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MG TD TF 1500 - DOT 5 and rubber

I think I read here that if switching to DOT-5 brake fluid all the old rubber in the MC and wheel cylinders must be replaced. My project is getting all new and/or re-sleeved components. When the previous owner
took the car off the road in 1964 he began a brake job. Included with the car were unopened Lockheed rebuild kits for all the wheel cylinders and the master cylinder. I purchased new kits for the wheel cylinders but was hoping to use the MC kit that came with the car. Circa '64
Does anyone know if I will have problems with the rubber cups in this kit? Everything seems a little "beefier" in the older kit so I would prefer that over a new kit.
Thanks- Mark
Mark Butler

I converted mine 3 years ago with all new wheel cylinder kits etc. Ends up the rear wheel cylinders leaked. Just so happened I had to old original Lockheed kits that my Uncle must have purchased years and years ago. I put those in and re-bled the system. They have been working fine for the last 1.5 years.

The real issue with old systems is just making sure all the dot 3 is out to avoid contamination and reaction with the 5. I've converted many many old systems over to 5 with no adverse effects.

You're call I guess but if it were me I wouldn't be afraid to use them.

L Rutt

I would have counseled to get new rubber, but Larry's experience mad me rethink the use of old, unused seals. I would suggest that the rubber pieces be flexed to reveal any cracks that might have occurred over the years and if there are none showing, go ahead and use them. Cheers - Dave
David DuBois

mark, best of luck with the brake build.
my thought on this is as follows. as you know, our cars have a single circuit drum brake the only thing between you and the trunk of the ABS equipped scooter in front of you are the seals in the brake cylinders and the master cylinder. it is not a good feeling pulling up on the emergency brake handle as you head toward a pick up truck stopped in front of you...don't ask me how i know...soooo..choose wisely, be conservative on your criteria as to what is serviceable and what is not.. regards, tom
tom peterson

In every brake fluid spec some amount of swelling is required of rubber seals. The rap on Silicone fluids and rubber was due to seals which already were pretty perished with DOT 3.

The old Lockheed tins of brake fitting lube was basically a silicone grease. I don't think you will have a problem with new silicone and new/old seals.

Dave Braun

Look into DOT 5.1 brake fluid; I have had it in my MGA for over a year now with no ill effects. There is no need to drain old fluid or replace any seals if they are good before adding DOT 5.1.
David Werblow


Everything I've read recommends replacing the seals that have been soaked in DOT 3 before filling the system with DOT 5. The DOT 5 will cause the seals to swell if any DOT 3 is in the system. Also, when you remove the seals from the brake actuators you'll normally find them filled with a mixture of rust and what appears to be some type of grease...why not replace them? They're cheap and it's easy.

Gene Gillam

Thanks to all. As this car had been sitting for 48 years I replaced/rebuilt everything. The master and wheel cylinders were sleeved by Mark Frappier. (excellent job I might add...Thanks Mark if you are reading).
New brake lines. Diligently cleaned every brass. I am comfortable that any old brake fluid is gone.
The rubber I put in the master cylinder was literally "like new".
Mark Butler


DOT 5.1, like DOT 3 and DOT 4, is a polyethylene glycol-based fluid (contrasted with DOT 5 which is silicone-based). The three can be mixed, although I can't think of a good reason to do so. DOT 5 of course, can not be mixed with the other three.

Dave Braun

Even if "like new", I wouldn't use seals that weren't *literally* new. For one thing, rubber compounds have changed over the years. New rubber is compatible with *all* current fluids. Old rubber -- who knows? It used to be that the old natural rubber seals weren't compatible with DOT 3. That hasn't been an issue in many years because all the old seals are gone (so we thought. ;-) And DOT 5 didn't exist then. Not worth risking your car or your life to save a few tens of dollars!
Rob Edwards

What a minute Dave! Why is 5.1 not silicone? I thought silicone was the best thing since white bread? Are we going backwards now that I did my whole system with silicone??? Sounds like a trick Microsoft would play with windows! Bring back something old and call it a "new feature".
efh Haskell

My reason for using DOT 5.1 is that it will NOT strip the paint like DOT 3 and 4 will, and you don't have to drain the brake system and clean or replace everything.
David Werblow

I'm pretty sure DOT 5.1 (glycol) will strip paint. DOT 5 (silicone) will not.
Rob Edwards

Correct me if I am wrong (often) but I believe DOT5.1 has a higher boiling point than 3 and 4 as well as being less hygroscopic (absorbs less moisture). So for our cars, it is a more desirable fluid to use if you dont want to use the silicone product.
Jim Merz

That's correct. My understanding is that it has similar wet and dry boiling points to DOT 5 (hence the DOT 5.1 designation) but glycol based.
Rob Edwards

And Ed, I use DOT 5 Silicone, not DOT 5.1 Ethylene Glycol. Technically, you can mix any of them, but the experience says that mixing DOT 5 with old DOT 3 or 4, contaminated with dirt, rubber bits and moisture, makes a gunky mess.

You wouldn't believe the extents I go to to clean up a system for DOT 5 silicone brake fluid. But I'm paranoid.

Dave Braun

This thread was discussed between 03/07/2012 and 09/07/2012

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