Re: Brakes

Posted by BH On 2008/1/16 10:21:10

I never bought into the notion that the TreadleVac hydraulic section can get sucked completely dry by engine vacuum.

Unlike more modern systems, the old master cylinders (and not just the TreadleVac) were not completely sealed to atmosphere. Left unused and unmaintained, DOT3 brake fluid will condense to a thick sludge over a long period of time; eventually, you end up with a crusty wax-like substance. The solvent base of the fluid simply evaporated.

It would take a failure of multiple seals for hydraulic fluid to leak out of the hydraulic chamber and into the vacuum shell, but even then, very little would get sucked into the engine. You'd have some warning of that through pedal feel.

If you've topped the system off, bled the brakes and have a nice firm pedal, you probably don't have a leak.

However, if you've simply refilled the master cylinder (no matter who made it) on an ancient garage/barn-find, seals could fail in short order. For vheciles like that, I recommned a complete overhaul of the brake system with new seals and hoses, flushing any steel lines that do not need to be replaced, and refilling with DOT5. You won't have the problem of evaporation with DOT5.