Techie question ... (Jeep)

[Mike Cecil]

Gents,

I'm consulting the brains trust, please ...

Willys MB:

New master cylinder installed about 2 years ago (about 1,000 miles ago). Everything working fine. The cheap no-name brand brake switch started needing more and more foot pedal to make the brake light work, so this morning I:
(1) removed the switch
(2) inserted a name brand switch
(3) bled the brakes.
(4) went for a short drive.

Problem: brake light remained on after the first push of the pedal. Checked the electrical circuit and the brake switch was live across the two terminals even when the brake was not depressed.

Solution: replace the new switch with another new switch from stock. The first switch removed, once out of the vehicle, was no longer live across the terminals once, so the problem was not the switch.

Problem: New replacement switch is like the first one: the brake lamp stays on after first application, but now turns off again by itself after about 45 seconds to 1 minute.

Anyone have any thoughts about what I'm doing wrong? Brakes are working fine and are not remaining on once the pedal is no longer depressed. I bled the brakes using a pressure bleeder, which leaves the master cylinder very full: is that the problem?


Mike

[rob love]

Do you have some amount of freeplay on the pedal? If you push the pedal with your fingers, there shoudl be a half inch or so before the pushrod actually contacts the piston.
As to overfilling, there is normally a vent on top of the master cylinder, so it should not be retaining any pressure in the reservoir, unless you have blocked off the vent. Without the vent, the heating of the cylinder from the nearby exhaust pipe can result in reservoir pressure and dragging brakes.

[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.