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By Bruce Compton - 7 Years Ago
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My '54 Merc brakes are building pressure even sitting in the garage and the car not running. If I leave it running the brakes will almost lock up and only release after the car is shut off for quite a while. Today I turned on the furnace in my shop and within two hours the brake lights were on (car not running). It has a Midland power booster ('55-57 T-Bird type). Any suggestions?? Thanks : Bruce
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By paul2748 - 7 Years Ago
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Did you made any changes just before this started ?
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By miker - 7 Years Ago
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I’d be looking for a lack of free play between the pedal and the booster, or the booster and the master cylinder. Very unusual from just turning the heat on in the garage unless you set it up at 20 below 0 or something. Maybe something slipped or hung up on the pedal itself?
Another long shot. Some master cylinders have a “plug” in the bore to allow them to work with both power and manual brakes. Maybe too much free play, and the rod slipped out of the bore in the plug and is hitting the edge. Resulting in no free play or even pressure.
I had that happen with too little free play between my hydroboost unit and the master, but that took 10 miles of driving on a 60 degree day.
Even with that booster, it’s possible, but a real long shot. I’d be looking under the dash and the firewall master first.
Edit. One more thought, is there a residual pressure valve in the system that could be hanging up at higher pressure?
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By Bruce Compton - 7 Years Ago
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The master is totally mechanically separated from the booster (mounted on the left side inner fender). I started doing this last fall when I was putting cars away for the winter. Thanks : Bruce
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By Bruce Compton - 7 Years Ago
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No, no changes, and everything worked perfect for three years. Bruce
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By miker - 7 Years Ago
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I’m familiar with that booster, and after 3 years I’m out of suggestions. Except if there is a residual pressure valve (typically 10lbs for drums), that’s somehow holding more pressure. Could be built in to either the firewall master or the booster master. I just don’t remember from when I had that unit in my bird.
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By Sandbird - 7 Years Ago
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Brake fluid attracts moisture. Maybe an excessive amount has built up over a period of time and is expanding when the temperature rises.
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By Hoosier Hurricane - 7 Years Ago
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There is a very small hole between the master cylinder bore and the reservoir that relieves the pressure in the system when the brake pedal is released. If that hole is plugged, the brakes will not release, or will build up pressure with temperature increase. It could be plugged with dirt or rust, or can be covered if there is no free play in the pedal. It is visible with the cap off if the fluid is clean or the reservoir is empty.
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By Bruce Compton - 7 Years Ago
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Thanks John, and the rest of you guys for helping me in this. I think I;ll pull the master off and go through it again since it hasn't been touched in 4 years.
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By MoonShadow - 7 Years Ago
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Did you do a line flush when you changed the master cylinder? Could have dislodged some old crap out of the lines that blocked that pesky hole.
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By 55 GLASS TOP - 7 Years Ago
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I would look at the flexible rubber hoses in the brake system sometime the close up internally and restrict the return flow of the brake fluid
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By Bruce Compton - 7 Years Ago
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Thanks everyone for your great suggestions. The problem was that the tiny vent hole in the master was plugged with crud. I rebuilt the master and all is well.Thanks again : Bruce
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