Thread: brake pressure switch sticking

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I replaced the almost unreachable brake light switch with a pressure switch on my '51 Chevy. The switch is on the passanger side frame rail and I replaced a connector with a "T". Brakes bled out and it worked fine. Then started sticking on, so I replaced the switch figuring it was bad, but it's sticking on again. The hydraulic brake system is otherwise stock, with a rebuilt original single pot M/C..

what can be wrong?

There is a possibility that the master has residual valve(s) in it and holding pressure in the lines..

That crossed my mind. I checked the '49-'53 shop manual but there is no mention ither way... I did however just notice that the brake pedal was quite stiff. I cracked a bleeder and it had a little pressure on it. Re-bled it all out and it's back to normal.. Old fluid possibly? or was that DOT-3 synthetic a bad idea? The DOT-3 syn was a fresh bottle I had on the shelf when I redid the front brakes. Said compatible with all DOT-3 and DOT-4 systems, so I figured it was ok.. I'v pumped a crap load of fluid thru it now. master has been filled 3 or 4 times out of that DOT-3 syn bottle

What psi is required to close the switch and turn on the brake lights??? Could be a low pressure switch, like you'd use for an oil pressure light, and the residual pressure as mentioned by 34_40 is closing the switch.......

PS--Please don't ask how I learned this!!!!!!!!!!!!!!!!!!!!!!

My 57 Chevy dump truck has the pedal throgh the floor with m/c under floor. The brake lights were staying on and killing the battery ..After a lot of changng this and that. I found that the return spring in the pedal was the problem. seems the weight of the pedal after a time was enough to turn on the switch... New spring no lights on when not wanted.

How about your MC pushrod length? If it's adjustable it needs a bit of free play when off the brake, to allow the MC to return all the way and relieve pressure, right?

Originally Posted by Dave Severson

What psi is required to close the switch and turn on the brake lights??? Could be a low pressure switch, like you'd use for an oil pressure light, and the residual pressure as mentioned by 34_40 is closing the switch.......

PS--Please don't ask how I learned this!!!!!!!!!!!!!!!!!!!!!!

It takes a touch of pressure to turn it on. I tested it that way to see how sensitive it was. The switch is 'correct' for a 1965 Rambler American, which I know from having owned 1, the switch is right under the master cylinder in it's own port.. However, the switch is a stock item at my local carquest so it may double as an oil light sender.

Originally Posted by cffisher

My 57 Chevy dump truck has the pedal throgh the floor with m/c under floor. The brake lights were staying on and killing the battery ..After a lot of changng this and that. I found that the return spring in the pedal was the problem. seems the weight of the pedal after a time was enough to turn on the switch... New spring no lights on when not wanted.

I'll have to see if it's weak or distressed, but it does come back up pretty easy

Originally Posted by rspears

How about your MC pushrod length? If it's adjustable it needs a bit of free play when off the brake, to allow the MC to return all the way and relieve pressure, right?

I'll have to check that and slacken it up a little if it is.

I had this happen on a 53 plym. years ago it turned out the vent hole in the cap was pluged up try washing the m/c cap......ted

The cap is a 3/4" plug that takes a 1" wrench to remove/ install.. However it won't tighten down. It spins out and while it won't come out, it's loose. Looking at the cap, I noticed the area near the hex face is recessed with no threads. I fixed the issue with a copper washer. Could that have caused it? if there's a vacuum on it because it's sealed so tight? I think there is supposed to be some type of washer but possibly somthing breathable. I bought an extra washer so what I think I'll do is knock a few notch lines in the washer with a chisel so that air can escape and ride it around/ see if that fixes it.

Yep, tha's probably it.. the cover needs to breathe.