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MG MGA - Synthetic Brake Fluid

Does Synthetic Brake Fluid have different expansion characteristics than regular when heated?

I switched over this winter (as many of you will recall)and I've noticed that the brake pedal bottoms at a different spot when the car has been running for a while vs. when its cold.

I'm fairly certain its not an air in the line problem as the pedal doesn't "pump up" it seems to be directly linked to when the brakes warm up.

Any thoughts?

Thanks,

Tysen
T McCarthy

Should have no effect at all. The extra fluid (if any) should come back into the reservoir.
dominic clancy

Tysen

The fluid contains a greater % of air and will have a different expansion characteristic, will also aerate, hence not used with ABS.
Paul

Sorry Paul, that's one of the myths about Si fluid. Si Fluid, properly installed and bled will have NO air in the system- just like normal hydraulic fluid. Si doesn't absorb moisture, that's the big difference to normal brake fluid. To aerate, it would have to find the air from somewhere, which would entail a leak in the system. It will not aerate in the reservoir.

When installing Si fluid, because it does have a tendency to form small bubbles in suspension, you should never put it back in the reservoir, but leave it standing for a few days before possible re-use of the clean fluid.
dominic clancy

Tysen

On my 58 A the brake pedal is "high" when I first start out after releasing the hand brake. One or two uses and the activates a bit lower. All in the adjustment of the rear drums.

FWIW

Larry
Larry Hallanger

Is this about silicone DOT5 or Castrol's synthetic DOT4? I've been using Castrol's DOT4 for a couple of years now, says it's compatible with regular (better be compatible with Girling) fluids. I haven't had any problems.
Tom

Dominic

Although not a specific DOT specification silicone brake fluids (NOT Castrol SRF which is a Silicon ester) can contain dissolved air at a level of 16%±3% by volume at standard temperature and pressure. This compares with a typical level of 5%±2%by volume of dissolved air for glycol fluids. An increase in brake pedal travel may be experienced under severe operating conditions, especially at higher altitudes and high temperature conditions.
Paul

Paul

That is a possible 19% on the worst case - which means bubbles in the fluid as I said. If you pour bubbles into the master cylinder, then you must expect to have air in the lines too!

In it's NORMAL state, (ie not shaken like crazy in the bottle before you use it) Si fluid should contain no dissolved air (as is the case with Glycol fluids).
dominic clancy

Hmmm, I can't make heads nor tails of it. I've thoroughly rebled the system (which yielded no bubbles)I can't find any evidence of leaks.

Its very consistent, when it cold the pedal is fully engaging the brakes at one point, when warmed up they engage at a point about 1 inch higher.

There is no pumping up or stiffening and ebbing, just when hot its as if the volume of the fluid in the system between the mc piston and the brake piston has expanded.
Tysen

Tysen,
What fluid are you using? There are a lot of different synthetic fluids, all of them different. There are synthetic Dot 3 and 4 and 5.1 which are not silicone, and there is Dot 5 which is silicone.

Also, is this a 1500 or a 1600, ie does it have disks or drums on the front? If this were a fluid or bleeding problem, the pedal would have a spongy feel, and would probably pump up to the same point every time. I am thinking that this must be something mechanical. Perhaps one shoe is not returning completely when warm. Maybe weak springs? Also, make sure you have enough free play in the piston rod. It could be that the piston is not returning far enough to uncover the return ports and keeping some pressure in the system. I have read old reports of silicone swelling the seals and preventing the piston from returning completely, but have never experienced it.

Jeff
Jeff Schultz

Jeff,

Its Dot 5. It has disk front brakes, which were rebuilt this past winter.
Tysen

Has anyone tried to reconfigure the rear 90 degree banjo fitting so that the bleeder is pointing up?
Fred H

This thread was discussed between 12/06/2006 and 14/06/2006

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