Mysterious "no brakes" problem on '48 Cadillac convertible

[49caddyman]

I recently had a very strange occurrence...one I haven't encountered in 47 years collecting & restoring vintake autos.  A customer of mine asked me if I would help him bleed the old brake fluid from the system on his '48 Cadillac conv. & replace it with new.  Sounds straightforward enough, doesn't it?!
   We successfully expelled the old fluid from the system by opening one bleeder screw at a time while he pumped the brake pedal.  Once all the old fluid appeared to be dispelled from the system, we installed fresh brake fluid.  The fluid we removed was the OEM non-silicone type, & naturally, we replaced it with the same type.
   We bled the brakes, and seemed to have a nice firm pedal, and thought we were done.  Wrongee again, thanks to the infamous Murphy's Law.  A couple of days later, I happened to notice a trickle of brake fluid on the garage floor next to the left rear wheel.  Thinking it was probably merely some residue dripping off the backing plate from when we bled the brakes, I didn't think much of it & wiped it up.
   About a week or two later, the customer was preparing to pick up the car.  I discovered, much to my dismay, consternation, and bewilderment, that the brake pedal went right to the floor...no brakes whatsoever!
   I'm surmising that the LR wheel cylinder must be leaking.  It wasn't leaking before we changed the brake fluid.  Has anyone ever had this happen to them?  I'm wondering if it's something that occurred from us expelling the old fluid from the system, or if this is another coincidental "fluke" that occurred against astronomical odds?!  Because of the bitter cold & no heat in the garage, I haven't been able to remove the rear drum to ascertain what actually happened here.  Joe Cutler

[Jon S]

I never heard of removing all the old fluid and then adding new fluid.  I always watch the master and add new fluid as it gets lower while removing the old fluid.

[TJ Hopland]

I don't know specifics about a 48 but the way I have always done it and heard of it done is first you remove almost all the oil fluid from the MC.  I say almost because you leave the ports still covered.   You then refill with new and start bleeding always making sure there is plenty of fluid in the MC.  If you got air in you may need to 'bench' bleed the MC again.   Also assuming there is some age there it may not take much exposure to air and then the movement without fluid to finish off the seals in the MC and wheel cylinders. 

[joeceretti]

That's the first thing I thought. Pumping the brakes with no fluid can easily damage old seals in the master cylinder but I never heard of it causing a problem in the wheel cylinders.

[Jon S]

The addition of new fluid to the Master pushes the old fluid out.  Once the MC ports get exposed to air, you're dead.  that is why you have to constantly check the MC level as you're bleeding the system.

[Steve Passmore]

Each one of these techniques work, if your doing a restoration you start with no fluid at all anyway.
It could be that that wheel cylinder was full of crud and frozen anyway, bleeding it out cleared it and caused the leak.

Easiest way is just peel back a wheel cylinder dust boot and see if its filled with a sugary substance.
If I ever had doubts about the fluid in my cars such that I thought I needed to change it then I would also to have every wheel cylinder off for inspection, Period.

[Gene Beaird]

It's also possible that 'exercising' the brake system by pumping the brake pedal and then letting the pressure off by opening a bleeder screw may have caused the wheel cylinders to move more than they normally-do under normal operating conditions. 

But as others have said, I never try to purposely empty the system of old fluid when bleeding the brakes.  Just suck out the old fluid from the MC, and fill with fresh.  Then start bleeding the brakes.  You'll usually see the difference between the old and new fluid as you bleed the brakes. 

[harry s]

As Steve mentioned there is probably a corrosion build up in the wheel cylinder and a small particle has gotten under the rubber cup allowing the fluid to leak. Been there, done that. Another mentod for changing brake fluid is to use a vacuum pump with the fluid capture cup and start at the furtherest wheel and work toward the master cylinder. It is a one person job, just keep the master cylinder full.    Harry