Techie question ... (Jeep)

[rob love]

I am thinking you are meaning to say a battery, two wires and a lightbulb. Two wires from a battery will just short out if you make the switch engage, likely burning out the switch.



If you have an air regulator with gauge, you can set it up onto the switch, and start increasing the air pressure until the light comes on. Removing the pressure should have the switch come off immediately. If it does, then your problem is the hydraulic system. Perhaps the residual pressure in the master is enough to engage the switch, or borderline to release the switch. There are identical switches that take higher pressure to engage. I had a quick look on NAPA which shows 4 similar switches for the Jeep. Two of them are listed as being for 60 to 120 pressure to close the contacts. I have seen other switches with much lower pressures listed but I'll have to check some of the NSN's at work tomorrow to confirm. If the residual pressure in your system was actually at 60 psi, I think that would have the brakes dragging.

You could also put a gauge onto the brake switch port and see what the pressure actually is after brake application, and how fast it returns to minimum. Any chance of a piece of something blocking the return port in the master cylinder? Luckily, if the switches prove to be OK, master cylinders are quite cheap in the US, usually around $40.

[Mike Cecil]

I have a circuit tester for that, and have tested the sensitivity of the switch. Seems to take a reasonable pressure to activate, and is crisp in its on-off operation.

Rob, you mentioned the return port in the M/cylinder, and I was thinking along those lines: crud in the m/cylinder well, partially blocking a port. Looks like I'm at the stage of removing the m/cylinder, giving it a good clean and reassemble. What a pain! Might pull a quick one and do a change over from the new spares box, then pull the old one apart to see what's what.

They are great when they are working fine, but ...

Thanks again, Gents, for your very helpful comments.

Mike

[rob love]

Yeah, for $40, how long do you want to spend figuring it all out. For about $60 you can replace both the master and the switch and be done with it.
One other point is what type of brake fluid are you using. I did note that some of the switches specifically mention they are not for use with silicone brake fluid. If the silicone causes the diaghragm within the switch to swell up and stick, that could also be your problem.

[Mike Cecil]

Rob,

Good points ... thanks.

Mike

[Lang]

Then of course you could get rid of the pressure switch and for a few dollars and very little work put in a foolproof spring loaded switch on the brake pedal.

[Mike Cecil]

That's true, Lang, but then, where's the challenge in that?? As my better half has often said: how hard can it be??

Tomorrow, it's m/cylinder change over time. The day after: that might be spring-loaded switch time!!

Mike

[Mike Cecil]

Despite being as careful as I could in re-filling the master cylinder, I must have knocked a minute amount of crap from under the vehicle into the open m/cylinder.

Today, I changed over the master cylinder, bled the brakes and bingo! everything works fine.

On flushing the removed m/cylinder with de-natured alchohol into a clean wash tray, I noticed some very small amounts of crap floating about. I suspect it was that which was being sucked into the holes in the bottom of the m/cylinder, partially blocking one or both, hence the time taken for the pressure to reduce and the switch to go off.

Lesson learned! Be more than careful when scrabbling around under the vehicle through the master cylinder access hole with the top off the m/cylinder!!

Thanks for the help, Gents: all contributions to the discussion greatly appreciated.

Mike