Techie question ... (Jeep)

[Richard Farrant]

Hi Mike,
I recall this used to happen a lot with Ferret scout cars. They had an extension tube on the top of the master cylinder, for convenience of filling, but if filled to the top of the tube they will activate the pressure switch, usually after a drive, suspect that heat of brakes cause the fluid to expand a little. Also filler cap had a vent in it and can get paint or dirt blocking the holes.

regards, Richard

[Mike Cecil]

Thanks for the replies.

Remember this was a new m/cylinder that was working fine since installation circa 2 years ago and before I replaced the switch, refilled and bled the system, so I doubt it has the wrong spring.

I've checked the brake pedal freeplay, lowered the fluid level in the cylinder, checked the tightness of the end bolt and changed the pressure switch yet again. No change. Press the pedal, and upon release, the brake lamp 'blips' off as the pedal recovers, but then remains on for a minute or so.

Can't think of what to do next. B****y annoying...

Mike

[Mike K]

Strange.

Each time you remove the switch: Is there any indication of the fluid squirting out the hole under slight pressure ?

Maybe you can get somebody to press the brake pedal slightly with the switch removed just to get an idea of what is going on at the switch port/hole. With the pedal fully up, there should not be enough fluid pressure to activate the switch,

Another idea , remove the switch and test the switch on the bench with a battery and two wires , poke something down the hole while the battery is connected and see how sensitive the on/off mechanism is. It's probably a rubber diaphram or similar ?

[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