Brake lights that wont go off

[DBT]

Two auto electricians, a new brake light switch and shed loads of WD40 and still after a good run my brake lights will not switch off until well after the car has stood for a while - what is happening and what am I missing?

I read somewhere that the brake light switch in prewar Buicks is tied into the pressure in the hoses rather than the mechanical action of the brake pedal... is the system telling me I have a brake problem?

[jeff]

In your master cylinder there is a check valve that is supposed to trap something like 6 or 8 psi in the brake lines. This is intended to minimize excess pedal travel. Could this be sticking or restrictive, and only slowly letting the pressure drop off?

Jeff

[Guest brh]

This could be a real stupid suggestion-- I remember Fords had an issue where if you were doing something on the brakes the light use to stay on. The cure was to open a bleeder till the light came off, with someone applying pressure to the brake pedal. Might work??

[Guest imported_JPIndusi]

I suspect that the system is retaining pressure in the lines. Could possibly be caused by internal collapse or narrowing of flexible hoses or a defect in master cylinder.

Joe, BCA 33493

[The Old Guy]

You might try tapping the hydraulic stop light switch lightly and see if it is sticking.

[Glassesguy]

Try tapping the hydraulic switch as Joe suggested. If no good, try loosening the switch and have someone press the pedal to squirt out some fluid. Frankly, I think you have a partially plugged or partially covered compensating port in the master cylinder bore. The compensating port is a small hole drilled between the reservoir and cylinder bore. This is in addition to the main hole through which the fluid flows from the reservoir to the cylinder bore. If that port is plugged with rust or if it is partially covered by an improper brake pedal adjustment, excess pressure(courtesy of the check valve already mentioned) will remain in the system. Your brakes may drag also. Bleeding a little fluid as suggested may temporarily help. I went through this back in 1974 with my first old Buick which was a 38 Special. Finally spotted a little rust in the compensating port. Removed the master, thoughly cleaned it, carefully reassembled it, adjusted the pedal height and had no further trouble. See pages 5-2 and 5-3 in the 1938 Shop Manual. Good luck.

[DBT]

Thanks folks.

Will follow up as suggested on the master cylinder.

[DBT]

Just a followup to this post.

Had the master cylinder overhauled and the problem has gone so that indeed was the cause.

Interestingly I ordered parts over the web for a repair kit and the kit that came was for a 1 1/8" cylinder. The one in the car is 1 1/4" so we ended up sourcing parts localy. The kit rubbers too were pretty weak.

I wonder now what is meant to be the original sizing - 1/4 or 1/8th?

Thanks everyone for their advice!

Regards

David