Packard Forum Index » Pre-War (1899-1942)

I recently had a brake and clutch master cylinder re-sleeved for an MGA, and the guidance I received back from Apple Hydraulics was to not use silicone as it was unknown whether the rubber parts/seals are incompatible with silicone.

That's ridiculous and suggests a company looking for loopholes of disputes over shoddy work. Because we know of no one driving any vintage car using DOT 5 silicone brake fluid in the proscribed manner having a problem. Ever.

Follow the product's directions.

Several rebuilders of power brake boosters will not honor their warranties if DOT 5 is used.

DOT 5 is also responsible for very short service life of some hydraulic brake light switches, most notably the one used on torsion level 55-56 Packards.

DOT 5 has a swelling agent added to mimic the effect of DOT 3 & 4. I have yet to see a manufacturer's bulletin regarding changing the brake fluid in a modern car from DOT 3 & 4 to DOT 5. DOT 5 is OEM for Harley-Davidson and AMG.

One rumor is that mixing fluids will cause problems. That is not true. When I was at DOD we found personnel mixing brake fluids with no ill effects.

As for the brake switches I had problems with those switches with modern DOT 3 which is not the same as the old stuff - that is the DOT 3 that required metal master cylinders. I did work on a 56 that came out of long term storage and had the old DOT 3 in it. I saved some of that old fluid in a vial to use as a reference. Exposed to light it turned brown. So I think those switches are responding to the new fluids.

The bottom line is that I suspect those warnings are more a legal liability issue the same way as the labels on a modern car oil and master cylinder. Motor rebuilders don't want to warrantee their work if someone is putting the wrong weight oil in their motors.