Safety Fast!

Jim Haskins

1953 MG TD
J. M. Haskins

Had similar issues with my TD-MKII. Did a Full rebuild of the system, MC WCs shoes hoses....and a conversion to silicone. All was well for a little while and then the same symptoms as Gord describes appeared. Did the shave off the piston and all resolved. Assertions are that the combination of the newer MCs and the silicone to a degree, result in a failure to retract far enough to release the pressure. Safety Fast. Paul
Paul

This thread was discussed between 21/09/2004 and 30/09/2004

MG TD TF 1500 index

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