Techie question ... (Jeep)

[Mike K]

Some master cylinders have a check valve located down at the output end of the cylinder , the end where the brass union for the pipes is located. The theory of the valve being there: it is to provide a small amount of permanent pressure in the hydraulic system. In your case, the little amount of permanent pressure might be enough to turn the brake light switch on.

[Harry Moon]

Those switches have been the bain of my existence, (Not really just adding some drama). New ones from the parts store fail altogether in short order either not working or on all the time. So i found an old one, cleaned it up and it's working better than the new ones and has been on and working for about 3 years now. Others in the club say the new made ones are all crap and have the same issues.

[Mike Cecil]

Gents,

Thanks - a couple of things for me to check. I'll do that tomorrow, and let you know the results.

Mike

[Lynn Eades]

Mike C., If it is not a bunch of faulty switches, and you have toe board clearance, then there is another long shot. It might be that your m/cyl has the wrong spring. The residual line pressure valve holds around 1.5 psi. It is a one way valve that lets fluid pass through the ports in the valve when the brake is applied. On release of the pedal, pressure in the lines lifts the whole valve from the rubber seal in the end of the bore, against spring pressure, until the pressure drops and the valve lands back on the rubber seal, back into a state of equalibrium. If the spring is from a different set up, it might level out at a higher pressure. This is admitedly a long shot.
Btw. the residual pressure is important. It holds the cup lips against the bores and prevents weeping and it holds fluid in the lines to enable a quicker response/ better pedal.
I believe your issue to be one of the first two things.

[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 ?