My car was stored for two years, and while on a long shake-down drive yesterday (high 80s weather), suddenly the brake pedal began sinking while stopped at intersections.  A few pumps always brought back a hard pedal, but obviously I figured the ride was best cut short, and soon turned tail for home.  I had a modern jap driver with anti-lock a few years back that exhibited the same symptoms, until the brakes gradually failed totally.  A new master cyl. cured that issue, but I am not certain how the complexities of a modern dual master system might translate to my '55 OEM part (replaced 17 years ago).  Sitting in a cool garage now, the pedal is hard, and will not sink with constant pressure. But, due to the T-Bird master's proximity to the exhaust manifold, only another test drive will indicate if extreme heat is a contributing factor.  While I DO plan to pull the drums and inspect all wheel cylinders, banjo connectors, lines, etc. for leaks, I notice the firewall pad/carpet is dry, and the brake fluid level is unchanged.  The fluid IS nearly 4 yrs. old, so upgrading any worn components and bleeding (I have a pressurized canister system) won't constitute much inconvenience.  Regardless, I really know very little about the mechanical construction & operational details of master cylinders, not to mention installation prep. tips, etc., and am just wondering if anyone out there with experience/knowledge could offer any pertinent advice?  

6 VOLTS/POS. GRD. NW INDIANA

It's possible for the internal seal to fail and not show a leak out the bore. While temperature probably has something to do with it, I'd say it's a failing master, just like a modern car. My other thought would be water coming to a boil. Either way, I think you should look at the master, and as you said, everything else. If the fluid in the master isn't going down. It's most likely bypassing internally, it's going to fail or leak before it gets better.

I replaced all the brake lines on my 55 bird 15 years ago or so, and they were pretty sad internally. Same with the hoses to the front and rear axle. The fittings were brass, they cleaned up ok.

There's a lot of talk about poor quality offshore rebuilds, so maybe someone else can chime in with a reliable source. I'm running a dual master, so I haven't dealt with OEM parts.

I had a total failure in a 55 bird back in 68-69. Fortunately, I'd just left heavy traffic and pulled into a level residential street. Got it home ok. Chafed line under the clamp on the rear axle had split. No worse feeling in the world than the pedal hitting the floor and nothing else happening. (I guess it would have been worse with something in front of me.)

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Go figure.  Another long, hot drive today (90 degrees), and the brakes worked perfectly (consistently hard pedal) all the way.  I'm hoping it was air or water that seeped into the system and was boiled away or expelled (?).  Or maybe built up crud that temporarily stuck some internal bypass valve?  Whatever, any sign of a sinking pedal again in future will automatically trigger a master cyl. change.  In the meantime, I'll make sure my emergency brake is properly adjusted!

6 VOLTS/POS. GRD. NW INDIANA

"Internal bypass valve". That rings a bell almost 50 years back. Something in a brake master that allows you to pump up a low pedal. I was told that was why you shouldn't use a brake master on a hydraulic clutch. No particulars, just that one old comment out of the cobwebs.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Do you have the heat guard that goes under the master cylinder?


54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ

Yes.  The later '55 smaller one.  

6 VOLTS/POS. GRD. NW INDIANA

I've never heard of one or seen one and surely don't remember one on either our 55 or 56 when new. Neither gad power prakes.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

[quote]miker (7/6/2016)

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