Brake puzzle

[GLAmes]

My car has a factory vacuum brake booster that was recently repaired by White Post Restorations under warranty because it leaked. Great service, no complaints. Yesterday, while checking for a power steering leak the brake lights lit up. I hadn't applied the brake pedal. My immediate suspicion went to the booster but even after I shut off the engine the lights stayed lit. I cracked the bleeder screw and the lights went off. Today I disconnected the vacuum hose and hooked up a gauge. 18 inches at an idle, drops to 15 or 16 when the engine revs and jumps to about 22 when the throttle is released, then back to 18. That seems OK to me. Here is the weird part. With the vacuum hose disconnected during my check the brake lights came on. I jacked one front wheel off the ground and the brakes were, in fact, applied. I waited a few minutes and they went out. I reached into the window and started the engine again without touching anything except the key. In a couple minutes the brake lights illuminated again. Anyone care to take a crack at what would cause the brakes to build pressure while the engine is idling with the vacuum hose disconnected?

[miker98038]

I’m not familiar with the ‘54 power booster, but the first thing I’d be checking is the free play between the pedal rod and the booster and the booster rod to the master cylinder. If it’s a Treadle Vac in it not sure how that would play out. Why with the vaccine disconnected, no clue. But I’ve personally not left enough free play, and as things warm up the brakes apply.

This would still apply to all those parts if it’s the remote style booster like my 55 bird had.

[GLAmes]

It is a remote booster, mounted on the inner fender. It is connected to the MC with a brake line and the brake line to the wheels come out of it. I did some more testing. I let it sit for over an hour. The brake pressure to the front tire decreased to normal. With the vacuum hose plugged I started it up again with no brake pedal application. I let the engine idle and after a few minutes the brake lights came on again while I stood behind the car watching. I didn't time it but it wasn't very long. So I shut the engine off and waited for the lights to extinguish. I plugged the vacuum port on the booster, leaving the plug in the vacuum hose so that the engine would idle normally. Again, after a few minutes the lights came on. Still scratching my head.

[Gene F]

Call White Post. They work on restorations too. People are human, possibly something in the unit is amiss. They may have seen this before. Remember, tone counts...

Is there a check valve in the vac assist system some place that may be hanging up?

[GLAmes]

White Post also rebuilt my MC, and I at first thought perhaps the internals were sticking. I'm not leaning that way since the lights come on without touching the brake pedal. I am, however, on my way out the door to check the rod free play.

[GLAmes]

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I checked the free play. It is too much. Should be 1/4 to 7/16 inch. It is over an inch. I'll correct that the next time I work on the car, but it isn't the cause of the brake light puzzle.

I'll have to rig up a fitting to check the check valve. Not sure how to accomplish that.

[Gene F]

Originally Posted by GLAmes

I checked the free play. It is too much. Should be 1/4 to 7/16 inch. It is over an inch. I'll correct that the next time I work on the car, but it isn't the cause of the brake light puzzle.

I'll have to rig up a fitting to check the check valve. Not sure how to accomplish that.

Sometimes start the engine, and a light tap with a wrench. Some simple soluitions are often the best.

[GLAmes]

I've come to the conclusion that the piston of the MC is stuck in the bore. There isn't anyway to put the free play back into spec, the eccentric bolt can't move the rod an inch. My theory is that once the thermostat is open the heat blown back from the radiator heats and expands the brake fluid in the line out of the MC. I hate to do it, but it is MC rebuild time, again.

[GLAmes]

I've removed, cleaned and reassembled the MC. The piston appeared to be completing it's full travel but I took it apart anyway. Nothing was obviously corroded or damaged so I put it back together. It was a good time to apply a coat of MC paint so it is drying.

Concerning the free play. The manual calls for 1/4 -7/16 inch. I get a full inch and a half of pedal movement before the rod contacts the piston. Are there two different rod lengths or possibly a plug that goes in front of the rod or am I reading the manual incorrectly? The parts manual doesn't show anything but I learned long ago to take it's information with a grain of salt.